{"id":92,"date":"2019-08-09T21:01:03","date_gmt":"2019-08-09T21:01:03","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/chapter\/unit-2-the-cardiovascular-system\/"},"modified":"2025-05-30T22:32:58","modified_gmt":"2025-05-30T22:32:58","slug":"unit-4-blood-vessels-and-circulation","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/chapter\/unit-4-blood-vessels-and-circulation\/","title":{"raw":"Unit 4: Blood Vessels and Circulation","rendered":"Unit 4: Blood Vessels and Circulation"},"content":{"raw":"<div class=\"unit-2:-the-cardiovascular-system-\">\r\n<div class=\"textbox shaded\">\r\n\r\n<strong>Unit Outline<\/strong>\r\n\r\n<a href=\"#4-1\"><strong>Part 1:<\/strong> Structure and function of blood vessels<\/a>\r\n<ul>\r\n \t<li><a href=\"#4-1a\">Shared structures<\/a><\/li>\r\n \t<li><a href=\"#4-1b\">Arteries<\/a><\/li>\r\n \t<li><a href=\"#4-1c\">Arterioles<\/a><\/li>\r\n \t<li><a href=\"#4-1d\">Capillaries<\/a><\/li>\r\n \t<li><a href=\"#4-1e\">Venules<\/a><\/li>\r\n \t<li><a href=\"#4-1f\">Veins<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#4-2\"><strong>Part 2:<\/strong> Capillary Exchange<\/a>\r\n\r\n<a href=\"#4-3\"><strong>Part 3:<\/strong>\u00a0Blood flow, blood pressure, and resistance<\/a>\r\n<ul>\r\n \t<li><a href=\"#4-3a\">Components of arterial blood pressure<\/a><\/li>\r\n \t<li><a href=\"#4-3b\">Pulse<\/a><\/li>\r\n \t<li><a href=\"#4-3c\">Variables affecting blood flow and blood pressure<\/a><\/li>\r\n \t<li><a href=\"#4-3d\">Venous system<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#4-4\"><strong>Part 4:<\/strong> Hemostatic Regulation of the Vascular System<\/a>\r\n<ul>\r\n \t<li><a href=\"#4-4a\">Neural regulation<\/a><\/li>\r\n \t<li><a href=\"#4-4b\">The cardiovascular centres in the brain<\/a><\/li>\r\n \t<li><a href=\"#4-4c\">Baroreceptor reflexes<\/a><\/li>\r\n \t<li><a href=\"#4-4d\">Endocrine regulation<\/a><\/li>\r\n \t<li><a href=\"#4-4e\">Autoregulation of perfusion<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#4-5\"><strong>Part 5:<\/strong> Circulatory Pathways<\/a>\r\n<ul>\r\n \t<li><a href=\"#4-5a\">Pulmonary circulation<\/a><\/li>\r\n \t<li><a href=\"#4-5b\">Overview of systemic arteries<\/a><\/li>\r\n \t<li><a href=\"#4-5c\">The aorta<\/a><\/li>\r\n \t<li><a href=\"#4-5d\">Coronary circulation<\/a><\/li>\r\n \t<li><a href=\"#4-5e\">Aortic arch branches<\/a><\/li>\r\n \t<li><a href=\"#4-5f\">Thoracic aorta and major branches<\/a><\/li>\r\n \t<li><a href=\"#4-5g\">Abdominal aorta and major branches<\/a><\/li>\r\n \t<li><a href=\"#4-5h\">Arteries serving the upper and lower limbs<\/a><\/li>\r\n \t<li><a href=\"#4-5i\">Overview of systemic veins<\/a><\/li>\r\n \t<li><a href=\"#4-5j\">The superior and inferior vena cavae<\/a><\/li>\r\n \t<li><a href=\"#4-5k\">Veins draining the lower limbs<\/a><\/li>\r\n<\/ul>\r\n<h2><a href=\"#P\">Practice Questions<\/a><\/h2>\r\n<\/div>\r\n<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\"><strong>Learning Objectives<\/strong><\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nAt the end of this unit, you should be able to:\r\n<p class=\"hanging-indent\"><strong>I.<\/strong> Describe relationships between the following components of the cardiovascular system and explain their functions: blood, artery, vein, capillary, atria, and ventricles.<\/p>\r\n<p class=\"hanging-indent\"><strong>II. <\/strong>Compare the structure and function of arteries, veins, and capillaries.<\/p>\r\n<p class=\"hanging-indent\"><strong>III.<\/strong> Describe what is meant by blood pressure and specify the following: five factors which affect blood pressure, the major mechanisms that control blood pressure, and the average blood pressure of a young adult.<\/p>\r\n<p class=\"hanging-indent\"><strong>IV.<\/strong> Describe what is felt when a pulse is located, and specify four points where an arterial pulse may be felt.<\/p>\r\n<p class=\"hanging-indent\"><strong>V.<\/strong> Describe the following components of the cardiovascular system: the main arteries leaving the heart, and those serving the trunk, appendages, and heart; the main veins entering the heart, and those draining the trunk, appendages, and heart.<\/p>\r\n\r\n<\/div>\r\n<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\"><strong>Learning Objectives and Guiding Questions<\/strong><\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nAt the end of this unit, you should be able to complete all the following tasks, including answering the guiding questions associated with each task.\r\n<p class=\"hanging-indent\"><strong>I.<\/strong> Describe relationships between the following components of the cardiovascular system and explain their functions: blood, artery, vein, capillary, atria, and ventricles.<\/p>\r\n<p class=\"hanging-indent\"><strong>II. <\/strong>Compare the structure and function of arteries, veins, and capillaries.<\/p>\r\n\r\n<ol>\r\n \t<li>In general, which arteries and veins carry oxygenated and deoxygenated blood?<\/li>\r\n \t<li>Define: artery, arteriole, vein, venule, capillary.<\/li>\r\n \t<li>Compare and contrast the structure of the walls of arteries, veins, and capillaries.<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>III.<\/strong> Describe what is meant by blood pressure and specify the following: five factors which affect blood pressure, the major mechanisms that control blood pressure, and the average blood pressure of a young adult.<\/p>\r\n\r\n<ol>\r\n \t<li>Define the term \"blood pressure\".<\/li>\r\n \t<li class=\"hanging-indent\">Describe how blood pressure is measured.<\/li>\r\n \t<li>Define cardiac output and describe how each of the following physiological factors affect blood pressure:\r\n<ul>\r\n \t<li class=\"hanging-indent\">Heart rate<\/li>\r\n \t<li class=\"hanging-indent\">Contractility (strength of contraction) of the heart<\/li>\r\n \t<li class=\"hanging-indent\">Blood volume<\/li>\r\n \t<li class=\"hanging-indent\">Peripheral resistance<\/li>\r\n \t<li class=\"hanging-indent\">Blood viscosity<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>How does maintaining blood pressure contribute to homeostasis.<\/li>\r\n \t<li>Describe how blood pressure is regulated by:<\/li>\r\n<\/ol>\r\n<ul>\r\n \t<li style=\"list-style-type: none\">\r\n<ul>\r\n \t<li class=\"hanging-indent\">The nervous system<\/li>\r\n \t<li class=\"hanging-indent\">The endocrine system<\/li>\r\n \t<li class=\"hanging-indent\">Autoregulation<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ul>\r\n<p class=\"hanging-indent\"><strong>IV.<\/strong> Describe what is felt when a pulse is located, and specify four points where an arterial pulse may be felt.<\/p>\r\n\r\n<ol>\r\n \t<li class=\"hanging-indent\">When you manually \u201ctake someone\u2019s pulse\u201d, what is causing the pulsing pressure waves you feel?<\/li>\r\n \t<li class=\"hanging-indent\">List four locations on the human body where a pulse can be taken manually and explain why an arterial pulse can be felt at specific locations rather than just anywhere on the human body.<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe, using examples, how capillaries use simple diffusion, facilitated diffusion and osmosis to exchange material with tissues.<\/p>\r\n<p class=\"hanging-indent\"><strong>VI.<\/strong> Describe the following components of the cardiovascular system: the main arteries leaving the heart, and those serving the trunk, appendages, and heart; the main veins entering the heart, and those draining the trunk, appendages, and heart.<\/p>\r\n\r\n<ol>\r\n \t<li class=\"hanging-indent\">Draw a flow chart showing the components of the cardiovascular system. Start with the three main components (heart, blood vessels, and blood), and continue by specifying all the constituent parts of each.<\/li>\r\n \t<li class=\"hanging-indent\">Compare and contrast (clearly!) the anatomical structure and function of arteries, veins, and blood capillaries.<\/li>\r\n \t<li>Draw a simple diagram of the human cardiovascular system that shows both circuits, indicating the vessels blood is moved through as it is passed to and from the head, arms, organs of the abdomen, and lungs. Your diagram should include:\r\n<ul>\r\n \t<li>The main arteries leaving the heart:\r\n<ul>\r\n \t<li>Pulmonary trunk<\/li>\r\n \t<li>Pulmonary arteries<\/li>\r\n \t<li>Pulmonary veins<\/li>\r\n \t<li>Aorta<\/li>\r\n \t<li>Ascending aorta<\/li>\r\n \t<li>Aortic arch<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>The main arteries serving the trunk, appendages and the heart:\r\n<ul>\r\n \t<li>Descending aorta<\/li>\r\n \t<li>Thoracic aorta<\/li>\r\n \t<li>Abdominal aorta<\/li>\r\n \t<li>Brachiocephalic artery<\/li>\r\n \t<li>Left common carotid artery<\/li>\r\n \t<li>Right common carotid artery<\/li>\r\n \t<li>Left subclavian artery<\/li>\r\n \t<li>Right subclavian artery<\/li>\r\n \t<li>Common iliac artery<\/li>\r\n \t<li>Axillary artery<\/li>\r\n \t<li>Femoral artery<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>The main veins entering the heart\r\n<ul>\r\n \t<li>Superior vena cava<\/li>\r\n \t<li>Inferior vena cava<\/li>\r\n \t<li>Coronary sinus<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>The main veins draining the trunk, appendages and the heart\r\n<ul>\r\n \t<li>Subclavian vein<\/li>\r\n \t<li>Axillary vein<\/li>\r\n \t<li>Brachiocephalic vein<\/li>\r\n \t<li>Femoral vein<\/li>\r\n \t<li>Common iliac vein<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<p style=\"text-align: justify\">In this unit, you will learn about the vascular part of the cardiovascular system; that is, the vessels that transport blood throughout the body and provide the physical site where gases, nutrients, and other substances are exchanged with body cells. When vessel functioning is reduced, blood-borne substances do not circulate effectively throughout the body. As a result, tissue injury occurs, metabolism is impaired, and the functions of every bodily system are threatened.<\/p>\r\n\r\n<h2 style=\"text-align: justify\"><strong><a id=\"4-1\"><\/a>Part 1: Structure and Function of Blood Vessels<\/strong><\/h2>\r\n<p style=\"text-align: justify\">Blood is carried through the body via blood vessels. An [pb_glossary id=\"976\"]artery[\/pb_glossary] is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels. Eventually, the smallest arteries, vessels called [pb_glossary id=\"598\"]arterioles[\/pb_glossary], further branch into tiny [pb_glossary id=\"977\"]capillaries[\/pb_glossary], where nutrients and wastes are exchanged, and then combine with other vessels that exit capillaries to form [pb_glossary id=\"599\"]venules[\/pb_glossary], small blood vessels that carry blood to a [pb_glossary id=\"978\"]vein[\/pb_glossary], a larger blood vessel that returns blood to the heart.<\/p>\r\n<p style=\"text-align: justify\">Arteries and veins transport blood in two distinct circuits: the [pb_glossary id=\"421\"]systemic circuit[\/pb_glossary] and the [pb_glossary id=\"420\"]pulmonary circuit[\/pb_glossary] (Figure 1). Systemic arteries provide blood rich in oxygen to the body\u2019s tissues. The blood returned to the heart through systemic veins has less oxygen, since much of the oxygen carried by the arteries has been delivered to the cells. In contrast, in the pulmonary circuit, arteries carry blood low in oxygen exclusively to the lungs for gas exchange. Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. Although arteries and veins differ structurally and functionally, they share certain features.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1534\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image37.png\" alt=\"image\" width=\"1534\" height=\"1024\" \/> <strong>Figure 1. Cardiovascular Circulation.<\/strong> The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart to repeat the cycle. The arrows indicate the direction of blood flow, and the colours show the relative levels of oxygen concentration.[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1a\"><\/a>Shared Structures<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0Different types of blood vessels vary slightly in their structures, but they share the same general features. Arteries and arterioles have thicker walls than veins and venules because they are closer to the heart and receive blood that is surging at a far greater pressure (Figure 2). Each type of vessel has a <strong>[pb_glossary id=\"777\"]lumen[\/pb_glossary]<\/strong>\u2014a hollow passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system. Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins.<\/p>\r\n<p style=\"text-align: justify\">By the time blood has passed through capillaries and entered venules, the pressure initially exerted upon it by heart contractions has diminished. In other words, in comparison to arteries, venules and veins withstand a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance. In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity.<\/p>\r\n<p style=\"text-align: justify\">Both arteries and veins have the same three distinct tissue layers, called tunics (from the Latin term tunica), for the garments first worn by ancient Romans; the term tunic is also used for some modern garments. From the most interior layer to the outer, these tunics are the [pb_glossary id=\"982\"]tunica intima[\/pb_glossary], the [pb_glossary id=\"983\"]tunica media[\/pb_glossary], and the [pb_glossary id=\"984\"]tunica externa[\/pb_glossary] (Figure 2 and Table 1).<\/p>\r\n<p style=\"text-align: justify\"><strong>Tunica Intima:<\/strong> The tunica intima (also called the tunica interna) is composed of epithelial and connective tissue layers. Lining the tunica intima is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the [pb_glossary id=\"783\"]collagenous[\/pb_glossary] fibres beneath is one of the primary causes of clot formation. Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels. Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow. The endothelium releases local chemicals called [pb_glossary id=\"580\"]endothelins[\/pb_glossary] that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. Uncompensated overproduction of endothelins may contribute to hypertension (high blood pressure) and cardiovascular disease.<\/p>\r\n<p style=\"text-align: justify\">Next to the endothelium is the basement membrane, or basal lamina, that effectively binds the endothelium to the connective tissue. The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it. The thin outer layer of the tunica intima contains a small amount of [pb_glossary id=\"985\"]areolar connective tissue[\/pb_glossary] that consists primarily of elastic fibres to provide the vessel with additional flexibility; it also contains some collagenous fibres to provide additional strength.<\/p>\r\n<p style=\"text-align: justify\">In larger arteries, there is also a thick, distinct layer of elastic fibres known as <strong>the internal elastic membrane<\/strong> (also called the internal elastic lamina) at the boundary with the tunica media. Like the other components of the tunica intima, the internal elastic membrane provides structure while allowing the vessel to stretch. It is permeated with small openings that allow exchange of materials between the tunics. The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen.<\/p>\r\n\r\n\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"754\"]<img class=\"wp-image-74 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image38-OpenStax-blood-vessel-structure-artery-vein-754x1024.png\" alt=\"\" width=\"754\" height=\"1024\" \/> <strong>Figure 2. Structure of Blood Vessels.<\/strong> (a) Arteries and (b) veins share the same general features, but the walls of arteries are much thicker because of the higher pressure of the blood that flows through them. (c) A micrograph shows the relative differences in thickness. LM \u00d7 160. (Micrograph provided by the Regents of the University of Michigan Medical School \u00a9 2012)[\/caption]\r\n<p style=\"text-align: justify\"><strong>Tunica Media:<\/strong> The <strong>tunica media<\/strong> is the substantial middle layer of the vessel wall (Figure 2). It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibres, most of which are arranged in circular sheets. Toward the outer portion of the tunic, there are also layers of longitudinal muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively. Specifically, in arteries,<strong> [pb_glossary id=\"986\"]vasoconstriction[\/pb_glossary]<\/strong> decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, <strong>[pb_glossary id=\"755\"]vasodilation[\/pb_glossary]<\/strong> increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop.<\/p>\r\n<p style=\"text-align: justify\">The smooth muscle layers of the tunica media are supported by a framework of collagenous fibres that also binds the tunica media to the inner and outer tunics. Along with the collagenous fibres are large numbers of elastic fibres that appear as wavy lines in prepared slides.<\/p>\r\n\r\n<table style=\"border-collapse: collapse;width: 100%\" border=\"0\"><caption>Table 1: Comparison of wall layers in arteries, veins, and capillaries<\/caption>\r\n<tbody>\r\n<tr>\r\n<td style=\"width: 20.0385%\"><\/td>\r\n<th style=\"width: 26.3969%\" scope=\"col\"><strong>Arteries<\/strong><\/th>\r\n<th style=\"width: 26.7823%\" scope=\"col\"><strong>Veins<\/strong><\/th>\r\n<th style=\"width: 26.7822%\"><strong>Capillaries<\/strong><\/th>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><strong>General appearance<\/strong><\/td>\r\n<td style=\"width: 26.3969%\">Thick walls with small lumens<\/td>\r\n<td style=\"width: 26.7823%\">Thin walls with large lumens<\/td>\r\n<td style=\"width: 26.7822%\">Very (microscopically) thin walls and very small lumens<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\r\n<td style=\"width: 26.3969%\">Generally appear rounded<\/td>\r\n<td style=\"width: 26.7823%\">Generally appear flattened<\/td>\r\n<td style=\"width: 26.7822%\">Generally round<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><strong>Tunica intima<\/strong><\/td>\r\n<td style=\"width: 26.3969%\">Endothelium usually appears wavy due to constriction of smooth muscle<\/td>\r\n<td style=\"width: 26.7823%\">Endothelium appears smooth<\/td>\r\n<td style=\"width: 26.7822%\">Endothelium appears smooth<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\r\n<td style=\"width: 26.3969%\">Internal elastic membrane present in larger vessels<\/td>\r\n<td style=\"width: 26.7823%\">Internal elastic membrane absent<\/td>\r\n<td style=\"width: 26.7822%\">Internal elastic membrane absent<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><strong>Tunica media<\/strong><\/td>\r\n<td style=\"width: 26.3969%\">Normally the thickest layer in arteries<\/td>\r\n<td style=\"width: 26.7823%\">Normally thinner than the tunica externa<\/td>\r\n<td style=\"width: 26.7822%\">Tunica media absent<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\r\n<td style=\"width: 26.3969%\">Smooth muscle cells and elastic fibres predominate (exact proportions vary with distance from the heart)<\/td>\r\n<td style=\"width: 26.7823%\">Smooth muscle cells and collagenous fibres predominate<\/td>\r\n<td style=\"width: 26.7822%\"><\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\r\n<td style=\"width: 26.3969%\">External elastic membrane present in larger vessels<\/td>\r\n<td style=\"width: 26.7823%\">External elastic membrane absent<\/td>\r\n<td style=\"width: 26.7822%\"><\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\r\n<td style=\"width: 26.3969%\"><\/td>\r\n<td style=\"width: 26.7823%\">Nervi vasorum and vasa vasorum present<\/td>\r\n<td style=\"width: 26.7822%\"><\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><strong>Tunica externa<\/strong><\/td>\r\n<td style=\"width: 26.3969%\">Normally thinner than tunica media in all but the largest arteries<\/td>\r\n<td style=\"width: 26.7823%\">Normally the thickest layer in veins<\/td>\r\n<td style=\"width: 26.7822%\">Tunica externa absent<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\r\n<td style=\"width: 26.3969%\">Collagenous and elastic fibres<\/td>\r\n<td style=\"width: 26.7823%\">Collagenous and smooth fibres predominate<\/td>\r\n<td style=\"width: 26.7822%\"><\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\r\n<td style=\"width: 26.3969%\">Nervi vasorum and vasa vasorum present<\/td>\r\n<td style=\"width: 26.7823%\">Nervi vasorum and vasa vasorum present<\/td>\r\n<td style=\"width: 26.7822%\"><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<p style=\"text-align: justify\"><strong>Tunica Externa:<\/strong> The outer tunic, the <strong>tunica externa<\/strong> (also called the tunica adventitia), is a substantial sheath of connective tissue composed primarily of collagenous fibres. Some bands of elastic fibres are found here as well. The tunica externa in veins also contains groups of smooth muscle fibres. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1b\"><\/a>Arteries<\/strong><\/h5>\r\n<p style=\"text-align: justify\">An<strong> artery<\/strong> is a blood vessel that conducts blood away from the heart. All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1c\"><\/a>Arterioles<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0An <strong>arteriole <\/strong>is a very small artery that leads to a capillary. Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin (Figure 39).<\/p>\r\n<p style=\"text-align: justify\">The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure. The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1d\"><\/a>Capillaries<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called <strong>[pb_glossary id=\"987\"]perfusion[\/pb_glossary]<\/strong>. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid ([pb_glossary id=\"595\"]interstitial fluid[\/pb_glossary]). The diameter of a capillary lumen is from 5-10 \u03bcm; the smallest are just barely wide enough for an [pb_glossary id=\"543\"]erythrocyte[\/pb_glossary] to squeeze through. Flow through capillaries is often described as microcirculation.<\/p>\r\n<p style=\"text-align: justify\">Unlike the walls of veins and arteries, the wall of a capillary consists of an endothelial layer surrounded by a basement membrane with occasional smooth muscle fibres. There is some variation in wall structure: in a large capillary, several endothelial cells bordering each other may line the lumen; in a small capillary, there may be only a single cell layer that wraps around to contact itself.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1e\"><\/a>Venules<\/strong><\/h5>\r\n<p style=\"text-align: justify\">A venule is an extremely small vein, generally 8\u2013100 \u03bcm in diameter. Postcapillary venules join multiple capillaries exiting from a capillary bed. Multiple venules join to form veins. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibres, plus an outer layer of connective tissue fibres that constitute a very thin [pb_glossary id=\"984\"]tunica externa[\/pb_glossary]. Venules as well as capillaries are the primary sites of emigration or [pb_glossary id=\"567\"]diapedesis[\/pb_glossary], in which the [pb_glossary id=\"544\"]leukocytes[\/pb_glossary] adhere to the endothelial lining of the vessels and then squeeze through adjacent cells to enter the tissue fluid.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1f\"><\/a>Veins<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0A vein is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens (Figure 42). Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity. Table 2 compares the features of arteries and veins.<\/p>\r\n\r\n<table style=\"border-collapse: collapse;width: 100%\" border=\"0\"><caption>Table 2: Comparison of arteries and veins<\/caption>\r\n<tbody>\r\n<tr>\r\n<td style=\"width: 33.3333%\"><\/td>\r\n<th style=\"width: 33.3333%\" scope=\"col\"><strong>Arteries<\/strong><\/th>\r\n<th style=\"width: 33.3333%\" scope=\"col\"><strong>Veins<\/strong><\/th>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Direction of blood flow<\/strong><\/th>\r\n<td style=\"width: 33.3333%\">Conducts blood away from the heart<\/td>\r\n<td style=\"width: 33.3333%\">Conducts blood toward the heart<\/td>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>General appearance<\/strong><\/th>\r\n<td style=\"width: 33.3333%\">Rounded<\/td>\r\n<td style=\"width: 33.3333%\">Irregular, often collapsed<\/td>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Pressure<\/strong><\/th>\r\n<td style=\"width: 33.3333%\">High<\/td>\r\n<td style=\"width: 33.3333%\">Low<\/td>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Wall thickness<\/strong><\/th>\r\n<td style=\"width: 33.3333%\">Thick<\/td>\r\n<td style=\"width: 33.3333%\">Thin<\/td>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Relative oxygen concentration<\/strong><\/th>\r\n<td style=\"width: 33.3333%\">Higher in systemic arteries; lower in pulmonary arteries<\/td>\r\n<td style=\"width: 33.3333%\">Lower in systemic veins; h<span style=\"text-indent: 1em;font-family: inherit;font-size: inherit\">igher in pulmonary venis<\/span><\/td>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Valves<\/strong><\/th>\r\n<td style=\"width: 33.3333%\">Not present<\/td>\r\n<td style=\"width: 33.3333%\">Present most commonly in limbs and in veins inferior to the heart<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<h2><strong><a id=\"4-2\"><\/a>Part 2: Capillary Exchange<\/strong><\/h2>\r\nThe primary purpose of the cardiovascular system is to circulate gases, nutrients, wastes, and other substances to and from the cells of the body. Small molecules, such as gases, lipids, and lipid-soluble molecules, can diffuse directly through the membranes of the endothelial cells of the capillary wall. Glucose, amino acids, and ions\u2014including sodium, potassium, calcium, and chloride\u2014use transporters to move through specific channels in the membrane by [pb_glossary id=\"988\"]facilitated diffusion[\/pb_glossary]. Glucose, ions, and larger molecules may also leave the blood through intercellular clefts. Larger molecules can pass through the pores of [pb_glossary id=\"990\"]fenestrated capillaries[\/pb_glossary], and even large plasma proteins can pass through the great gaps in the [pb_glossary id=\"989\"]sinusoid capillaries[\/pb_glossary]. Some large proteins in blood plasma can move into and out of the endothelial cells packaged within vesicles by [pb_glossary id=\"991\"]endocytosis[\/pb_glossary] and [pb_glossary id=\"992\"]exocytosis[\/pb_glossary]. Water moves by [pb_glossary id=\"903\"]osmosis[\/pb_glossary].\r\n<h2 style=\"text-align: justify\"><strong><a id=\"4-3\"><\/a>Part 3: Blood Flow, Blood Pressure, and Resistance<\/strong><\/h2>\r\n<p style=\"text-align: justify\"><strong>Blood flow<\/strong> refers to the movement of blood through a vessel, tissue, or organ, and is usually expressed in terms of volume of blood per unit of time. It is initiated by the contraction of the ventricles of the heart. Ventricular contraction ejects blood into the major arteries, resulting in flow from regions of higher pressure to regions of lower pressure, as blood encounters smaller arteries and arterioles, then capillaries, then the venules and veins of the venous system. This section discusses a number of critical variables that contribute to blood flow throughout the body. It also discusses the factors that impede or slow blood flow, a phenomenon known as <strong>resistance<\/strong>.<\/p>\r\n<p style=\"text-align: justify\">As noted earlier, hydrostatic pressure is the force exerted by a fluid due to gravitational pull, usually against the wall of the container in which it is located. One form of hydrostatic pressure is <strong>blood pressure<\/strong>, the force exerted by blood upon the walls of the blood vessels or the chambers of the heart. Blood pressure may be measured in capillaries and veins, as well as the vessels of the pulmonary circulation; however, the term blood pressure without any specific descriptors typically refers to systemic arterial blood pressure\u2014that is, the pressure of blood flowing in the arteries of the systemic circulation. In clinical practice, this pressure is measured in mm Hg and is usually obtained using the brachial artery of the arm.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-3a\"><\/a>Components of Arterial Blood Pressure<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0Arterial blood pressure in the larger vessels consists of several distinct components (Figure 3): systolic and diastolic pressures, pulse pressure, and mean arterial pressure.<\/p>\r\n<p style=\"text-align: justify\"><strong>Systolic and Diastolic Pressures:<\/strong> When systemic arterial blood pressure is measured, it is recorded as a ratio of two numbers (e.g., 120\/80 is a normal adult blood pressure), expressed as systolic pressure over diastolic pressure. The <strong>[pb_glossary id=\"994\"]systolic pressure[\/pb_glossary]<\/strong> is the higher value (typically around 120 mm Hg) and reflects the arterial pressure resulting from the ejection of blood during ventricular contraction, or systole. The <strong>[pb_glossary id=\"993\"]diastolic pressure[\/pb_glossary]<\/strong> is the lower value (usually about 80 mm Hg) and represents the arterial pressure of blood during ventricular relaxation, or diastole.<\/p>\r\n\r\n\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"1024\"]<img class=\"wp-image-75 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image41-OpenStax-systemic-blood-pressure-in-blood-vessels-1024x708.png\" alt=\"\" width=\"1024\" height=\"708\" \/> <strong>Figure 3. Systemic Blood Pressure.<\/strong> The graph shows the components of blood pressure throughout the blood vessels, including systolic, diastolic, mean arterial, and pulse pressures.[\/caption]\r\n<p style=\"text-align: justify\"><strong>Mean Arterial Pressure: [pb_glossary id=\"995\"]Mean arterial pressure (MAP)[\/pb_glossary]<\/strong> represents the \u201caverage\u201d pressure of blood in the arteries, that is, the average force driving blood into vessels that serve the tissues. Mean is a statistical concept and is calculated by taking the sum of the values divided by the number of values. Although complicated to measure directly and complicated to calculate, MAP can be approximated by adding the diastolic pressure to one-third of the pulse pressure or systolic pressure minus the diastolic pressure:<\/p>\r\n<p style=\"text-align: center\">MAP = diastolic BP + ((systolic-diastolic BP) \/ 3)<\/p>\r\n<p style=\"text-align: justify\">Normally, the MAP falls within the range of 70\u2013110 mm Hg. If the value falls below 60 mm Hg for an extended time, blood pressure will not be high enough to ensure circulation to and through the tissues, which results in <strong>[pb_glossary id=\"996\"]ischemia[\/pb_glossary]<\/strong>, or insufficient blood flow. A condition called [pb_glossary id=\"997\"]hypoxia[\/pb_glossary], inadequate oxygenation of tissues, commonly accompanies ischemia. The term hypoxemia refers to low levels of oxygen in systemic arterial blood.<\/p>\r\n<p style=\"text-align: justify\"><strong>Measurement of Blood Pressure:<\/strong> Blood pressure is one of the critical parameters measured on virtually every patient in every healthcare setting. The technique used today was developed more than 100 years ago by a pioneering Russian physician, Dr. Nikolai Korotkoff. Turbulent blood flow through the vessels can be heard as a soft ticking while measuring blood pressure; these sounds are known as <strong>[pb_glossary id=\"998\"]Korotkoff sounds[\/pb_glossary]<\/strong>. The technique of measuring blood pressure requires the use of a <strong>sphygmomanometer<\/strong> (a blood pressure cuff attached to a measuring device) and a stethoscope. The technique is as follows:<\/p>\r\n\r\n<ul>\r\n \t<li style=\"text-align: justify\">The clinician wraps an inflatable cuff tightly around the patient\u2019s arm at about the level of the heart.<\/li>\r\n \t<li style=\"text-align: justify\">The clinician squeezes a rubber pump to inject air into the cuff, raising pressure around the artery and temporarily cutting off blood flow into the patient\u2019s arm.<\/li>\r\n \t<li style=\"text-align: justify\">The clinician places the stethoscope on the patient\u2019s antecubital region and, while gradually allowing air within the cuff to escape, listens for the Korotkoff sounds.<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">The first sound heard through the stethoscope\u2014the first Korotkoff sound\u2014indicates [pb_glossary id=\"994\"]systolic pressure[\/pb_glossary]. As more air is released from the cuff, blood is able to flow freely through the brachial artery and all sounds disappear. The point at which the last sound is heard is recorded as the patient\u2019s [pb_glossary id=\"993\"]diastolic pressure[\/pb_glossary].<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-3b\"><\/a>Pulse<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0After blood is ejected from the heart, elastic fibres in the arteries help maintain a high-pressure gradient as they expand to accommodate the blood, then recoil. This expansion and recoiling effect, known as the <strong>pulse<\/strong>, can be palpated manually or measured electronically. Although the effect diminishes over distance from the heart, elements of the systolic and diastolic components of the pulse are still evident down to the level of the arterioles.<\/p>\r\n<p style=\"text-align: justify\">Because pulse indicates heart rate, it is measured clinically to provide clues to a patient\u2019s state of health. It is recorded as beats per minute. Both the rate and the strength of the pulse are important clinically. A high or irregular pulse rate can be caused by physical activity or other temporary factors, but it may also indicate a heart condition. The pulse strength indicates the strength of ventricular contraction and cardiac output. If the pulse is strong, then systolic pressure is high. If it is weak, systolic pressure has fallen, and medical intervention may be warranted.<\/p>\r\n\r\n\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"664\"]<img class=\"wp-image-76 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image42-OpenStax-pulse-points-664x1024.png\" alt=\"\" width=\"664\" height=\"1024\" \/> <strong>Figure 4. Pulse Sites.<\/strong> The pulse is most readily measured at the radial artery, but can be measured at any of the pulse points shown.[\/caption]\r\n<p style=\"text-align: justify\">Pulse can be palpated manually by placing the tips of the fingers across an artery that runs close to the body surface and pressing lightly. While this procedure is normally performed using the radial artery in the wrist or the [pb_glossary id=\"999\"]common carotid artery[\/pb_glossary] in the neck, any superficial artery that can be palpated may be used (Figure 4). Common sites to find a pulse include temporal and facial arteries in the head, [pb_glossary id=\"1000\"]brachial arteries[\/pb_glossary] in the upper arm, [pb_glossary id=\"1001\"]femoral arteries[\/pb_glossary] in the thigh, [pb_glossary id=\"1002\"]popliteal arteries[\/pb_glossary] behind the knees, [pb_glossary id=\"1003\"]posterior tibial arteries[\/pb_glossary] near the medial [pb_glossary id=\"1004\"]tarsal[\/pb_glossary] regions, and [pb_glossary id=\"1005\"]dorsalis pedis arteries[\/pb_glossary] in the feet. A variety of commercial electronic devices are also available to measure pulse.<\/p>\r\n\r\n\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"1024\"]<img class=\"wp-image-77 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph-1024x635.png\" alt=\"\" width=\"1024\" height=\"635\" \/> <strong>Figure 5. Blood Pressure Measurement.<\/strong> When pressure in a sphygmomanometer cuff is released, a clinician can hear the Korotkoff sounds. In this graph, a blood pressure tracing is aligned to a measurement of systolic and diastolic pressures.[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-3c\"><\/a>Variables Affecting Blood Flow and Blood Pressure<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0Five variables influence blood flow and blood pressure:<\/p>\r\n\r\n<ul>\r\n \t<li style=\"text-align: justify\">Cardiac output<\/li>\r\n \t<li style=\"text-align: justify\">Compliance<\/li>\r\n \t<li style=\"text-align: justify\">Volume of the blood<\/li>\r\n \t<li style=\"text-align: justify\">Viscosity of the blood<\/li>\r\n \t<li style=\"text-align: justify\">Blood vessel length and diameter<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">Recall that blood moves from higher pressure to lower pressure. It is pumped from the heart into the arteries at high pressure. If you increase pressure in the arteries (afterload), and cardiac function does not compensate, blood flow will actually decrease. In the venous system, the opposite relationship is true. Increased pressure in the veins does not decrease flow as it does in arteries, but actually increases flow. Since pressure in the veins is normally relatively low, for blood to flow back into the heart, the pressure in the atria during atrial diastole must be even lower. It normally approaches zero, except when the atria contract (Figure 5).<\/p>\r\n<p style=\"text-align: justify\"><strong>Cardiac Output:<\/strong> Cardiac output is the measurement of blood flow from the heart through the ventricles, and is usually measured in liters per minute. Any factor that causes cardiac output to increase, by elevating heart rate or stroke volume or both, will elevate blood pressure and promote blood flow. These factors include sympathetic stimulation, the catecholamines [pb_glossary id=\"449\"]epinephrine[\/pb_glossary] and [pb_glossary id=\"454\"]norepinephrine[\/pb_glossary], [pb_glossary id=\"1006\"]thyroid hormones[\/pb_glossary], and increased calcium ion levels. Conversely, any factor that decreases cardiac output, by decreasing heart rate or stroke volume or both, will decrease arterial pressure and blood flow. These factors include [pb_glossary id=\"536\"]parasympathetic[\/pb_glossary] stimulation, elevated or decreased potassium ion levels, decreased calcium levels, anoxia, and acidosis.<\/p>\r\n<p style=\"text-align: justify\"><strong>Compliance:<\/strong> Compliance is the ability of any compartment to expand to accommodate increased content. A metal pipe, for example, is not compliant, whereas a balloon is. The greater the compliance of an artery, the more effectively it is able to expand to accommodate surges in blood flow without increased resistance or blood pressure. Veins are more compliant than arteries and can expand to hold more blood. When vascular disease causes stiffening of arteries, compliance is reduced and resistance to blood flow is increased. The result is more turbulence, higher pressure within the vessel, and reduced blood flow. This increases the work of the heart.<\/p>\r\n<p style=\"text-align: justify\"><strong>Blood Volume:<\/strong> The relationship between blood volume, blood pressure, and blood flow is intuitively obvious. Water may merely trickle along a creek bed in a dry season, but rush quickly and under great pressure after a heavy rain. Similarly, as blood volume decreases, pressure and flow decrease. As blood volume increases, pressure and flow increase.<\/p>\r\n<p style=\"text-align: justify\"><strong>Blood Viscosity:<\/strong> Viscosity is the thickness of fluids that affects their ability to flow. Clean water, for example, is less viscous than mud. The viscosity of blood is directly proportional to resistance and inversely proportional to flow; therefore, any condition that causes viscosity to increase will also increase resistance (and therefore blood pressure) and decrease flow. For example, imagine sipping milk, then a milkshake, through the same size straw. You experience more resistance and therefore less flow from the milkshake. Conversely, any condition that causes viscosity to decrease (such as when the milkshake melts) will decrease resistance and increase flow.<\/p>\r\n<p style=\"text-align: justify\">Normally the viscosity of blood does not change over short periods of time. The two primary determinants of blood viscosity are the formed elements and [pb_glossary id=\"546\"]plasma[\/pb_glossary] proteins. Since the vast majority of formed elements are erythrocytes, any condition affecting [pb_glossary id=\"1007\"]erythropoiesis[\/pb_glossary], such as [pb_glossary id=\"565\"]polycythemia [\/pb_glossary]or [pb_glossary id=\"564\"]anemia[\/pb_glossary], can alter viscosity. Viscosity generally increases with increasing numbers of formed elements relative to the amount of plasma.\u00a0 If the concentration of proteins in the plasma is increased, this would also increase viscosity.\u00a0 Since most plasma proteins are produced by the liver, any condition affecting liver function can also change the viscosity and therefore affect blood flow. Liver abnormalities include hepatitis, cirrhosis, alcohol damage, and drug toxicities. While [pb_glossary id=\"544\"]leukocytes[\/pb_glossary] and [pb_glossary id=\"545\"]platelets[\/pb_glossary] are normally a small component of the formed elements, there are some rare conditions in which there is such a great overproduction of these that viscosity increases.<\/p>\r\n<p style=\"text-align: justify\"><strong>Vessel Length and Diameter:<\/strong> The length of a vessel is directly proportional to its resistance: the longer the vessel, the greater the resistance and the lower the flow. As with blood volume, this makes intuitive sense, since the increased surface area of the vessel will impede the flow of blood. Likewise, if the vessel is shortened, the resistance will decrease and flow will increase.<\/p>\r\n<p style=\"text-align: justify\">In contrast to length, the diameter of blood vessels changes throughout the body, according to the type of vessel, as we discussed earlier. The diameter of any given vessel may also change frequently throughout the day in response to neural and chemical signals that trigger vasodilation and vasoconstriction. The <strong>vascular tone<\/strong> of the vessel is the contractile state of the smooth muscle and the primary determinant of diameter, and thus of resistance and flow. The effect of vessel diameter on resistance is inverse: Given the same volume of blood, an increased diameter means there is less blood contacting the vessel wall, thus lower friction and lower resistance, subsequently increasing flow. A decreased diameter means more of the blood contacts the vessel wall, and resistance increases, subsequently decreasing flow.<\/p>\r\n<p style=\"text-align: justify\">[pb_glossary id=\"755\"]Vasodilation[\/pb_glossary] and [pb_glossary id=\"986\"]vasoconstriction[\/pb_glossary] of arterioles play more significant roles in regulating blood pressure than do the vasodilation and vasoconstriction of other vessels.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong>Venous System<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart. Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during [pb_glossary id=\"529\"]diastole[\/pb_glossary] is very low, often approaching zero when the atria are relaxed (atrial diastole). Second, two physiologic \u201cpumps\u201d increase pressure in the venous system. The use of the term \u201cpump\u201d implies a physical device that speeds flow. These physiological pumps are less obvious.<\/p>\r\n<p style=\"text-align: justify\"><strong>Skeletal Muscle Pump:<\/strong> In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle. This mechanism, known as the <strong>skeletal muscle pump<\/strong> (Figure 6), helps the lower-pressure veins counteract the force of gravity, increasing pressure to move blood back to the heart. As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves. This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through. Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet. Military recruits are trained to flex their legs slightly while standing at attention for prolonged periods. Failure to do so may allow blood to pool in the lower limbs rather than returning to the heart. Consequently, the brain will not receive enough oxygenated blood, and the individual may lose consciousness.<\/p>\r\n<p style=\"text-align: justify\"><strong>Respiratory Pump:<\/strong> The respiratory pump aids blood flow through the veins of the thorax and abdomen. During inhalation, the volume of the thorax increases, largely through the contraction of the diaphragm, which moves downward and compresses the abdominal cavity. The elevation of the chest caused by the contraction of the external intercostal muscles also contributes to the increased volume of the thorax. The volume increase causes air pressure within the thorax to decrease, allowing us to inhale. Additionally, as air pressure within the thorax drops, blood pressure in the thoracic veins also decreases, falling below the pressure in the abdominal veins. This causes blood to flow along its pressure gradient from veins outside the thorax, where pressure is higher, into the thoracic region, where pressure is now lower. This in turn promotes the return of blood from the thoracic veins to the atria. During exhalation, when air pressure increases within the thoracic cavity, pressure in the thoracic veins increases, speeding blood flow into the heart while valves in the veins prevent blood from flowing backward from the thoracic and abdominal veins. Also notice that, as blood moves from venules to veins, the average blood pressure drops.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1193\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image44.png\" alt=\"image\" width=\"1193\" height=\"1036\" \/> <strong>Figure 6. Skeletal Muscle Pump.<\/strong> The contraction of skeletal muscles surrounding a vein compresses the blood and increases the pressure in that area. This action forces blood closer to the heart where venous pressure is lower. Note the importance of the one-way valves to assure that blood flows only in the proper direction.[\/caption]\r\n<h2 style=\"text-align: justify\"><strong><a id=\"4-4\"><\/a>Part 4: Homeostatic Regulation of the Vascular System<\/strong><\/h2>\r\n<p style=\"text-align: justify\">To maintain homeostasis in the cardiovascular system and provide adequate blood to the tissues, blood flow must be redirected continually to the tissues as they become more active. In a very real sense, the cardiovascular system engages in resource allocation, because there is not enough blood flow to distribute blood equally to all tissues simultaneously. For example, when an individual is exercising, more blood will be directed to skeletal muscles, the heart, and the lungs. Following a meal, more blood is directed to the digestive system. Only the brain receives a more or less constant supply of blood whether you are active, resting, thinking, or engaged in any other activity.<\/p>\r\n<p style=\"text-align: justify\">Table 3 provides the distribution of systemic blood at rest and during exercise. Although most of the data appears logical, the values for the distribution of blood to the integument may seem surprising. During exercise, the body distributes more blood to the body surface where it can dissipate the excess heat generated by increased activity into the environment.\u00a0 Three homeostatic mechanisms ensure adequate blood flow, blood pressure, distribution, and ultimately perfusion: neural, endocrine, and autoregulatory mechanisms (Figure 7).<\/p>\r\n\r\n<table style=\"border-collapse: collapse;width: 100%\" border=\"0\"><caption>Table 3: Systemic blood flow during rest, mild exercise, and maximal exercise in a healthy young individual<\/caption>\r\n<tbody>\r\n<tr>\r\n<th style=\"width: 25%\" scope=\"col\"><strong>Organ<\/strong><\/th>\r\n<th style=\"width: 25%\" scope=\"col\"><strong>Resting (mL\/min)<\/strong><\/th>\r\n<th style=\"width: 25%\" scope=\"col\"><strong>Mild exercise (mL\/min)<\/strong><\/th>\r\n<th style=\"width: 25%\" scope=\"col\"><strong>Maximal exercise (mL\/min)<\/strong><\/th>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 25%\" scope=\"row\">Skeletal muscle<\/td>\r\n<td style=\"width: 25%\">1200<\/td>\r\n<td style=\"width: 25%\">4500<\/td>\r\n<td style=\"width: 25%\">12,500<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 25%\" scope=\"row\">Heart<\/td>\r\n<td style=\"width: 25%\">250<\/td>\r\n<td style=\"width: 25%\">350<\/td>\r\n<td style=\"width: 25%\">750<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 25%\" scope=\"row\">Brain<\/td>\r\n<td style=\"width: 25%\">750<\/td>\r\n<td style=\"width: 25%\">750<\/td>\r\n<td style=\"width: 25%\">750<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 25%\" scope=\"row\">Integument<\/td>\r\n<td style=\"width: 25%\">500<\/td>\r\n<td style=\"width: 25%\">1500<\/td>\r\n<td style=\"width: 25%\">1900<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 25%\" scope=\"row\">Kidney<\/td>\r\n<td style=\"width: 25%\">1100<\/td>\r\n<td style=\"width: 25%\">900<\/td>\r\n<td style=\"width: 25%\">600<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 25%\" scope=\"row\">Gastrointestinal<\/td>\r\n<td style=\"width: 25%\">1400<\/td>\r\n<td style=\"width: 25%\">1100<\/td>\r\n<td style=\"width: 25%\">600<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 25%\" scope=\"row\">Others (e.g., liver, spleen)<\/td>\r\n<td style=\"width: 25%\">600<\/td>\r\n<td style=\"width: 25%\">400<\/td>\r\n<td style=\"width: 25%\">400<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 25%\" scope=\"row\">Total<\/td>\r\n<td style=\"width: 25%\">5800<\/td>\r\n<td style=\"width: 25%\">9500<\/td>\r\n<td style=\"width: 25%\">17,500<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"789\"]<img class=\"wp-image-79 size-full\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/2115_Vascular_Homeostasis_Flow_Art.png\" alt=\"\" width=\"789\" height=\"1125\" \/> <strong><strong>Figure 7. Summary of Factors Maintaining Vascular Homeostasis. <\/strong><\/strong>Adequate blood flow, blood pressure, distribution, and perfusion involve autoregulatory, neural, and endocrine mechanisms.[\/caption]\r\n\r\n&nbsp;\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4a\"><\/a>Neural Regulation<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0The nervous system plays a critical role in the regulation of vascular homeostasis. The primary regulatory sites include the cardiovascular centres in the brain that control both cardiac and vascular functions. In addition, more generalized neural responses from the limbic system and the autonomic nervous system are factors.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4b\"><\/a>The Cardiovascular Centres in the Brain<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0Neurological regulation of blood pressure and flow depends on the cardiovascular centres located in the [pb_glossary id=\"725\"]medulla oblongata[\/pb_glossary]. This cluster of neurons responds to changes in blood pressure as well as blood concentrations of oxygen, carbon dioxide, and hydrogen ions. The cardiovascular centre contains three distinct components:<\/p>\r\n\r\n<ul>\r\n \t<li style=\"text-align: justify\">The cardioacceleratory centre stimulates cardiac function by regulating heart rate and stroke volume via sympathetic stimulation from the cardiac accelerator nerve.<\/li>\r\n \t<li style=\"text-align: justify\">The cardioinhibitory centre slows cardiac function by decreasing heart rate via [pb_glossary id=\"536\"]parasympathetic[\/pb_glossary] stimulation from the [pb_glossary id=\"538\"]vagus nerve[\/pb_glossary].<\/li>\r\n \t<li style=\"text-align: justify\">The vasomotor centre controls vessel tone or contraction of the smooth muscle in the tunica media. Changes in diameter affect peripheral resistance, pressure, and flow, which affect cardiac output. The majority of these neurons act via the release of the neurotransmitter norepinephrine from sympathetic neurons.<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">Although each centre functions independently, they are not anatomically distinct.<\/p>\r\n<p style=\"text-align: justify\">There is also a small population of neurons that control vasodilation in the vessels of the brain and skeletal muscles by relaxing the smooth muscle fibres in the vessel tunics. Many of these are cholinergic neurons, that is, they release acetylcholine, which in turn stimulates the vessels\u2019 endothelial cells to release [pb_glossary id=\"1016\"]nitric oxide (NO)[\/pb_glossary], which causes vasodilation. Others release norepinephrine that binds to \u03b2<sub>2<\/sub> receptors. A few neurons release [pb_glossary id=\"1016\"]NO[\/pb_glossary] directly as a neurotransmitter.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4c\"><\/a>Baroreceptor Reflexes<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Baroreceptors are specialized stretch receptors located within thin areas of blood vessels and heart chambers that respond to the degree of stretch caused by the presence of blood. They send impulses to the cardiovascular centres to regulate blood pressure. Vascular baroreceptors are found primarily in sinuses (small cavities) within the aorta and carotid arteries: The <strong>[pb_glossary id=\"1009\"]aortic sinuses[\/pb_glossary]<\/strong> are found in the walls of the ascending aorta just superior to the aortic valve, whereas the <strong>[pb_glossary id=\"1010\"]carotid sinuses[\/pb_glossary]<\/strong> are in the base of the [pb_glossary id=\"1011\"]internal carotid arteries[\/pb_glossary]. There are also low-pressure baroreceptors located in the walls of the venae cavae and right atrium.<\/p>\r\n<p style=\"text-align: justify\">When blood pressure increases, the baroreceptors are stretched more tightly and initiate [pb_glossary id=\"1012\"]action potentials[\/pb_glossary] at a higher rate. At lower blood pressures, the degree of stretch is lower and the rate of firing is slower. When the cardiovascular centres in the medulla oblongata receives this input, they triggers a reflex that maintains homeostasis (Figure 8):<\/p>\r\n\r\n<ul>\r\n \t<li style=\"text-align: justify\">When blood pressure rises too high, the baroreceptors fire at a higher rate and trigger parasympathetic stimulation of the heart. As a result, cardiac output falls. [pb_glossary id=\"535\"]Sympathetic[\/pb_glossary] stimulation of the peripheral [pb_glossary id=\"598\"]arterioles[\/pb_glossary] will also decrease, resulting in vasodilation. Combined, these activities cause blood pressure to fall.<\/li>\r\n \t<li style=\"text-align: justify\">When blood pressure drops too low, the rate of [pb_glossary id=\"1013\"]baroreceptor[\/pb_glossary] firing decreases. This will trigger an increase in sympathetic stimulation of the heart, causing cardiac output to increase. It will also trigger sympathetic stimulation of the peripheral vessels, resulting in vasoconstriction. Combined, these activities cause blood pressure to rise.<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">The baroreceptors in the venae cavae and right atrium monitor blood pressure as the blood returns to the heart from the systemic circulation. Normally, blood flow into the aorta is the same as blood flow back into the right atrium. If blood is returning to the right atrium more rapidly than it is being ejected from the left ventricle, the atrial receptors will stimulate the cardiovascular centres to increase sympathetic firing and increase cardiac output until homeostasis is achieved. The opposite is also true. This mechanism is referred to as the atrial reflex.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1390\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image47.png\" alt=\"image\" width=\"1390\" height=\"1036\" \/> <strong>Figure 8. Baroreceptor Reflexes for Maintaining Vascular Homeostasis.<\/strong> Increased blood pressure results in increased rates of baroreceptor firing, whereas decreased blood pressure results in slower rates of fire, both initiating the homeostatic mechanism to restore blood pressure.[\/caption]\r\n<p style=\"text-align: justify\"><strong>Chemoreceptor Reflexes:<\/strong> In addition to the baroreceptors are [pb_glossary id=\"734\"]chemoreceptors[\/pb_glossary] that monitor levels of oxygen, carbon dioxide, and hydrogen ions (pH), and thereby contribute to vascular homeostasis. Chemoreceptors monitoring the blood are located in close proximity to the [pb_glossary id=\"1013\"]baroreceptors[\/pb_glossary] in the aortic and carotid sinuses. They signal the cardiovascular centres as well as the respiratory centres in the [pb_glossary id=\"725\"]medulla oblongata[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\">Since tissues consume oxygen and produce carbon dioxide and acids as waste products, when the body is more active, oxygen levels fall and carbon dioxide levels rise as cells undergo cellular respiration to meet the energy needs of activities. This causes more hydrogen ions to be produced, causing the blood pH to drop. When the body is resting, oxygen levels are higher, carbon dioxide levels are lower, more hydrogen is bound, and pH rises.<\/p>\r\n<p style=\"text-align: justify\">The chemoreceptors respond to increasing carbon dioxide and hydrogen ion levels (falling pH) by stimulating the cardioacceleratory and vasomotor centres, increasing cardiac output and constricting peripheral vessels. The cardioinhibitory centre is suppressed. With falling carbon dioxide and hydrogen ion levels (increasing pH), the cardioinhibitory centre is stimulated, and the cardioacceleratory and vasomotor centres are suppressed, decreasing cardiac output and causing peripheral vasodilation. In order to maintain adequate supplies of oxygen to the cells and remove waste products such as carbon dioxide, it is essential that the respiratory system respond to changing metabolic demands. In turn, the cardiovascular system will transport these gases to the lungs for exchange, again in accordance with metabolic demands. This interrelationship of cardiovascular and respiratory control cannot be overemphasized.<\/p>\r\n<p style=\"text-align: justify\">Other neural mechanisms can also have affect cardiovascular function. These include the limbic system that links physiological responses to psychological stimuli, as well as generalized sympathetic and parasympathetic stimulation.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4d\"><\/a>Endocrine Regulation<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0Endocrine control over the cardiovascular system involves the catecholamines, epinephrine and norepinephrine, as well as several hormones that interact with the kidneys in the regulation of blood volume.<\/p>\r\n<p style=\"text-align: justify\"><strong>Epinephrine and Norepinephrine:<\/strong> The catecholamines [pb_glossary id=\"449\"]epinephrine[\/pb_glossary] and [pb_glossary id=\"454\"]norepinephrine[\/pb_glossary] are released by the adrenal medulla, and enhance and extend the body\u2019s sympathetic or \u201cfight-or-flight\u201d response (Figure 9). They increase heart rate and force of contraction, while temporarily constricting blood vessels to organs not essential for flight-or-fight responses and redirecting blood flow to the liver, muscles, and heart.<\/p>\r\n<p style=\"text-align: justify\"><strong>Antidiuretic Hormone:<\/strong> [pb_glossary id=\"473\"]Antidiuretic hormone[\/pb_glossary] (ADH), also known as vasopressin, is secreted by the cells in the hypothalamus and transported via the hypothalamic-hypophyseal tracts to the posterior pituitary where it is stored until released upon nervous stimulation. The primary trigger prompting the hypothalamus to release antiduiretic hormone is increasing osmolarity of tissue fluid, usually in response to significant loss of blood volume (Figure 10). ADH signals its target cells in the kidneys to reabsorb more water, thus preventing the loss of additional fluid in the urine. This will increase overall fluid levels and help restore blood volume and pressure. In addition, antiduiretic hormone constricts peripheral vessels.<\/p>\r\n<p style=\"text-align: justify\"><strong>Renin-Angiotensin-Aldosterone Mechanism:<\/strong> The renin-angiotensin-aldosterone mechanism has a major effect upon the cardiovascular system (Figure 9). Renin is an [pb_glossary id=\"464\"]enzyme[\/pb_glossary], although because of its importance in the renin-angiotensin-aldosterone pathway, some sources identify it as a hormone. Specialized cells in the kidneys found in the [pb_glossary id=\"1014\"]juxtaglomerular apparatus[\/pb_glossary] respond to decreased blood flow by secreting renin into the blood. Renin converts the plasma protein angiotensinogen, which is produced by the liver, into its active form\u2014angiotensin I. Angiotensin I circulates in the blood and is then converted into angiotensin II in the lungs. This reaction is catalyzed by the enzyme angiotensin-converting enzyme (ACE).<\/p>\r\n<p style=\"text-align: justify\">Angiotensin II is a powerful vasoconstrictor, greatly increasing blood pressure. It also stimulates the release of antiduiretic hormone and [pb_glossary id=\"1015\"]aldosterone[\/pb_glossary], a hormone produced by the adrenal cortex. Aldosterone increases the reabsorption of sodium into the blood by the kidneys. Since water follows sodium, this increases the reabsorption of water. This in turn increases blood volume, raising blood pressure. Angiotensin II also stimulates the thirst centre in the [pb_glossary id=\"392\"]hypothalamus[\/pb_glossary], so an individual will likely consume more fluids, again increasing blood volume and pressure.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1670\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image48.png\" alt=\"image\" width=\"1670\" height=\"722\" \/> <strong>Figure 9. Hormones Involved in Renal Control of Blood Pressure.<\/strong> In the renin-angiotensin-aldosterone mechanism, increasing angiotensin II will stimulate the production of antidiuretic hormone and aldosterone. In addition to renin, the kidneys produce erythropoietin, which stimulates the production of red blood cells, further increasing blood volume.[\/caption]\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1667\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image49.png\" alt=\"image\" width=\"1667\" height=\"877\" \/> <strong><strong>Figure 10. Homeostatic Responses to Loss of Blood Volume<\/strong><\/strong>[\/caption]\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\"><strong>Erythropoietin:<\/strong> [pb_glossary id=\"562\"]Erythropoietin (EPO)[\/pb_glossary] is released by the kidneys when blood flow and\/or oxygen levels decrease. Erythropoietin stimulates the production of erythrocytes within the bone marrow. Erythrocytes are the major formed element of the blood and may contribute 40% or more to blood volume, a significant factor of viscosity, resistance, pressure, and flow. In addition, erythropoietin is a vasoconstrictor. Overproduction of erythropoietin or excessive intake of synthetic erythropoietin, often to enhance athletic performance, will increase viscosity, resistance, and pressure, and decrease flow in addition to its contribution as a vasoconstrictor.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4e\"><\/a>Autoregulation of Perfusion<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Autoregulation mechanisms require neither specialized nervous stimulation nor endocrine control. Rather, these are local, self-regulatory mechanisms that allow each region of tissue to adjust its blood flow, and thus its perfusion. These local mechanisms include chemical signals and myogenic controls.<\/p>\r\n<p style=\"text-align: justify\"><strong>Chemical Signals Involved in Autoregulation:<\/strong> Chemical signals work at the level of the precapillary [pb_glossary id=\"866\"]sphincters[\/pb_glossary] to trigger either constriction or relaxation. Opening a precapillary sphincter allows blood to flow into that particular capillary, whereas constricting a precapillary sphincter temporarily shuts off blood flow to that region. The factors involved in regulating the precapillary sphincters include the following:<\/p>\r\n\r\n<ul>\r\n \t<li style=\"text-align: justify\">Opening of the sphincter is triggered in response to decreased oxygen concentrations; increased carbon dioxide concentrations; increasing levels of lactic acid or other byproducts of cellular metabolism; increasing concentrations of potassium ions or hydrogen ions (falling pH); inflammatory chemicals such as histamines; and increased body temperature. These conditions in turn stimulate the release of [pb_glossary id=\"1016\"]NO[\/pb_glossary], a powerful vasodilator, from endothelial cells.<\/li>\r\n \t<li style=\"text-align: justify\">Contraction of the precapillary sphincter is triggered by the opposite levels of the regulators, which prompt the release of endothelins, powerful vasoconstricting peptides secreted by endothelial cells. [pb_glossary id=\"545\"]Platelet[\/pb_glossary] secretions and certain [pb_glossary id=\"1017\"]prostaglandins[\/pb_glossary] may also trigger constriction.<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">Again, these factors alter tissue [pb_glossary id=\"987\"]perfusion[\/pb_glossary] via their effects on the precapillary sphincter mechanism, which regulates blood flow to capillaries. Since the amount of blood is limited, not all capillaries can fill at once, so blood flow is allocated based upon the needs and metabolic state of the tissues as reflected in these parameters. Bear in mind, however, that dilation and constriction of the arterioles feeding the capillary beds is the primary control mechanism.<\/p>\r\n<p style=\"text-align: justify\"><strong>The Myogenic Response:<\/strong> The myogenic response is a reaction to the stretching of the smooth muscle in the walls of arterioles as changes in blood flow occur through the vessel. This may be viewed as a largely protective function against dramatic fluctuations in blood pressure and blood flow to maintain homeostasis. If [pb_glossary id=\"987\"]perfusion[\/pb_glossary] of an organ is too low (ischemia), the tissue will experience low levels of oxygen (hypoxia). In contrast, excessive perfusion could damage the organ\u2019s smaller and more fragile vessels. The myogenic response is a localized process that serves to stabilize blood flow in the capillary network that follows that arteriole. When blood flow is low, the vessel\u2019s smooth muscle will be only minimally stretched. In response, it relaxes, allowing the vessel to dilate and thereby increase the movement of blood into the tissue. When blood flow is too high, the smooth muscle will contract in response to the increased stretch, prompting vasoconstriction that reduces blood flow.<\/p>\r\n\r\n<h2 style=\"text-align: justify\"><strong><a id=\"4-5\"><\/a>Part 5: Circulatory Pathways<\/strong><\/h2>\r\n<p style=\"text-align: justify\">Virtually every cell, tissue, organ, and system in the body is impacted by the circulatory system. This includes the generalized and more specialized functions of transport of materials, capillary exchange, maintaining health by transporting leukocytes and various immunoglobulins ([pb_glossary id=\"633\"]antibodies[\/pb_glossary]), [pb_glossary id=\"1018\"]hemostasis[\/pb_glossary], regulation of body temperature, and helping to maintain acid-base balance. In addition to these shared functions, many systems enjoy a unique relationship with the circulatory system (Figure 11).<\/p>\r\n<p style=\"text-align: justify\">As you learn about the vessels of the systemic and pulmonary circuits, notice that many arteries and veins share the same names, parallel one another throughout the body, and are very similar on the right and left sides of the body. For example, you will find a pair of [pb_glossary id=\"1001\"]femoral arteries[\/pb_glossary] and a pair of femoral veins, with one vessel on each side of the body. In contrast, some vessels closer to the midline of the body, such as the aorta, are unique. Another phenomenon that can make the study of vessels challenging is that names of vessels can change with location. Like a street that changes name as it passes through an intersection, an artery or vein can change names as it passes an anatomical landmark. For example, the left [pb_glossary id=\"1019\"]subclavian artery[\/pb_glossary] becomes the [pb_glossary id=\"1020\"]axillary artery[\/pb_glossary] as it passes through the body wall and into the [pb_glossary id=\"1025\"]axillary region[\/pb_glossary], and then becomes the brachial artery as it flows from the axillary region into the upper arm (or brachium).<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"4-5a\"><\/a>Pulmonary Circulation<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0Recall that blood returning from the systemic circuit enters the right atrium (Figure 12) via the <strong>[pb_glossary id=\"422\"]superior[\/pb_glossary] and [pb_glossary id=\"423\"]inferior venae cavae[\/pb_glossary]<\/strong> and the <strong>[pb_glossary id=\"504\"]coronary sinus[\/pb_glossary]<\/strong>, which drains the blood supply of the heart muscle. These vessels will be described more fully later in this section. This blood is relatively low in oxygen and relatively high in carbon dioxide, since much of the oxygen has been extracted for use by the tissues and the waste gas carbon dioxide was picked up to be transported to the lungs for elimination. From the right atrium, blood moves into the right ventricle, which pumps it to the lungs for gas exchange. This system of vessels is referred to as the <strong>[pb_glossary id=\"420\"]pulmonary circuit[\/pb_glossary]<\/strong>.<\/p>\r\n<p style=\"text-align: justify\">The single vessel exiting the right ventricle is the <strong>[pb_glossary id=\"497\"]pulmonary trunk[\/pb_glossary]<\/strong>. At the base of the pulmonary trunk is the pulmonary [pb_glossary id=\"508\"]semilunar valve[\/pb_glossary], which prevents backflow of blood into the right ventricle during ventricular diastole. As the pulmonary trunk reaches the superior surface of the heart, it curves posteriorly and rapidly bifurcates (divides) into two branches, a left and a right <strong>[pb_glossary id=\"1026\"]pulmonary artery[\/pb_glossary]<\/strong>. To prevent confusion between these vessels, it is important to refer to the vessel exiting the heart as the pulmonary trunk, rather than also calling it a pulmonary artery.<\/p>\r\n<p style=\"text-align: justify\">The pulmonary arteries in turn branch many times within the lung, forming a series of smaller arteries and arterioles that eventually lead to the pulmonary capillaries. The pulmonary capillaries surround lung structures known as [pb_glossary id=\"663\"]alveoli[\/pb_glossary] that are the sites of oxygen and carbon dioxide exchange.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"867\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image50.png\" alt=\"image\" width=\"867\" height=\"1049\" \/> <strong>Figure 11. Interaction of the Circulatory System with Other Body Systems<\/strong>[\/caption]\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1505\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image51.png\" alt=\"image\" width=\"1505\" height=\"860\" \/> <strong>Figure 12. Pulmonary Circuit.<\/strong> Blood exiting from the right ventricle flows into the pulmonary trunk, which bifurcates into the two pulmonary arteries. These vessels branch to supply blood to the pulmonary capillaries, where gas exchange occurs within the lung alveoli. Blood returns via the pulmonary veins to the left atrium.[\/caption]\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"156\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image52.png\" alt=\"image\" width=\"156\" height=\"158\" \/> Watch <a href=\"https:\/\/youtu.be\/v43ej5lCeBo\">this CrashCourse video<\/a> to learn more about the blood vessels! Direct link: <a href=\"https:\/\/youtu.be\/v43ej5lCeBo\">https:\/\/youtu.be\/v43ej5lCeBo<\/a>[\/caption]\r\n\r\n<\/div>\r\n<div><\/div>\r\n<div class=\"unit-2:-the-cardiovascular-system-\">\r\n\r\nOnce gas exchange is completed, oxygenated blood flows from the pulmonary capillaries into a series of pulmonary venules that eventually lead to a series of larger <strong>[pb_glossary id=\"1027\"]pulmonary veins[\/pb_glossary]<\/strong>. Four pulmonary veins, two on the left and two on the right, return blood to the left atrium. At this point, the pulmonary circuit is complete. Table 4 defines the major arteries and veins of the pulmonary circuit discussed in the text.\r\n<h5 style=\"text-align: justify\"><strong>Overview of Systemic Arteries<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Blood relatively high in oxygen concentration is returned from the pulmonary circuit to the left atrium via the four pulmonary veins. From the left atrium, blood moves into the left ventricle, which pumps blood into the aorta. The aorta and its branches\u2014the systemic arteries\u2014send blood to virtually every organ of the body (Figure 41).<\/p>\r\n\r\n<table style=\"border-collapse: collapse;width: 100%;height: 100px\" border=\"0\"><caption>Table 4: Pulmonary arteries and veins<\/caption>\r\n<tbody>\r\n<tr style=\"height: 16px\">\r\n<th style=\"width: 44.7368%;height: 16px\" scope=\"col\"><strong>Vessel<\/strong><\/th>\r\n<th style=\"width: 55.2632%;height: 16px\" scope=\"col\"><strong>Description<\/strong><\/th>\r\n<\/tr>\r\n<tr style=\"height: 52px\">\r\n<td style=\"width: 44.7368%;height: 52px\" scope=\"row\">Pulmonary trunk<\/td>\r\n<td style=\"width: 55.2632%;height: 52px\">Single large vessel exiting the right ventricle (divides to form the right and left pulmonary arteries)<\/td>\r\n<\/tr>\r\n<tr style=\"height: 16px\">\r\n<td style=\"width: 44.7368%;height: 16px\" scope=\"row\">Pulmonary arteries (left pulmonary artery, right pulmonary artery)<\/td>\r\n<td style=\"width: 55.2632%;height: 16px\">Two vessels that form from the pulmonary trunk and lead to smaller arterioles and eventually to the pulmonary capillaries<\/td>\r\n<\/tr>\r\n<tr style=\"height: 16px\">\r\n<td style=\"width: 44.7368%;height: 16px\" scope=\"row\">Pulmonary veins (left superior pulmonary vein, left inferior pulmonary vein, right superior pulmonary vein, right inferior pulmonary vein)<\/td>\r\n<td style=\"width: 55.2632%;height: 16px\">Two sets of paired vessels (one pair from each side) that are formed from venules, leading blood away from the pulmonary capillaries to flow into the left atrium<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<h5 style=\"text-align: justify\"><strong>The Aorta<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0The <strong>[pb_glossary id=\"1028\"]aorta[\/pb_glossary]<\/strong> is the largest artery in the body (Figure 13). It arises from the left ventricle and eventually descends to the abdominal region, where it bifurcates at the level of the fourth lumbar vertebra into the two common iliac arteries. The aorta consists of the ascending aorta, the aortic arch, and the descending aorta (Table 5) which passes through the diaphragm, a landmark that divides into the superior thoracic and inferior abdominal components. Arteries originating from the aorta ultimately distribute blood to virtually all tissues of the body. At the base of the aorta is the aortic semilunar valve that prevents backflow of blood into the left ventricle while the heart is relaxing.<\/p>\r\n\r\n\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"631\"]<img class=\"wp-image-86 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image54-OpenStax-systemic-arteries-631x1024.png\" alt=\"\" width=\"631\" height=\"1024\" \/> <strong>Figure 13. Systemic Arteries.<\/strong> The major systemic arteries shown here deliver oxygenated blood throughout the body.[\/caption]\r\n<p style=\"text-align: justify\">After exiting the heart, the <strong>[pb_glossary id=\"1032\"]ascending aorta[\/pb_glossary]<\/strong> moves in a [pb_glossary id=\"1033\"]superior[\/pb_glossary] direction for approximately 5 cm and ends at the sternal angle. Following this ascent, it reverses direction, forming a graceful arc to the left, called the <strong>[pb_glossary id=\"731\"]aortic arch[\/pb_glossary]<\/strong>. The aortic arch descends toward the inferior portions of the body and ends at the level of the intervertebral disk between the fourth and fifth thoracic vertebrae. Beyond this point, the <strong>[pb_glossary id=\"1031\"]descending aorta[\/pb_glossary]<\/strong> continues close to the bodies of the vertebrae and passes through an opening in the diaphragm. Superior to the diaphragm, the aorta is called the <strong>[pb_glossary id=\"1030\"]thoracic aorta[\/pb_glossary]<\/strong>, and inferior to the diaphragm, it is called the <strong>[pb_glossary id=\"1029\"]abdominal aorta[\/pb_glossary]<\/strong>. The abdominal aorta terminates when it bifurcates into the two common iliac arteries at the level of the fourth lumbar vertebra. See Figure 55 for an illustration of the ascending aorta, the aortic arch, and the initial segment of the descending aorta plus major branches.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong>Coronary Circulation<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The first vessels that branch from the ascending aorta are the paired [pb_glossary id=\"513\"]coronary arteries[\/pb_glossary] (see Figure 42), which arise from two of the three sinuses in the ascending aorta just superior to the aortic [pb_glossary id=\"508\"]semilunar valve[\/pb_glossary]. These sinuses contain the aortic baroreceptors and chemoreceptors critical to maintain cardiac function. The left coronary artery arises from the left posterior [pb_glossary id=\"1009\"]aortic sinus[\/pb_glossary]. The right coronary artery arises from the anterior aortic sinus. Normally, the right posterior aortic sinus does not give rise to a vessel.<\/p>\r\n<p style=\"text-align: justify\">The coronary arteries encircle the heart, forming a ring-like structure that divides into the next level of branches that supplies blood to the heart tissues.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong>Aortic Arch Branches<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0There are three major branches of the aortic arch: the [pb_glossary id=\"1034\"]<strong>brachiocephalic<\/strong> artery[\/pb_glossary], the <strong>left common carotid artery<\/strong>, and the <strong>left [pb_glossary id=\"1019\"]subclavian[\/pb_glossary]<\/strong>\u00a0(literally \u201cunder the clavicle\u201d) <strong>artery<\/strong>. As you would expect based upon proximity to the heart, each of these vessels is classified as an elastic artery.<\/p>\r\n<p style=\"text-align: justify\">The brachiocephalic artery is located only on the right side of the body; there is no corresponding artery on the left. The brachiocephalic artery branches into the <strong>right subclavian artery<\/strong> and the <strong>right common carotid artery<\/strong>. The left subclavian and left common carotid arteries arise independently from the aortic arch but otherwise follow a similar pattern and distribution to the corresponding arteries on the right side (see Figure 14).<\/p>\r\n<p style=\"text-align: justify\">Each <strong>subclavian artery<\/strong> supplies blood to the arms, chest, shoulders, back, and central nervous system.<\/p>\r\n<p style=\"text-align: justify\">The <strong>common carotid<\/strong> artery divides into internal and external carotid arteries. The right common carotid artery arises from the brachiocephalic artery and the left common carotid artery arises directly from the aortic arch. The <strong>branches of the carotid arteries<\/strong> supply blood to numerous structures within the head and neck. Each internal carotid artery initially forms an expansion known as the carotid sinus, containing the carotid baroreceptors and chemoreceptors. Like their counterparts in the aortic sinuses, the information provided by these receptors is critical to maintaining cardiovascular homeostasis (see Figure 13).<\/p>\r\n&nbsp;\r\n\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"972\"]<img class=\"wp-image-87 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image55-OpenStax-aorta-arch-and-arteries-972x1024.png\" alt=\"\" width=\"972\" height=\"1024\" \/> <strong>Figure 14. Aorta.<\/strong> The aorta has distinct regions, including the ascending aorta, aortic arch, and the descending aorta, which includes the thoracic and abdominal regions.[\/caption]\r\n<table style=\"border-collapse: collapse;width: 100%\" border=\"0\"><caption>Table 5: Components of the aorta<\/caption>\r\n<tbody>\r\n<tr>\r\n<th style=\"width: 13.7427%\" scope=\"col\"><strong>Vessel<\/strong><\/th>\r\n<th style=\"width: 86.2573%\" scope=\"col\"><strong>Description<\/strong><\/th>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 13.7427%\" scope=\"row\">Aorta<\/td>\r\n<td style=\"width: 86.2573%\">Largest artery in the body; originates from the left ventricle and descends to the abdominal region then bifurcates into the left and right common iliac arteries at the level of the fourth lumbar vertebra<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 13.7427%\" scope=\"row\">Ascending aorta<\/td>\r\n<td style=\"width: 86.2573%\">Initial portion of the aorta; rises superiorly from the left ventricle for a distance of approximately 5 cm<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 13.7427%\" scope=\"row\">Aortic arch<\/td>\r\n<td style=\"width: 86.2573%\">Graceful arc to the left that connects the ascending aorta to the descending aorta; ends at the intervertebral disk between the fourth and fifth thoracic vertebrae<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 13.7427%\" scope=\"row\">Descending aorta<\/td>\r\n<td style=\"width: 86.2573%\">Continues inferiorly from the end of the aortic arch; subdivided into the thoracic aorta and the abdominal aorta<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 13.7427%\" scope=\"row\">Thoracic aorta<\/td>\r\n<td style=\"width: 86.2573%\">Portion of the descending aorta superior to the aortic hiatus<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 13.7427%\" scope=\"row\">Abdominal aorta<\/td>\r\n<td style=\"width: 86.2573%\">Portion of the aorta inferior to the aortic hiatus; ends at its bifurcation into the left common iliac artery and the right common iliac artery<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<h5 style=\"text-align: justify\"><strong>Thoracic Aorta and Major Branches<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0The [pb_glossary id=\"1030\"]thoracic aorta[\/pb_glossary] begins at the level of vertebra T5 and continues through to the diaphragm at the level of T12, initially traveling within the [pb_glossary id=\"494\"]mediastinum[\/pb_glossary] to the left of the vertebral column. As it passes through the thoracic region, the thoracic aorta gives rise to several branches (Figure 15).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1100\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image57.png\" alt=\"image\" width=\"1100\" height=\"1037\" \/> <strong>Figure 15. Arteries of the Thoracic and Abdominal Regions.<\/strong> The thoracic aorta gives rise to the arteries of the visceral and parietal branches.[\/caption]\r\n<h5 style=\"text-align: justify\"><strong>Abdominal Aorta and Major Branches<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0After crossing through the diaphragm, the thoracic aorta is called the abdominal aorta. This vessel remains to the left of the vertebral column and is embedded in adipose tissue behind the peritoneal cavity. It formally ends at approximately the level of vertebra L4, where it bifurcates to form the two (left and right)\u00a0<strong>[pb_glossary id=\"1036\"]common iliac arteries[\/pb_glossary].<\/strong> Before this division, the abdominal aorta gives rise to several important branches.\u00a0 The common iliac arteries provide blood to the pelvic region and ultimately to the lower limbs.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong>Arteries Serving the Upper and Lower Limbs<\/strong><\/h5>\r\n<p style=\"text-align: justify\"><strong>Arteries Serving the Upper Limbs:<\/strong> As each subclavian artery exits the thorax into the [pb_glossary id=\"1025\"]axillary region[\/pb_glossary], it is renamed the <strong>[pb_glossary id=\"1020\"]axillary artery[\/pb_glossary]<\/strong>. Although each axillary artery does branch and supply blood to the region near the head of the humerus (via the humeral circumflex arteries), the majority of the vessel continues into the upper arm, or brachium, and becomes the brachial artery.<\/p>\r\n\r\n\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"676\"]<img class=\"wp-image-89 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image58-OpenStax-systemic-veins-676x1024.png\" alt=\"\" width=\"676\" height=\"1024\" \/> <strong>Figure 16. Major Systemic Veins of the Body.<\/strong> The major systemic veins of the body are shown here in anterior view.[\/caption]\r\n<p style=\"text-align: justify\"><strong>Arteries Serving the Lower Limbs:<\/strong> Each external iliac artery exits the body cavity and enters the femoral region of the lower leg. As it passes through the body wall, it is renamed the <strong>[pb_glossary id=\"1037\"]femoral artery[\/pb_glossary]<\/strong>. Each femoral artery gives rise to the genicular artery, which provides blood to the region of the knee. As each femoral artery passes posterior to the knee near the popliteal fossa, it is called the popliteal artery. Each popliteal artery branches into anterior and posterior tibial arteries.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong>Overview of Systemic Veins<\/strong><\/h5>\r\n<p style=\"text-align: justify\">\u00a0Systemic veins return blood to the right atrium. Since the blood has already passed through the systemic capillaries, it will be relatively low in oxygen concentration (Figure 16).<\/p>\r\n<p style=\"text-align: justify\">The right atrium receives all of the systemic venous return. Most of the blood flows into either the <strong>[pb_glossary id=\"422\"]superior vena cava[\/pb_glossary] <\/strong>or <strong>[pb_glossary id=\"423\"]inferior vena cava[\/pb_glossary].<\/strong> If you draw an imaginary line at the level of the diaphragm, systemic venous circulation from above that line will generally flow into the superior vena cava; this includes blood from the head, neck, chest, shoulders, and upper limbs. The exception to this is that most venous blood flow from the coronary veins flows directly into the coronary sinus and from there directly into the right atrium. Beneath the diaphragm, systemic venous flow enters the inferior vena cava, that is, blood from the abdominal and pelvic regions and the lower limbs.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong>The Superior and Inferior Vena Cavae<\/strong><\/h5>\r\n<p style=\"text-align: justify\"><strong>The Superior Vena Cava:<\/strong> The <strong>superior vena cava<\/strong> drains most of the body superior to the diaphragm (Figure 17). On both the left and right sides, the <strong>[pb_glossary id=\"602\"]subclavian vein[\/pb_glossary]<\/strong> forms when the <strong>[pb_glossary id=\"1038\"]axillary vein[\/pb_glossary]<\/strong> passes through the body wall from the axillary region. Each subclavian vein joins with the external and internal jugular veins from the head and neck to form the <strong>[pb_glossary id=\"1039\"]brachiocephalic vein[\/pb_glossary]<\/strong>.<\/p>\r\n\r\n\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"1024\"]<img class=\"wp-image-90 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image59-OpenStax-systemic-veins-thoracic-abdominal-region-1024x923.png\" alt=\"\" width=\"1024\" height=\"923\" \/> <strong>Figure 17. Veins of the Thoracic and Abdominal Regions.<\/strong> Veins of the thoracic and abdominal regions drain blood from the area above the diaphragm, returning it to the right atrium via the superior vena cava.[\/caption]\r\n<p style=\"text-align: justify\"><strong>The Inferior Vena Cava:<\/strong> Most of the blood inferior to the diaphragm drains into the <strong>inferior vena cava<\/strong> before it is returned to the heart (see Figure 17). Lying just beneath the parietal peritoneum in the abdominal cavity, the inferior vena cava parallels the abdominal aorta, where it can receive blood from abdominal veins.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong>Veins Draining the Lower Limbs<\/strong><\/h5>\r\n<p style=\"text-align: justify\">As each <strong>[pb_glossary id=\"1040\"]femoral vein[\/pb_glossary]<\/strong> penetrates the body wall from the femoral portion of the upper limb, it becomes the external iliac vein, a large vein that drains blood from the leg to the common iliac vein (Figure 18). The pelvic organs and integument drain into the internal iliac vein on either side of the body, which forms from several smaller veins in the region, including the umbilical veins that run on either side of the bladder. The external and internal iliac veins combine near the inferior portion of the sacroiliac joint on either side to form the <strong>[pb_glossary id=\"1041\"]common iliac vein[\/pb_glossary]<\/strong>. In addition to blood supply from the external and internal iliac veins, the middle sacral vein drains the sacral region into the common iliac vein. Similar to the common iliac arteries, the two common iliac veins come together at the level of L5 to form the <strong>inferior vena cava<\/strong>.<\/p>\r\n\r\n<\/div>\r\n&nbsp;\r\n<div class=\"unit-2:-the-cardiovascular-system-\">\r\n\r\n[caption id=\"attachment_91\" align=\"alignnone\" width=\"1024\"]<img class=\"wp-image-91 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image60-OpenStax-systemic-veins-lower-limbs-1024x946.png\" alt=\"\" width=\"1024\" height=\"946\" \/> <strong>Figure 18. The Major Veins of the Lower Limbs.<\/strong>[\/caption]\r\n\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\"><a id=\"P\"><\/a>Practice Questions<\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\n<strong>Part 1:<\/strong> Structure and function of blood vessels\r\n\r\n[h5p id=\"102\"]\r\n\r\n[h5p id=\"101\"]\r\n\r\n<strong>Part 2:<\/strong> Capillary Exchange\r\n\r\n[h5p id=\"103\"]\r\n\r\n<strong>Part 3:<\/strong>\u00a0Blood flow, blood pressure, and resistance\r\n\r\n[h5p id=\"104\"]\r\n\r\n<strong>Part 4:<\/strong> Hemostatic Regulation of the Vascular System\r\n\r\n[h5p id=\"106\"]\r\n\r\n<strong>Part 5:<\/strong> Circulatory Pathways\r\n\r\n[h5p id=\"107\"]\r\n\r\n[h5p id=\"105\"]\r\n\r\n[h5p id=\"108\"]\r\n\r\n<\/div>\r\n<\/div>\r\n&nbsp;\r\n\r\n<\/div>","rendered":"<div class=\"unit-2:-the-cardiovascular-system-\">\n<div class=\"textbox shaded\">\n<p><strong>Unit Outline<\/strong><\/p>\n<p><a href=\"#4-1\"><strong>Part 1:<\/strong> Structure and function of blood vessels<\/a><\/p>\n<ul>\n<li><a href=\"#4-1a\">Shared structures<\/a><\/li>\n<li><a href=\"#4-1b\">Arteries<\/a><\/li>\n<li><a href=\"#4-1c\">Arterioles<\/a><\/li>\n<li><a href=\"#4-1d\">Capillaries<\/a><\/li>\n<li><a href=\"#4-1e\">Venules<\/a><\/li>\n<li><a href=\"#4-1f\">Veins<\/a><\/li>\n<\/ul>\n<p><a href=\"#4-2\"><strong>Part 2:<\/strong> Capillary Exchange<\/a><\/p>\n<p><a href=\"#4-3\"><strong>Part 3:<\/strong>\u00a0Blood flow, blood pressure, and resistance<\/a><\/p>\n<ul>\n<li><a href=\"#4-3a\">Components of arterial blood pressure<\/a><\/li>\n<li><a href=\"#4-3b\">Pulse<\/a><\/li>\n<li><a href=\"#4-3c\">Variables affecting blood flow and blood pressure<\/a><\/li>\n<li><a href=\"#4-3d\">Venous system<\/a><\/li>\n<\/ul>\n<p><a href=\"#4-4\"><strong>Part 4:<\/strong> Hemostatic Regulation of the Vascular System<\/a><\/p>\n<ul>\n<li><a href=\"#4-4a\">Neural regulation<\/a><\/li>\n<li><a href=\"#4-4b\">The cardiovascular centres in the brain<\/a><\/li>\n<li><a href=\"#4-4c\">Baroreceptor reflexes<\/a><\/li>\n<li><a href=\"#4-4d\">Endocrine regulation<\/a><\/li>\n<li><a href=\"#4-4e\">Autoregulation of perfusion<\/a><\/li>\n<\/ul>\n<p><a href=\"#4-5\"><strong>Part 5:<\/strong> Circulatory Pathways<\/a><\/p>\n<ul>\n<li><a href=\"#4-5a\">Pulmonary circulation<\/a><\/li>\n<li><a href=\"#4-5b\">Overview of systemic arteries<\/a><\/li>\n<li><a href=\"#4-5c\">The aorta<\/a><\/li>\n<li><a href=\"#4-5d\">Coronary circulation<\/a><\/li>\n<li><a href=\"#4-5e\">Aortic arch branches<\/a><\/li>\n<li><a href=\"#4-5f\">Thoracic aorta and major branches<\/a><\/li>\n<li><a href=\"#4-5g\">Abdominal aorta and major branches<\/a><\/li>\n<li><a href=\"#4-5h\">Arteries serving the upper and lower limbs<\/a><\/li>\n<li><a href=\"#4-5i\">Overview of systemic veins<\/a><\/li>\n<li><a href=\"#4-5j\">The superior and inferior vena cavae<\/a><\/li>\n<li><a href=\"#4-5k\">Veins draining the lower limbs<\/a><\/li>\n<\/ul>\n<h2><a href=\"#P\">Practice Questions<\/a><\/h2>\n<\/div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\"><strong>Learning Objectives<\/strong><\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p>At the end of this unit, you should be able to:<\/p>\n<p class=\"hanging-indent\"><strong>I.<\/strong> Describe relationships between the following components of the cardiovascular system and explain their functions: blood, artery, vein, capillary, atria, and ventricles.<\/p>\n<p class=\"hanging-indent\"><strong>II. <\/strong>Compare the structure and function of arteries, veins, and capillaries.<\/p>\n<p class=\"hanging-indent\"><strong>III.<\/strong> Describe what is meant by blood pressure and specify the following: five factors which affect blood pressure, the major mechanisms that control blood pressure, and the average blood pressure of a young adult.<\/p>\n<p class=\"hanging-indent\"><strong>IV.<\/strong> Describe what is felt when a pulse is located, and specify four points where an arterial pulse may be felt.<\/p>\n<p class=\"hanging-indent\"><strong>V.<\/strong> Describe the following components of the cardiovascular system: the main arteries leaving the heart, and those serving the trunk, appendages, and heart; the main veins entering the heart, and those draining the trunk, appendages, and heart.<\/p>\n<\/div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\"><strong>Learning Objectives and Guiding Questions<\/strong><\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p>At the end of this unit, you should be able to complete all the following tasks, including answering the guiding questions associated with each task.<\/p>\n<p class=\"hanging-indent\"><strong>I.<\/strong> Describe relationships between the following components of the cardiovascular system and explain their functions: blood, artery, vein, capillary, atria, and ventricles.<\/p>\n<p class=\"hanging-indent\"><strong>II. <\/strong>Compare the structure and function of arteries, veins, and capillaries.<\/p>\n<ol>\n<li>In general, which arteries and veins carry oxygenated and deoxygenated blood?<\/li>\n<li>Define: artery, arteriole, vein, venule, capillary.<\/li>\n<li>Compare and contrast the structure of the walls of arteries, veins, and capillaries.<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>III.<\/strong> Describe what is meant by blood pressure and specify the following: five factors which affect blood pressure, the major mechanisms that control blood pressure, and the average blood pressure of a young adult.<\/p>\n<ol>\n<li>Define the term &#8220;blood pressure&#8221;.<\/li>\n<li class=\"hanging-indent\">Describe how blood pressure is measured.<\/li>\n<li>Define cardiac output and describe how each of the following physiological factors affect blood pressure:\n<ul>\n<li class=\"hanging-indent\">Heart rate<\/li>\n<li class=\"hanging-indent\">Contractility (strength of contraction) of the heart<\/li>\n<li class=\"hanging-indent\">Blood volume<\/li>\n<li class=\"hanging-indent\">Peripheral resistance<\/li>\n<li class=\"hanging-indent\">Blood viscosity<\/li>\n<\/ul>\n<\/li>\n<li>How does maintaining blood pressure contribute to homeostasis.<\/li>\n<li>Describe how blood pressure is regulated by:<\/li>\n<\/ol>\n<ul>\n<li style=\"list-style-type: none\">\n<ul>\n<li class=\"hanging-indent\">The nervous system<\/li>\n<li class=\"hanging-indent\">The endocrine system<\/li>\n<li class=\"hanging-indent\">Autoregulation<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p class=\"hanging-indent\"><strong>IV.<\/strong> Describe what is felt when a pulse is located, and specify four points where an arterial pulse may be felt.<\/p>\n<ol>\n<li class=\"hanging-indent\">When you manually \u201ctake someone\u2019s pulse\u201d, what is causing the pulsing pressure waves you feel?<\/li>\n<li class=\"hanging-indent\">List four locations on the human body where a pulse can be taken manually and explain why an arterial pulse can be felt at specific locations rather than just anywhere on the human body.<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe, using examples, how capillaries use simple diffusion, facilitated diffusion and osmosis to exchange material with tissues.<\/p>\n<p class=\"hanging-indent\"><strong>VI.<\/strong> Describe the following components of the cardiovascular system: the main arteries leaving the heart, and those serving the trunk, appendages, and heart; the main veins entering the heart, and those draining the trunk, appendages, and heart.<\/p>\n<ol>\n<li class=\"hanging-indent\">Draw a flow chart showing the components of the cardiovascular system. Start with the three main components (heart, blood vessels, and blood), and continue by specifying all the constituent parts of each.<\/li>\n<li class=\"hanging-indent\">Compare and contrast (clearly!) the anatomical structure and function of arteries, veins, and blood capillaries.<\/li>\n<li>Draw a simple diagram of the human cardiovascular system that shows both circuits, indicating the vessels blood is moved through as it is passed to and from the head, arms, organs of the abdomen, and lungs. Your diagram should include:\n<ul>\n<li>The main arteries leaving the heart:\n<ul>\n<li>Pulmonary trunk<\/li>\n<li>Pulmonary arteries<\/li>\n<li>Pulmonary veins<\/li>\n<li>Aorta<\/li>\n<li>Ascending aorta<\/li>\n<li>Aortic arch<\/li>\n<\/ul>\n<\/li>\n<li>The main arteries serving the trunk, appendages and the heart:\n<ul>\n<li>Descending aorta<\/li>\n<li>Thoracic aorta<\/li>\n<li>Abdominal aorta<\/li>\n<li>Brachiocephalic artery<\/li>\n<li>Left common carotid artery<\/li>\n<li>Right common carotid artery<\/li>\n<li>Left subclavian artery<\/li>\n<li>Right subclavian artery<\/li>\n<li>Common iliac artery<\/li>\n<li>Axillary artery<\/li>\n<li>Femoral artery<\/li>\n<\/ul>\n<\/li>\n<li>The main veins entering the heart\n<ul>\n<li>Superior vena cava<\/li>\n<li>Inferior vena cava<\/li>\n<li>Coronary sinus<\/li>\n<\/ul>\n<\/li>\n<li>The main veins draining the trunk, appendages and the heart\n<ul>\n<li>Subclavian vein<\/li>\n<li>Axillary vein<\/li>\n<li>Brachiocephalic vein<\/li>\n<li>Femoral vein<\/li>\n<li>Common iliac vein<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<p style=\"text-align: justify\">In this unit, you will learn about the vascular part of the cardiovascular system; that is, the vessels that transport blood throughout the body and provide the physical site where gases, nutrients, and other substances are exchanged with body cells. When vessel functioning is reduced, blood-borne substances do not circulate effectively throughout the body. As a result, tissue injury occurs, metabolism is impaired, and the functions of every bodily system are threatened.<\/p>\n<h2 style=\"text-align: justify\"><strong><a id=\"4-1\"><\/a>Part 1: Structure and Function of Blood Vessels<\/strong><\/h2>\n<p style=\"text-align: justify\">Blood is carried through the body via blood vessels. An <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_976\">artery<\/a> is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels. Eventually, the smallest arteries, vessels called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_598\">arterioles<\/a>, further branch into tiny <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_977\">capillaries<\/a>, where nutrients and wastes are exchanged, and then combine with other vessels that exit capillaries to form <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_599\">venules<\/a>, small blood vessels that carry blood to a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_978\">vein<\/a>, a larger blood vessel that returns blood to the heart.<\/p>\n<p style=\"text-align: justify\">Arteries and veins transport blood in two distinct circuits: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_421\">systemic circuit<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_420\">pulmonary circuit<\/a> (Figure 1). Systemic arteries provide blood rich in oxygen to the body\u2019s tissues. The blood returned to the heart through systemic veins has less oxygen, since much of the oxygen carried by the arteries has been delivered to the cells. In contrast, in the pulmonary circuit, arteries carry blood low in oxygen exclusively to the lungs for gas exchange. Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. Although arteries and veins differ structurally and functionally, they share certain features.<\/p>\n<figure style=\"width: 1534px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image37.png\" alt=\"image\" width=\"1534\" height=\"1024\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 1. Cardiovascular Circulation.<\/strong> The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart to repeat the cycle. The arrows indicate the direction of blood flow, and the colours show the relative levels of oxygen concentration.<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1a\"><\/a>Shared Structures<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0Different types of blood vessels vary slightly in their structures, but they share the same general features. Arteries and arterioles have thicker walls than veins and venules because they are closer to the heart and receive blood that is surging at a far greater pressure (Figure 2). Each type of vessel has a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_777\">lumen<\/a><\/strong>\u2014a hollow passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system. Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins.<\/p>\n<p style=\"text-align: justify\">By the time blood has passed through capillaries and entered venules, the pressure initially exerted upon it by heart contractions has diminished. In other words, in comparison to arteries, venules and veins withstand a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are correspondingly larger in diameter, allowing more blood to flow with less vessel resistance. In addition, many veins of the body, particularly those of the limbs, contain valves that assist the unidirectional flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity.<\/p>\n<p style=\"text-align: justify\">Both arteries and veins have the same three distinct tissue layers, called tunics (from the Latin term tunica), for the garments first worn by ancient Romans; the term tunic is also used for some modern garments. From the most interior layer to the outer, these tunics are the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_982\">tunica intima<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_983\">tunica media<\/a>, and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_984\">tunica externa<\/a> (Figure 2 and Table 1).<\/p>\n<p style=\"text-align: justify\"><strong>Tunica Intima:<\/strong> The tunica intima (also called the tunica interna) is composed of epithelial and connective tissue layers. Lining the tunica intima is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_783\">collagenous<\/a> fibres beneath is one of the primary causes of clot formation. Until recently, the endothelium was viewed simply as the boundary between the blood in the lumen and the walls of the vessels. Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow. The endothelium releases local chemicals called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_580\">endothelins<\/a> that can constrict the smooth muscle within the walls of the vessel to increase blood pressure. Uncompensated overproduction of endothelins may contribute to hypertension (high blood pressure) and cardiovascular disease.<\/p>\n<p style=\"text-align: justify\">Next to the endothelium is the basement membrane, or basal lamina, that effectively binds the endothelium to the connective tissue. The basement membrane provides strength while maintaining flexibility, and it is permeable, allowing materials to pass through it. The thin outer layer of the tunica intima contains a small amount of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_985\">areolar connective tissue<\/a> that consists primarily of elastic fibres to provide the vessel with additional flexibility; it also contains some collagenous fibres to provide additional strength.<\/p>\n<p style=\"text-align: justify\">In larger arteries, there is also a thick, distinct layer of elastic fibres known as <strong>the internal elastic membrane<\/strong> (also called the internal elastic lamina) at the boundary with the tunica media. Like the other components of the tunica intima, the internal elastic membrane provides structure while allowing the vessel to stretch. It is permeated with small openings that allow exchange of materials between the tunics. The internal elastic membrane is not apparent in veins. In addition, many veins, particularly in the lower limbs, contain valves formed by sections of thickened endothelium that are reinforced with connective tissue, extending into the lumen.<\/p>\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 754px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-74 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image38-OpenStax-blood-vessel-structure-artery-vein-754x1024.png\" alt=\"\" width=\"754\" height=\"1024\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image38-OpenStax-blood-vessel-structure-artery-vein-754x1024.png 754w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image38-OpenStax-blood-vessel-structure-artery-vein-221x300.png 221w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image38-OpenStax-blood-vessel-structure-artery-vein-65x88.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image38-OpenStax-blood-vessel-structure-artery-vein-225x305.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image38-OpenStax-blood-vessel-structure-artery-vein-350x475.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image38-OpenStax-blood-vessel-structure-artery-vein.png 766w\" sizes=\"auto, (max-width: 754px) 100vw, 754px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong>Figure 2. Structure of Blood Vessels.<\/strong> (a) Arteries and (b) veins share the same general features, but the walls of arteries are much thicker because of the higher pressure of the blood that flows through them. (c) A micrograph shows the relative differences in thickness. LM \u00d7 160. (Micrograph provided by the Regents of the University of Michigan Medical School \u00a9 2012)<\/figcaption><\/figure>\n<p style=\"text-align: justify\"><strong>Tunica Media:<\/strong> The <strong>tunica media<\/strong> is the substantial middle layer of the vessel wall (Figure 2). It is generally the thickest layer in arteries, and it is much thicker in arteries than it is in veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibres, most of which are arranged in circular sheets. Toward the outer portion of the tunic, there are also layers of longitudinal muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively. Specifically, in arteries,<strong> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_986\">vasoconstriction<\/a><\/strong> decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_755\">vasodilation<\/a><\/strong> increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop.<\/p>\n<p style=\"text-align: justify\">The smooth muscle layers of the tunica media are supported by a framework of collagenous fibres that also binds the tunica media to the inner and outer tunics. Along with the collagenous fibres are large numbers of elastic fibres that appear as wavy lines in prepared slides.<\/p>\n<table style=\"border-collapse: collapse;width: 100%\">\n<caption>Table 1: Comparison of wall layers in arteries, veins, and capillaries<\/caption>\n<tbody>\n<tr>\n<td style=\"width: 20.0385%\"><\/td>\n<th style=\"width: 26.3969%\" scope=\"col\"><strong>Arteries<\/strong><\/th>\n<th style=\"width: 26.7823%\" scope=\"col\"><strong>Veins<\/strong><\/th>\n<th style=\"width: 26.7822%\"><strong>Capillaries<\/strong><\/th>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><strong>General appearance<\/strong><\/td>\n<td style=\"width: 26.3969%\">Thick walls with small lumens<\/td>\n<td style=\"width: 26.7823%\">Thin walls with large lumens<\/td>\n<td style=\"width: 26.7822%\">Very (microscopically) thin walls and very small lumens<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\n<td style=\"width: 26.3969%\">Generally appear rounded<\/td>\n<td style=\"width: 26.7823%\">Generally appear flattened<\/td>\n<td style=\"width: 26.7822%\">Generally round<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><strong>Tunica intima<\/strong><\/td>\n<td style=\"width: 26.3969%\">Endothelium usually appears wavy due to constriction of smooth muscle<\/td>\n<td style=\"width: 26.7823%\">Endothelium appears smooth<\/td>\n<td style=\"width: 26.7822%\">Endothelium appears smooth<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\n<td style=\"width: 26.3969%\">Internal elastic membrane present in larger vessels<\/td>\n<td style=\"width: 26.7823%\">Internal elastic membrane absent<\/td>\n<td style=\"width: 26.7822%\">Internal elastic membrane absent<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><strong>Tunica media<\/strong><\/td>\n<td style=\"width: 26.3969%\">Normally the thickest layer in arteries<\/td>\n<td style=\"width: 26.7823%\">Normally thinner than the tunica externa<\/td>\n<td style=\"width: 26.7822%\">Tunica media absent<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\n<td style=\"width: 26.3969%\">Smooth muscle cells and elastic fibres predominate (exact proportions vary with distance from the heart)<\/td>\n<td style=\"width: 26.7823%\">Smooth muscle cells and collagenous fibres predominate<\/td>\n<td style=\"width: 26.7822%\"><\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\n<td style=\"width: 26.3969%\">External elastic membrane present in larger vessels<\/td>\n<td style=\"width: 26.7823%\">External elastic membrane absent<\/td>\n<td style=\"width: 26.7822%\"><\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\n<td style=\"width: 26.3969%\"><\/td>\n<td style=\"width: 26.7823%\">Nervi vasorum and vasa vasorum present<\/td>\n<td style=\"width: 26.7822%\"><\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><strong>Tunica externa<\/strong><\/td>\n<td style=\"width: 26.3969%\">Normally thinner than tunica media in all but the largest arteries<\/td>\n<td style=\"width: 26.7823%\">Normally the thickest layer in veins<\/td>\n<td style=\"width: 26.7822%\">Tunica externa absent<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\n<td style=\"width: 26.3969%\">Collagenous and elastic fibres<\/td>\n<td style=\"width: 26.7823%\">Collagenous and smooth fibres predominate<\/td>\n<td style=\"width: 26.7822%\"><\/td>\n<\/tr>\n<tr>\n<td style=\"width: 20.0385%\" scope=\"row\"><\/td>\n<td style=\"width: 26.3969%\">Nervi vasorum and vasa vasorum present<\/td>\n<td style=\"width: 26.7823%\">Nervi vasorum and vasa vasorum present<\/td>\n<td style=\"width: 26.7822%\"><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p style=\"text-align: justify\"><strong>Tunica Externa:<\/strong> The outer tunic, the <strong>tunica externa<\/strong> (also called the tunica adventitia), is a substantial sheath of connective tissue composed primarily of collagenous fibres. Some bands of elastic fibres are found here as well. The tunica externa in veins also contains groups of smooth muscle fibres. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1b\"><\/a>Arteries<\/strong><\/h5>\n<p style=\"text-align: justify\">An<strong> artery<\/strong> is a blood vessel that conducts blood away from the heart. All arteries have relatively thick walls that can withstand the high pressure of blood ejected from the heart.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1c\"><\/a>Arterioles<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0An <strong>arteriole <\/strong>is a very small artery that leads to a capillary. Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica intima is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin (Figure 39).<\/p>\n<p style=\"text-align: justify\">The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure. The precise diameter of the lumen of an arteriole at any given moment is determined by neural and chemical controls, and vasoconstriction and vasodilation in the arterioles are the primary mechanisms for distribution of blood flow.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1d\"><\/a>Capillaries<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_987\">perfusion<\/a><\/strong>. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_595\">interstitial fluid<\/a>). The diameter of a capillary lumen is from 5-10 \u03bcm; the smallest are just barely wide enough for an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_543\">erythrocyte<\/a> to squeeze through. Flow through capillaries is often described as microcirculation.<\/p>\n<p style=\"text-align: justify\">Unlike the walls of veins and arteries, the wall of a capillary consists of an endothelial layer surrounded by a basement membrane with occasional smooth muscle fibres. There is some variation in wall structure: in a large capillary, several endothelial cells bordering each other may line the lumen; in a small capillary, there may be only a single cell layer that wraps around to contact itself.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1e\"><\/a>Venules<\/strong><\/h5>\n<p style=\"text-align: justify\">A venule is an extremely small vein, generally 8\u2013100 \u03bcm in diameter. Postcapillary venules join multiple capillaries exiting from a capillary bed. Multiple venules join to form veins. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibres, plus an outer layer of connective tissue fibres that constitute a very thin <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_984\">tunica externa<\/a>. Venules as well as capillaries are the primary sites of emigration or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_567\">diapedesis<\/a>, in which the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_544\">leukocytes<\/a> adhere to the endothelial lining of the vessels and then squeeze through adjacent cells to enter the tissue fluid.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-1f\"><\/a>Veins<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0A vein is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens (Figure 42). Because they are low-pressure vessels, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries caused by the inherent low blood pressure in veins as well as the pull of gravity. Table 2 compares the features of arteries and veins.<\/p>\n<table style=\"border-collapse: collapse;width: 100%\">\n<caption>Table 2: Comparison of arteries and veins<\/caption>\n<tbody>\n<tr>\n<td style=\"width: 33.3333%\"><\/td>\n<th style=\"width: 33.3333%\" scope=\"col\"><strong>Arteries<\/strong><\/th>\n<th style=\"width: 33.3333%\" scope=\"col\"><strong>Veins<\/strong><\/th>\n<\/tr>\n<tr>\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Direction of blood flow<\/strong><\/th>\n<td style=\"width: 33.3333%\">Conducts blood away from the heart<\/td>\n<td style=\"width: 33.3333%\">Conducts blood toward the heart<\/td>\n<\/tr>\n<tr>\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>General appearance<\/strong><\/th>\n<td style=\"width: 33.3333%\">Rounded<\/td>\n<td style=\"width: 33.3333%\">Irregular, often collapsed<\/td>\n<\/tr>\n<tr>\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Pressure<\/strong><\/th>\n<td style=\"width: 33.3333%\">High<\/td>\n<td style=\"width: 33.3333%\">Low<\/td>\n<\/tr>\n<tr>\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Wall thickness<\/strong><\/th>\n<td style=\"width: 33.3333%\">Thick<\/td>\n<td style=\"width: 33.3333%\">Thin<\/td>\n<\/tr>\n<tr>\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Relative oxygen concentration<\/strong><\/th>\n<td style=\"width: 33.3333%\">Higher in systemic arteries; lower in pulmonary arteries<\/td>\n<td style=\"width: 33.3333%\">Lower in systemic veins; h<span style=\"text-indent: 1em;font-family: inherit;font-size: inherit\">igher in pulmonary venis<\/span><\/td>\n<\/tr>\n<tr>\n<th style=\"width: 33.3333%\" scope=\"row\"><strong>Valves<\/strong><\/th>\n<td style=\"width: 33.3333%\">Not present<\/td>\n<td style=\"width: 33.3333%\">Present most commonly in limbs and in veins inferior to the heart<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2><strong><a id=\"4-2\"><\/a>Part 2: Capillary Exchange<\/strong><\/h2>\n<p>The primary purpose of the cardiovascular system is to circulate gases, nutrients, wastes, and other substances to and from the cells of the body. Small molecules, such as gases, lipids, and lipid-soluble molecules, can diffuse directly through the membranes of the endothelial cells of the capillary wall. Glucose, amino acids, and ions\u2014including sodium, potassium, calcium, and chloride\u2014use transporters to move through specific channels in the membrane by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_988\">facilitated diffusion<\/a>. Glucose, ions, and larger molecules may also leave the blood through intercellular clefts. Larger molecules can pass through the pores of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_990\">fenestrated capillaries<\/a>, and even large plasma proteins can pass through the great gaps in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_989\">sinusoid capillaries<\/a>. Some large proteins in blood plasma can move into and out of the endothelial cells packaged within vesicles by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_991\">endocytosis<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_992\">exocytosis<\/a>. Water moves by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_903\">osmosis<\/a>.<\/p>\n<h2 style=\"text-align: justify\"><strong><a id=\"4-3\"><\/a>Part 3: Blood Flow, Blood Pressure, and Resistance<\/strong><\/h2>\n<p style=\"text-align: justify\"><strong>Blood flow<\/strong> refers to the movement of blood through a vessel, tissue, or organ, and is usually expressed in terms of volume of blood per unit of time. It is initiated by the contraction of the ventricles of the heart. Ventricular contraction ejects blood into the major arteries, resulting in flow from regions of higher pressure to regions of lower pressure, as blood encounters smaller arteries and arterioles, then capillaries, then the venules and veins of the venous system. This section discusses a number of critical variables that contribute to blood flow throughout the body. It also discusses the factors that impede or slow blood flow, a phenomenon known as <strong>resistance<\/strong>.<\/p>\n<p style=\"text-align: justify\">As noted earlier, hydrostatic pressure is the force exerted by a fluid due to gravitational pull, usually against the wall of the container in which it is located. One form of hydrostatic pressure is <strong>blood pressure<\/strong>, the force exerted by blood upon the walls of the blood vessels or the chambers of the heart. Blood pressure may be measured in capillaries and veins, as well as the vessels of the pulmonary circulation; however, the term blood pressure without any specific descriptors typically refers to systemic arterial blood pressure\u2014that is, the pressure of blood flowing in the arteries of the systemic circulation. In clinical practice, this pressure is measured in mm Hg and is usually obtained using the brachial artery of the arm.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-3a\"><\/a>Components of Arterial Blood Pressure<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0Arterial blood pressure in the larger vessels consists of several distinct components (Figure 3): systolic and diastolic pressures, pulse pressure, and mean arterial pressure.<\/p>\n<p style=\"text-align: justify\"><strong>Systolic and Diastolic Pressures:<\/strong> When systemic arterial blood pressure is measured, it is recorded as a ratio of two numbers (e.g., 120\/80 is a normal adult blood pressure), expressed as systolic pressure over diastolic pressure. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_994\">systolic pressure<\/a><\/strong> is the higher value (typically around 120 mm Hg) and reflects the arterial pressure resulting from the ejection of blood during ventricular contraction, or systole. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_993\">diastolic pressure<\/a><\/strong> is the lower value (usually about 80 mm Hg) and represents the arterial pressure of blood during ventricular relaxation, or diastole.<\/p>\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 1024px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-75 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image41-OpenStax-systemic-blood-pressure-in-blood-vessels-1024x708.png\" alt=\"\" width=\"1024\" height=\"708\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image41-OpenStax-systemic-blood-pressure-in-blood-vessels-1024x708.png 1024w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image41-OpenStax-systemic-blood-pressure-in-blood-vessels-300x207.png 300w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image41-OpenStax-systemic-blood-pressure-in-blood-vessels-768x531.png 768w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image41-OpenStax-systemic-blood-pressure-in-blood-vessels-65x45.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image41-OpenStax-systemic-blood-pressure-in-blood-vessels-225x156.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image41-OpenStax-systemic-blood-pressure-in-blood-vessels-350x242.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image41-OpenStax-systemic-blood-pressure-in-blood-vessels.png 1496w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong>Figure 3. Systemic Blood Pressure.<\/strong> The graph shows the components of blood pressure throughout the blood vessels, including systolic, diastolic, mean arterial, and pulse pressures.<\/figcaption><\/figure>\n<p style=\"text-align: justify\"><strong>Mean Arterial Pressure: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_995\">Mean arterial pressure (MAP)<\/a><\/strong> represents the \u201caverage\u201d pressure of blood in the arteries, that is, the average force driving blood into vessels that serve the tissues. Mean is a statistical concept and is calculated by taking the sum of the values divided by the number of values. Although complicated to measure directly and complicated to calculate, MAP can be approximated by adding the diastolic pressure to one-third of the pulse pressure or systolic pressure minus the diastolic pressure:<\/p>\n<p style=\"text-align: center\">MAP = diastolic BP + ((systolic-diastolic BP) \/ 3)<\/p>\n<p style=\"text-align: justify\">Normally, the MAP falls within the range of 70\u2013110 mm Hg. If the value falls below 60 mm Hg for an extended time, blood pressure will not be high enough to ensure circulation to and through the tissues, which results in <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_996\">ischemia<\/a><\/strong>, or insufficient blood flow. A condition called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_997\">hypoxia<\/a>, inadequate oxygenation of tissues, commonly accompanies ischemia. The term hypoxemia refers to low levels of oxygen in systemic arterial blood.<\/p>\n<p style=\"text-align: justify\"><strong>Measurement of Blood Pressure:<\/strong> Blood pressure is one of the critical parameters measured on virtually every patient in every healthcare setting. The technique used today was developed more than 100 years ago by a pioneering Russian physician, Dr. Nikolai Korotkoff. Turbulent blood flow through the vessels can be heard as a soft ticking while measuring blood pressure; these sounds are known as <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_998\">Korotkoff sounds<\/a><\/strong>. The technique of measuring blood pressure requires the use of a <strong>sphygmomanometer<\/strong> (a blood pressure cuff attached to a measuring device) and a stethoscope. The technique is as follows:<\/p>\n<ul>\n<li style=\"text-align: justify\">The clinician wraps an inflatable cuff tightly around the patient\u2019s arm at about the level of the heart.<\/li>\n<li style=\"text-align: justify\">The clinician squeezes a rubber pump to inject air into the cuff, raising pressure around the artery and temporarily cutting off blood flow into the patient\u2019s arm.<\/li>\n<li style=\"text-align: justify\">The clinician places the stethoscope on the patient\u2019s antecubital region and, while gradually allowing air within the cuff to escape, listens for the Korotkoff sounds.<\/li>\n<\/ul>\n<p style=\"text-align: justify\">The first sound heard through the stethoscope\u2014the first Korotkoff sound\u2014indicates <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_994\">systolic pressure<\/a>. As more air is released from the cuff, blood is able to flow freely through the brachial artery and all sounds disappear. The point at which the last sound is heard is recorded as the patient\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_993\">diastolic pressure<\/a>.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-3b\"><\/a>Pulse<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0After blood is ejected from the heart, elastic fibres in the arteries help maintain a high-pressure gradient as they expand to accommodate the blood, then recoil. This expansion and recoiling effect, known as the <strong>pulse<\/strong>, can be palpated manually or measured electronically. Although the effect diminishes over distance from the heart, elements of the systolic and diastolic components of the pulse are still evident down to the level of the arterioles.<\/p>\n<p style=\"text-align: justify\">Because pulse indicates heart rate, it is measured clinically to provide clues to a patient\u2019s state of health. It is recorded as beats per minute. Both the rate and the strength of the pulse are important clinically. A high or irregular pulse rate can be caused by physical activity or other temporary factors, but it may also indicate a heart condition. The pulse strength indicates the strength of ventricular contraction and cardiac output. If the pulse is strong, then systolic pressure is high. If it is weak, systolic pressure has fallen, and medical intervention may be warranted.<\/p>\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 664px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-76 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image42-OpenStax-pulse-points-664x1024.png\" alt=\"\" width=\"664\" height=\"1024\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image42-OpenStax-pulse-points-664x1024.png 664w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image42-OpenStax-pulse-points-194x300.png 194w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image42-OpenStax-pulse-points-65x100.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image42-OpenStax-pulse-points-225x347.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image42-OpenStax-pulse-points-350x540.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image42-OpenStax-pulse-points.png 670w\" sizes=\"auto, (max-width: 664px) 100vw, 664px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong>Figure 4. Pulse Sites.<\/strong> The pulse is most readily measured at the radial artery, but can be measured at any of the pulse points shown.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Pulse can be palpated manually by placing the tips of the fingers across an artery that runs close to the body surface and pressing lightly. While this procedure is normally performed using the radial artery in the wrist or the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_999\">common carotid artery<\/a> in the neck, any superficial artery that can be palpated may be used (Figure 4). Common sites to find a pulse include temporal and facial arteries in the head, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1000\">brachial arteries<\/a> in the upper arm, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1001\">femoral arteries<\/a> in the thigh, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1002\">popliteal arteries<\/a> behind the knees, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1003\">posterior tibial arteries<\/a> near the medial <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1004\">tarsal<\/a> regions, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1005\">dorsalis pedis arteries<\/a> in the feet. A variety of commercial electronic devices are also available to measure pulse.<\/p>\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 1024px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-77 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph-1024x635.png\" alt=\"\" width=\"1024\" height=\"635\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph-1024x635.png 1024w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph-300x186.png 300w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph-768x476.png 768w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph-1536x952.png 1536w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph-65x40.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph-225x139.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph-350x217.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image43-OpenStax-blood-pressure-measurement-graph.png 1641w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong>Figure 5. Blood Pressure Measurement.<\/strong> When pressure in a sphygmomanometer cuff is released, a clinician can hear the Korotkoff sounds. In this graph, a blood pressure tracing is aligned to a measurement of systolic and diastolic pressures.<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-3c\"><\/a>Variables Affecting Blood Flow and Blood Pressure<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0Five variables influence blood flow and blood pressure:<\/p>\n<ul>\n<li style=\"text-align: justify\">Cardiac output<\/li>\n<li style=\"text-align: justify\">Compliance<\/li>\n<li style=\"text-align: justify\">Volume of the blood<\/li>\n<li style=\"text-align: justify\">Viscosity of the blood<\/li>\n<li style=\"text-align: justify\">Blood vessel length and diameter<\/li>\n<\/ul>\n<p style=\"text-align: justify\">Recall that blood moves from higher pressure to lower pressure. It is pumped from the heart into the arteries at high pressure. If you increase pressure in the arteries (afterload), and cardiac function does not compensate, blood flow will actually decrease. In the venous system, the opposite relationship is true. Increased pressure in the veins does not decrease flow as it does in arteries, but actually increases flow. Since pressure in the veins is normally relatively low, for blood to flow back into the heart, the pressure in the atria during atrial diastole must be even lower. It normally approaches zero, except when the atria contract (Figure 5).<\/p>\n<p style=\"text-align: justify\"><strong>Cardiac Output:<\/strong> Cardiac output is the measurement of blood flow from the heart through the ventricles, and is usually measured in liters per minute. Any factor that causes cardiac output to increase, by elevating heart rate or stroke volume or both, will elevate blood pressure and promote blood flow. These factors include sympathetic stimulation, the catecholamines <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_449\">epinephrine<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_454\">norepinephrine<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1006\">thyroid hormones<\/a>, and increased calcium ion levels. Conversely, any factor that decreases cardiac output, by decreasing heart rate or stroke volume or both, will decrease arterial pressure and blood flow. These factors include <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_536\">parasympathetic<\/a> stimulation, elevated or decreased potassium ion levels, decreased calcium levels, anoxia, and acidosis.<\/p>\n<p style=\"text-align: justify\"><strong>Compliance:<\/strong> Compliance is the ability of any compartment to expand to accommodate increased content. A metal pipe, for example, is not compliant, whereas a balloon is. The greater the compliance of an artery, the more effectively it is able to expand to accommodate surges in blood flow without increased resistance or blood pressure. Veins are more compliant than arteries and can expand to hold more blood. When vascular disease causes stiffening of arteries, compliance is reduced and resistance to blood flow is increased. The result is more turbulence, higher pressure within the vessel, and reduced blood flow. This increases the work of the heart.<\/p>\n<p style=\"text-align: justify\"><strong>Blood Volume:<\/strong> The relationship between blood volume, blood pressure, and blood flow is intuitively obvious. Water may merely trickle along a creek bed in a dry season, but rush quickly and under great pressure after a heavy rain. Similarly, as blood volume decreases, pressure and flow decrease. As blood volume increases, pressure and flow increase.<\/p>\n<p style=\"text-align: justify\"><strong>Blood Viscosity:<\/strong> Viscosity is the thickness of fluids that affects their ability to flow. Clean water, for example, is less viscous than mud. The viscosity of blood is directly proportional to resistance and inversely proportional to flow; therefore, any condition that causes viscosity to increase will also increase resistance (and therefore blood pressure) and decrease flow. For example, imagine sipping milk, then a milkshake, through the same size straw. You experience more resistance and therefore less flow from the milkshake. Conversely, any condition that causes viscosity to decrease (such as when the milkshake melts) will decrease resistance and increase flow.<\/p>\n<p style=\"text-align: justify\">Normally the viscosity of blood does not change over short periods of time. The two primary determinants of blood viscosity are the formed elements and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_546\">plasma<\/a> proteins. Since the vast majority of formed elements are erythrocytes, any condition affecting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1007\">erythropoiesis<\/a>, such as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_565\">polycythemia <\/a>or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_564\">anemia<\/a>, can alter viscosity. Viscosity generally increases with increasing numbers of formed elements relative to the amount of plasma.\u00a0 If the concentration of proteins in the plasma is increased, this would also increase viscosity.\u00a0 Since most plasma proteins are produced by the liver, any condition affecting liver function can also change the viscosity and therefore affect blood flow. Liver abnormalities include hepatitis, cirrhosis, alcohol damage, and drug toxicities. While <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_544\">leukocytes<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_545\">platelets<\/a> are normally a small component of the formed elements, there are some rare conditions in which there is such a great overproduction of these that viscosity increases.<\/p>\n<p style=\"text-align: justify\"><strong>Vessel Length and Diameter:<\/strong> The length of a vessel is directly proportional to its resistance: the longer the vessel, the greater the resistance and the lower the flow. As with blood volume, this makes intuitive sense, since the increased surface area of the vessel will impede the flow of blood. Likewise, if the vessel is shortened, the resistance will decrease and flow will increase.<\/p>\n<p style=\"text-align: justify\">In contrast to length, the diameter of blood vessels changes throughout the body, according to the type of vessel, as we discussed earlier. The diameter of any given vessel may also change frequently throughout the day in response to neural and chemical signals that trigger vasodilation and vasoconstriction. The <strong>vascular tone<\/strong> of the vessel is the contractile state of the smooth muscle and the primary determinant of diameter, and thus of resistance and flow. The effect of vessel diameter on resistance is inverse: Given the same volume of blood, an increased diameter means there is less blood contacting the vessel wall, thus lower friction and lower resistance, subsequently increasing flow. A decreased diameter means more of the blood contacts the vessel wall, and resistance increases, subsequently decreasing flow.<\/p>\n<p style=\"text-align: justify\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_755\">Vasodilation<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_986\">vasoconstriction<\/a> of arterioles play more significant roles in regulating blood pressure than do the vasodilation and vasoconstriction of other vessels.<\/p>\n<h5 style=\"text-align: justify\"><strong>Venous System<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0The pumping action of the heart propels the blood into the arteries, from an area of higher pressure toward an area of lower pressure. If blood is to flow from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart. Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_529\">diastole<\/a> is very low, often approaching zero when the atria are relaxed (atrial diastole). Second, two physiologic \u201cpumps\u201d increase pressure in the venous system. The use of the term \u201cpump\u201d implies a physical device that speeds flow. These physiological pumps are less obvious.<\/p>\n<p style=\"text-align: justify\"><strong>Skeletal Muscle Pump:<\/strong> In many body regions, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle. This mechanism, known as the <strong>skeletal muscle pump<\/strong> (Figure 6), helps the lower-pressure veins counteract the force of gravity, increasing pressure to move blood back to the heart. As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves. This increased pressure causes blood to flow upward, opening valves superior to the contracting muscles so blood flows through. Simultaneously, valves inferior to the contracting muscles close; thus, blood should not seep back downward toward the feet. Military recruits are trained to flex their legs slightly while standing at attention for prolonged periods. Failure to do so may allow blood to pool in the lower limbs rather than returning to the heart. Consequently, the brain will not receive enough oxygenated blood, and the individual may lose consciousness.<\/p>\n<p style=\"text-align: justify\"><strong>Respiratory Pump:<\/strong> The respiratory pump aids blood flow through the veins of the thorax and abdomen. During inhalation, the volume of the thorax increases, largely through the contraction of the diaphragm, which moves downward and compresses the abdominal cavity. The elevation of the chest caused by the contraction of the external intercostal muscles also contributes to the increased volume of the thorax. The volume increase causes air pressure within the thorax to decrease, allowing us to inhale. Additionally, as air pressure within the thorax drops, blood pressure in the thoracic veins also decreases, falling below the pressure in the abdominal veins. This causes blood to flow along its pressure gradient from veins outside the thorax, where pressure is higher, into the thoracic region, where pressure is now lower. This in turn promotes the return of blood from the thoracic veins to the atria. During exhalation, when air pressure increases within the thoracic cavity, pressure in the thoracic veins increases, speeding blood flow into the heart while valves in the veins prevent blood from flowing backward from the thoracic and abdominal veins. Also notice that, as blood moves from venules to veins, the average blood pressure drops.<\/p>\n<figure style=\"width: 1193px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image44.png\" alt=\"image\" width=\"1193\" height=\"1036\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 6. Skeletal Muscle Pump.<\/strong> The contraction of skeletal muscles surrounding a vein compresses the blood and increases the pressure in that area. This action forces blood closer to the heart where venous pressure is lower. Note the importance of the one-way valves to assure that blood flows only in the proper direction.<\/figcaption><\/figure>\n<h2 style=\"text-align: justify\"><strong><a id=\"4-4\"><\/a>Part 4: Homeostatic Regulation of the Vascular System<\/strong><\/h2>\n<p style=\"text-align: justify\">To maintain homeostasis in the cardiovascular system and provide adequate blood to the tissues, blood flow must be redirected continually to the tissues as they become more active. In a very real sense, the cardiovascular system engages in resource allocation, because there is not enough blood flow to distribute blood equally to all tissues simultaneously. For example, when an individual is exercising, more blood will be directed to skeletal muscles, the heart, and the lungs. Following a meal, more blood is directed to the digestive system. Only the brain receives a more or less constant supply of blood whether you are active, resting, thinking, or engaged in any other activity.<\/p>\n<p style=\"text-align: justify\">Table 3 provides the distribution of systemic blood at rest and during exercise. Although most of the data appears logical, the values for the distribution of blood to the integument may seem surprising. During exercise, the body distributes more blood to the body surface where it can dissipate the excess heat generated by increased activity into the environment.\u00a0 Three homeostatic mechanisms ensure adequate blood flow, blood pressure, distribution, and ultimately perfusion: neural, endocrine, and autoregulatory mechanisms (Figure 7).<\/p>\n<table style=\"border-collapse: collapse;width: 100%\">\n<caption>Table 3: Systemic blood flow during rest, mild exercise, and maximal exercise in a healthy young individual<\/caption>\n<tbody>\n<tr>\n<th style=\"width: 25%\" scope=\"col\"><strong>Organ<\/strong><\/th>\n<th style=\"width: 25%\" scope=\"col\"><strong>Resting (mL\/min)<\/strong><\/th>\n<th style=\"width: 25%\" scope=\"col\"><strong>Mild exercise (mL\/min)<\/strong><\/th>\n<th style=\"width: 25%\" scope=\"col\"><strong>Maximal exercise (mL\/min)<\/strong><\/th>\n<\/tr>\n<tr>\n<td style=\"width: 25%\" scope=\"row\">Skeletal muscle<\/td>\n<td style=\"width: 25%\">1200<\/td>\n<td style=\"width: 25%\">4500<\/td>\n<td style=\"width: 25%\">12,500<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 25%\" scope=\"row\">Heart<\/td>\n<td style=\"width: 25%\">250<\/td>\n<td style=\"width: 25%\">350<\/td>\n<td style=\"width: 25%\">750<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 25%\" scope=\"row\">Brain<\/td>\n<td style=\"width: 25%\">750<\/td>\n<td style=\"width: 25%\">750<\/td>\n<td style=\"width: 25%\">750<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 25%\" scope=\"row\">Integument<\/td>\n<td style=\"width: 25%\">500<\/td>\n<td style=\"width: 25%\">1500<\/td>\n<td style=\"width: 25%\">1900<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 25%\" scope=\"row\">Kidney<\/td>\n<td style=\"width: 25%\">1100<\/td>\n<td style=\"width: 25%\">900<\/td>\n<td style=\"width: 25%\">600<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 25%\" scope=\"row\">Gastrointestinal<\/td>\n<td style=\"width: 25%\">1400<\/td>\n<td style=\"width: 25%\">1100<\/td>\n<td style=\"width: 25%\">600<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 25%\" scope=\"row\">Others (e.g., liver, spleen)<\/td>\n<td style=\"width: 25%\">600<\/td>\n<td style=\"width: 25%\">400<\/td>\n<td style=\"width: 25%\">400<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 25%\" scope=\"row\">Total<\/td>\n<td style=\"width: 25%\">5800<\/td>\n<td style=\"width: 25%\">9500<\/td>\n<td style=\"width: 25%\">17,500<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 789px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-79 size-full\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/2115_Vascular_Homeostasis_Flow_Art.png\" alt=\"\" width=\"789\" height=\"1125\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/2115_Vascular_Homeostasis_Flow_Art.png 789w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/2115_Vascular_Homeostasis_Flow_Art-210x300.png 210w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/2115_Vascular_Homeostasis_Flow_Art-718x1024.png 718w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/2115_Vascular_Homeostasis_Flow_Art-768x1095.png 768w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/2115_Vascular_Homeostasis_Flow_Art-65x93.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/2115_Vascular_Homeostasis_Flow_Art-225x321.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/2115_Vascular_Homeostasis_Flow_Art-350x499.png 350w\" sizes=\"auto, (max-width: 789px) 100vw, 789px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong><strong>Figure 7. Summary of Factors Maintaining Vascular Homeostasis. <\/strong><\/strong>Adequate blood flow, blood pressure, distribution, and perfusion involve autoregulatory, neural, and endocrine mechanisms.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4a\"><\/a>Neural Regulation<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0The nervous system plays a critical role in the regulation of vascular homeostasis. The primary regulatory sites include the cardiovascular centres in the brain that control both cardiac and vascular functions. In addition, more generalized neural responses from the limbic system and the autonomic nervous system are factors.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4b\"><\/a>The Cardiovascular Centres in the Brain<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0Neurological regulation of blood pressure and flow depends on the cardiovascular centres located in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_725\">medulla oblongata<\/a>. This cluster of neurons responds to changes in blood pressure as well as blood concentrations of oxygen, carbon dioxide, and hydrogen ions. The cardiovascular centre contains three distinct components:<\/p>\n<ul>\n<li style=\"text-align: justify\">The cardioacceleratory centre stimulates cardiac function by regulating heart rate and stroke volume via sympathetic stimulation from the cardiac accelerator nerve.<\/li>\n<li style=\"text-align: justify\">The cardioinhibitory centre slows cardiac function by decreasing heart rate via <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_536\">parasympathetic<\/a> stimulation from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_538\">vagus nerve<\/a>.<\/li>\n<li style=\"text-align: justify\">The vasomotor centre controls vessel tone or contraction of the smooth muscle in the tunica media. Changes in diameter affect peripheral resistance, pressure, and flow, which affect cardiac output. The majority of these neurons act via the release of the neurotransmitter norepinephrine from sympathetic neurons.<\/li>\n<\/ul>\n<p style=\"text-align: justify\">Although each centre functions independently, they are not anatomically distinct.<\/p>\n<p style=\"text-align: justify\">There is also a small population of neurons that control vasodilation in the vessels of the brain and skeletal muscles by relaxing the smooth muscle fibres in the vessel tunics. Many of these are cholinergic neurons, that is, they release acetylcholine, which in turn stimulates the vessels\u2019 endothelial cells to release <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1016\">nitric oxide (NO)<\/a>, which causes vasodilation. Others release norepinephrine that binds to \u03b2<sub>2<\/sub> receptors. A few neurons release <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1016\">NO<\/a> directly as a neurotransmitter.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4c\"><\/a>Baroreceptor Reflexes<\/strong><\/h5>\n<p style=\"text-align: justify\">Baroreceptors are specialized stretch receptors located within thin areas of blood vessels and heart chambers that respond to the degree of stretch caused by the presence of blood. They send impulses to the cardiovascular centres to regulate blood pressure. Vascular baroreceptors are found primarily in sinuses (small cavities) within the aorta and carotid arteries: The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1009\">aortic sinuses<\/a><\/strong> are found in the walls of the ascending aorta just superior to the aortic valve, whereas the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1010\">carotid sinuses<\/a><\/strong> are in the base of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1011\">internal carotid arteries<\/a>. There are also low-pressure baroreceptors located in the walls of the venae cavae and right atrium.<\/p>\n<p style=\"text-align: justify\">When blood pressure increases, the baroreceptors are stretched more tightly and initiate <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1012\">action potentials<\/a> at a higher rate. At lower blood pressures, the degree of stretch is lower and the rate of firing is slower. When the cardiovascular centres in the medulla oblongata receives this input, they triggers a reflex that maintains homeostasis (Figure 8):<\/p>\n<ul>\n<li style=\"text-align: justify\">When blood pressure rises too high, the baroreceptors fire at a higher rate and trigger parasympathetic stimulation of the heart. As a result, cardiac output falls. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_535\">Sympathetic<\/a> stimulation of the peripheral <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_598\">arterioles<\/a> will also decrease, resulting in vasodilation. Combined, these activities cause blood pressure to fall.<\/li>\n<li style=\"text-align: justify\">When blood pressure drops too low, the rate of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1013\">baroreceptor<\/a> firing decreases. This will trigger an increase in sympathetic stimulation of the heart, causing cardiac output to increase. It will also trigger sympathetic stimulation of the peripheral vessels, resulting in vasoconstriction. Combined, these activities cause blood pressure to rise.<\/li>\n<\/ul>\n<p style=\"text-align: justify\">The baroreceptors in the venae cavae and right atrium monitor blood pressure as the blood returns to the heart from the systemic circulation. Normally, blood flow into the aorta is the same as blood flow back into the right atrium. If blood is returning to the right atrium more rapidly than it is being ejected from the left ventricle, the atrial receptors will stimulate the cardiovascular centres to increase sympathetic firing and increase cardiac output until homeostasis is achieved. The opposite is also true. This mechanism is referred to as the atrial reflex.<\/p>\n<figure style=\"width: 1390px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image47.png\" alt=\"image\" width=\"1390\" height=\"1036\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 8. Baroreceptor Reflexes for Maintaining Vascular Homeostasis.<\/strong> Increased blood pressure results in increased rates of baroreceptor firing, whereas decreased blood pressure results in slower rates of fire, both initiating the homeostatic mechanism to restore blood pressure.<\/figcaption><\/figure>\n<p style=\"text-align: justify\"><strong>Chemoreceptor Reflexes:<\/strong> In addition to the baroreceptors are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_734\">chemoreceptors<\/a> that monitor levels of oxygen, carbon dioxide, and hydrogen ions (pH), and thereby contribute to vascular homeostasis. Chemoreceptors monitoring the blood are located in close proximity to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1013\">baroreceptors<\/a> in the aortic and carotid sinuses. They signal the cardiovascular centres as well as the respiratory centres in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_725\">medulla oblongata<\/a>.<\/p>\n<p style=\"text-align: justify\">Since tissues consume oxygen and produce carbon dioxide and acids as waste products, when the body is more active, oxygen levels fall and carbon dioxide levels rise as cells undergo cellular respiration to meet the energy needs of activities. This causes more hydrogen ions to be produced, causing the blood pH to drop. When the body is resting, oxygen levels are higher, carbon dioxide levels are lower, more hydrogen is bound, and pH rises.<\/p>\n<p style=\"text-align: justify\">The chemoreceptors respond to increasing carbon dioxide and hydrogen ion levels (falling pH) by stimulating the cardioacceleratory and vasomotor centres, increasing cardiac output and constricting peripheral vessels. The cardioinhibitory centre is suppressed. With falling carbon dioxide and hydrogen ion levels (increasing pH), the cardioinhibitory centre is stimulated, and the cardioacceleratory and vasomotor centres are suppressed, decreasing cardiac output and causing peripheral vasodilation. In order to maintain adequate supplies of oxygen to the cells and remove waste products such as carbon dioxide, it is essential that the respiratory system respond to changing metabolic demands. In turn, the cardiovascular system will transport these gases to the lungs for exchange, again in accordance with metabolic demands. This interrelationship of cardiovascular and respiratory control cannot be overemphasized.<\/p>\n<p style=\"text-align: justify\">Other neural mechanisms can also have affect cardiovascular function. These include the limbic system that links physiological responses to psychological stimuli, as well as generalized sympathetic and parasympathetic stimulation.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4d\"><\/a>Endocrine Regulation<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0Endocrine control over the cardiovascular system involves the catecholamines, epinephrine and norepinephrine, as well as several hormones that interact with the kidneys in the regulation of blood volume.<\/p>\n<p style=\"text-align: justify\"><strong>Epinephrine and Norepinephrine:<\/strong> The catecholamines <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_449\">epinephrine<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_454\">norepinephrine<\/a> are released by the adrenal medulla, and enhance and extend the body\u2019s sympathetic or \u201cfight-or-flight\u201d response (Figure 9). They increase heart rate and force of contraction, while temporarily constricting blood vessels to organs not essential for flight-or-fight responses and redirecting blood flow to the liver, muscles, and heart.<\/p>\n<p style=\"text-align: justify\"><strong>Antidiuretic Hormone:<\/strong> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_473\">Antidiuretic hormone<\/a> (ADH), also known as vasopressin, is secreted by the cells in the hypothalamus and transported via the hypothalamic-hypophyseal tracts to the posterior pituitary where it is stored until released upon nervous stimulation. The primary trigger prompting the hypothalamus to release antiduiretic hormone is increasing osmolarity of tissue fluid, usually in response to significant loss of blood volume (Figure 10). ADH signals its target cells in the kidneys to reabsorb more water, thus preventing the loss of additional fluid in the urine. This will increase overall fluid levels and help restore blood volume and pressure. In addition, antiduiretic hormone constricts peripheral vessels.<\/p>\n<p style=\"text-align: justify\"><strong>Renin-Angiotensin-Aldosterone Mechanism:<\/strong> The renin-angiotensin-aldosterone mechanism has a major effect upon the cardiovascular system (Figure 9). Renin is an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_464\">enzyme<\/a>, although because of its importance in the renin-angiotensin-aldosterone pathway, some sources identify it as a hormone. Specialized cells in the kidneys found in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1014\">juxtaglomerular apparatus<\/a> respond to decreased blood flow by secreting renin into the blood. Renin converts the plasma protein angiotensinogen, which is produced by the liver, into its active form\u2014angiotensin I. Angiotensin I circulates in the blood and is then converted into angiotensin II in the lungs. This reaction is catalyzed by the enzyme angiotensin-converting enzyme (ACE).<\/p>\n<p style=\"text-align: justify\">Angiotensin II is a powerful vasoconstrictor, greatly increasing blood pressure. It also stimulates the release of antiduiretic hormone and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1015\">aldosterone<\/a>, a hormone produced by the adrenal cortex. Aldosterone increases the reabsorption of sodium into the blood by the kidneys. Since water follows sodium, this increases the reabsorption of water. This in turn increases blood volume, raising blood pressure. Angiotensin II also stimulates the thirst centre in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_392\">hypothalamus<\/a>, so an individual will likely consume more fluids, again increasing blood volume and pressure.<\/p>\n<figure style=\"width: 1670px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image48.png\" alt=\"image\" width=\"1670\" height=\"722\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 9. Hormones Involved in Renal Control of Blood Pressure.<\/strong> In the renin-angiotensin-aldosterone mechanism, increasing angiotensin II will stimulate the production of antidiuretic hormone and aldosterone. In addition to renin, the kidneys produce erythropoietin, which stimulates the production of red blood cells, further increasing blood volume.<\/figcaption><\/figure>\n<figure style=\"width: 1667px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image49.png\" alt=\"image\" width=\"1667\" height=\"877\" \/><figcaption class=\"wp-caption-text\"><strong><strong>Figure 10. Homeostatic Responses to Loss of Blood Volume<\/strong><\/strong><\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><strong>Erythropoietin:<\/strong> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_562\">Erythropoietin (EPO)<\/a> is released by the kidneys when blood flow and\/or oxygen levels decrease. Erythropoietin stimulates the production of erythrocytes within the bone marrow. Erythrocytes are the major formed element of the blood and may contribute 40% or more to blood volume, a significant factor of viscosity, resistance, pressure, and flow. In addition, erythropoietin is a vasoconstrictor. Overproduction of erythropoietin or excessive intake of synthetic erythropoietin, often to enhance athletic performance, will increase viscosity, resistance, and pressure, and decrease flow in addition to its contribution as a vasoconstrictor.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-4e\"><\/a>Autoregulation of Perfusion<\/strong><\/h5>\n<p style=\"text-align: justify\">Autoregulation mechanisms require neither specialized nervous stimulation nor endocrine control. Rather, these are local, self-regulatory mechanisms that allow each region of tissue to adjust its blood flow, and thus its perfusion. These local mechanisms include chemical signals and myogenic controls.<\/p>\n<p style=\"text-align: justify\"><strong>Chemical Signals Involved in Autoregulation:<\/strong> Chemical signals work at the level of the precapillary <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_866\">sphincters<\/a> to trigger either constriction or relaxation. Opening a precapillary sphincter allows blood to flow into that particular capillary, whereas constricting a precapillary sphincter temporarily shuts off blood flow to that region. The factors involved in regulating the precapillary sphincters include the following:<\/p>\n<ul>\n<li style=\"text-align: justify\">Opening of the sphincter is triggered in response to decreased oxygen concentrations; increased carbon dioxide concentrations; increasing levels of lactic acid or other byproducts of cellular metabolism; increasing concentrations of potassium ions or hydrogen ions (falling pH); inflammatory chemicals such as histamines; and increased body temperature. These conditions in turn stimulate the release of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1016\">NO<\/a>, a powerful vasodilator, from endothelial cells.<\/li>\n<li style=\"text-align: justify\">Contraction of the precapillary sphincter is triggered by the opposite levels of the regulators, which prompt the release of endothelins, powerful vasoconstricting peptides secreted by endothelial cells. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_545\">Platelet<\/a> secretions and certain <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1017\">prostaglandins<\/a> may also trigger constriction.<\/li>\n<\/ul>\n<p style=\"text-align: justify\">Again, these factors alter tissue <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_987\">perfusion<\/a> via their effects on the precapillary sphincter mechanism, which regulates blood flow to capillaries. Since the amount of blood is limited, not all capillaries can fill at once, so blood flow is allocated based upon the needs and metabolic state of the tissues as reflected in these parameters. Bear in mind, however, that dilation and constriction of the arterioles feeding the capillary beds is the primary control mechanism.<\/p>\n<p style=\"text-align: justify\"><strong>The Myogenic Response:<\/strong> The myogenic response is a reaction to the stretching of the smooth muscle in the walls of arterioles as changes in blood flow occur through the vessel. This may be viewed as a largely protective function against dramatic fluctuations in blood pressure and blood flow to maintain homeostasis. If <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_987\">perfusion<\/a> of an organ is too low (ischemia), the tissue will experience low levels of oxygen (hypoxia). In contrast, excessive perfusion could damage the organ\u2019s smaller and more fragile vessels. The myogenic response is a localized process that serves to stabilize blood flow in the capillary network that follows that arteriole. When blood flow is low, the vessel\u2019s smooth muscle will be only minimally stretched. In response, it relaxes, allowing the vessel to dilate and thereby increase the movement of blood into the tissue. When blood flow is too high, the smooth muscle will contract in response to the increased stretch, prompting vasoconstriction that reduces blood flow.<\/p>\n<h2 style=\"text-align: justify\"><strong><a id=\"4-5\"><\/a>Part 5: Circulatory Pathways<\/strong><\/h2>\n<p style=\"text-align: justify\">Virtually every cell, tissue, organ, and system in the body is impacted by the circulatory system. This includes the generalized and more specialized functions of transport of materials, capillary exchange, maintaining health by transporting leukocytes and various immunoglobulins (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_633\">antibodies<\/a>), <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1018\">hemostasis<\/a>, regulation of body temperature, and helping to maintain acid-base balance. In addition to these shared functions, many systems enjoy a unique relationship with the circulatory system (Figure 11).<\/p>\n<p style=\"text-align: justify\">As you learn about the vessels of the systemic and pulmonary circuits, notice that many arteries and veins share the same names, parallel one another throughout the body, and are very similar on the right and left sides of the body. For example, you will find a pair of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1001\">femoral arteries<\/a> and a pair of femoral veins, with one vessel on each side of the body. In contrast, some vessels closer to the midline of the body, such as the aorta, are unique. Another phenomenon that can make the study of vessels challenging is that names of vessels can change with location. Like a street that changes name as it passes through an intersection, an artery or vein can change names as it passes an anatomical landmark. For example, the left <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1019\">subclavian artery<\/a> becomes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1020\">axillary artery<\/a> as it passes through the body wall and into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1025\">axillary region<\/a>, and then becomes the brachial artery as it flows from the axillary region into the upper arm (or brachium).<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"4-5a\"><\/a>Pulmonary Circulation<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0Recall that blood returning from the systemic circuit enters the right atrium (Figure 12) via the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_422\">superior<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_423\">inferior venae cavae<\/a><\/strong> and the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_504\">coronary sinus<\/a><\/strong>, which drains the blood supply of the heart muscle. These vessels will be described more fully later in this section. This blood is relatively low in oxygen and relatively high in carbon dioxide, since much of the oxygen has been extracted for use by the tissues and the waste gas carbon dioxide was picked up to be transported to the lungs for elimination. From the right atrium, blood moves into the right ventricle, which pumps it to the lungs for gas exchange. This system of vessels is referred to as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_420\">pulmonary circuit<\/a><\/strong>.<\/p>\n<p style=\"text-align: justify\">The single vessel exiting the right ventricle is the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_497\">pulmonary trunk<\/a><\/strong>. At the base of the pulmonary trunk is the pulmonary <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_508\">semilunar valve<\/a>, which prevents backflow of blood into the right ventricle during ventricular diastole. As the pulmonary trunk reaches the superior surface of the heart, it curves posteriorly and rapidly bifurcates (divides) into two branches, a left and a right <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1026\">pulmonary artery<\/a><\/strong>. To prevent confusion between these vessels, it is important to refer to the vessel exiting the heart as the pulmonary trunk, rather than also calling it a pulmonary artery.<\/p>\n<p style=\"text-align: justify\">The pulmonary arteries in turn branch many times within the lung, forming a series of smaller arteries and arterioles that eventually lead to the pulmonary capillaries. The pulmonary capillaries surround lung structures known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_663\">alveoli<\/a> that are the sites of oxygen and carbon dioxide exchange.<\/p>\n<figure style=\"width: 867px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image50.png\" alt=\"image\" width=\"867\" height=\"1049\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 11. Interaction of the Circulatory System with Other Body Systems<\/strong><\/figcaption><\/figure>\n<figure style=\"width: 1505px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image51.png\" alt=\"image\" width=\"1505\" height=\"860\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 12. Pulmonary Circuit.<\/strong> Blood exiting from the right ventricle flows into the pulmonary trunk, which bifurcates into the two pulmonary arteries. These vessels branch to supply blood to the pulmonary capillaries, where gas exchange occurs within the lung alveoli. Blood returns via the pulmonary veins to the left atrium.<\/figcaption><\/figure>\n<figure style=\"width: 156px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image52.png\" alt=\"image\" width=\"156\" height=\"158\" \/><figcaption class=\"wp-caption-text\">Watch <a href=\"https:\/\/youtu.be\/v43ej5lCeBo\">this CrashCourse video<\/a> to learn more about the blood vessels! Direct link: <a href=\"https:\/\/youtu.be\/v43ej5lCeBo\">https:\/\/youtu.be\/v43ej5lCeBo<\/a><\/figcaption><\/figure>\n<\/div>\n<div><\/div>\n<div class=\"unit-2:-the-cardiovascular-system-\">\n<p>Once gas exchange is completed, oxygenated blood flows from the pulmonary capillaries into a series of pulmonary venules that eventually lead to a series of larger <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1027\">pulmonary veins<\/a><\/strong>. Four pulmonary veins, two on the left and two on the right, return blood to the left atrium. At this point, the pulmonary circuit is complete. Table 4 defines the major arteries and veins of the pulmonary circuit discussed in the text.<\/p>\n<h5 style=\"text-align: justify\"><strong>Overview of Systemic Arteries<\/strong><\/h5>\n<p style=\"text-align: justify\">Blood relatively high in oxygen concentration is returned from the pulmonary circuit to the left atrium via the four pulmonary veins. From the left atrium, blood moves into the left ventricle, which pumps blood into the aorta. The aorta and its branches\u2014the systemic arteries\u2014send blood to virtually every organ of the body (Figure 41).<\/p>\n<table style=\"border-collapse: collapse;width: 100%;height: 100px\">\n<caption>Table 4: Pulmonary arteries and veins<\/caption>\n<tbody>\n<tr style=\"height: 16px\">\n<th style=\"width: 44.7368%;height: 16px\" scope=\"col\"><strong>Vessel<\/strong><\/th>\n<th style=\"width: 55.2632%;height: 16px\" scope=\"col\"><strong>Description<\/strong><\/th>\n<\/tr>\n<tr style=\"height: 52px\">\n<td style=\"width: 44.7368%;height: 52px\" scope=\"row\">Pulmonary trunk<\/td>\n<td style=\"width: 55.2632%;height: 52px\">Single large vessel exiting the right ventricle (divides to form the right and left pulmonary arteries)<\/td>\n<\/tr>\n<tr style=\"height: 16px\">\n<td style=\"width: 44.7368%;height: 16px\" scope=\"row\">Pulmonary arteries (left pulmonary artery, right pulmonary artery)<\/td>\n<td style=\"width: 55.2632%;height: 16px\">Two vessels that form from the pulmonary trunk and lead to smaller arterioles and eventually to the pulmonary capillaries<\/td>\n<\/tr>\n<tr style=\"height: 16px\">\n<td style=\"width: 44.7368%;height: 16px\" scope=\"row\">Pulmonary veins (left superior pulmonary vein, left inferior pulmonary vein, right superior pulmonary vein, right inferior pulmonary vein)<\/td>\n<td style=\"width: 55.2632%;height: 16px\">Two sets of paired vessels (one pair from each side) that are formed from venules, leading blood away from the pulmonary capillaries to flow into the left atrium<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h5 style=\"text-align: justify\"><strong>The Aorta<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1028\">aorta<\/a><\/strong> is the largest artery in the body (Figure 13). It arises from the left ventricle and eventually descends to the abdominal region, where it bifurcates at the level of the fourth lumbar vertebra into the two common iliac arteries. The aorta consists of the ascending aorta, the aortic arch, and the descending aorta (Table 5) which passes through the diaphragm, a landmark that divides into the superior thoracic and inferior abdominal components. Arteries originating from the aorta ultimately distribute blood to virtually all tissues of the body. At the base of the aorta is the aortic semilunar valve that prevents backflow of blood into the left ventricle while the heart is relaxing.<\/p>\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 631px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-86 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image54-OpenStax-systemic-arteries-631x1024.png\" alt=\"\" width=\"631\" height=\"1024\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image54-OpenStax-systemic-arteries-631x1024.png 631w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image54-OpenStax-systemic-arteries-185x300.png 185w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image54-OpenStax-systemic-arteries-65x106.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image54-OpenStax-systemic-arteries-225x365.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image54-OpenStax-systemic-arteries-350x568.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image54-OpenStax-systemic-arteries.png 643w\" sizes=\"auto, (max-width: 631px) 100vw, 631px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong>Figure 13. Systemic Arteries.<\/strong> The major systemic arteries shown here deliver oxygenated blood throughout the body.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">After exiting the heart, the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1032\">ascending aorta<\/a><\/strong> moves in a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1033\">superior<\/a> direction for approximately 5 cm and ends at the sternal angle. Following this ascent, it reverses direction, forming a graceful arc to the left, called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_731\">aortic arch<\/a><\/strong>. The aortic arch descends toward the inferior portions of the body and ends at the level of the intervertebral disk between the fourth and fifth thoracic vertebrae. Beyond this point, the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1031\">descending aorta<\/a><\/strong> continues close to the bodies of the vertebrae and passes through an opening in the diaphragm. Superior to the diaphragm, the aorta is called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1030\">thoracic aorta<\/a><\/strong>, and inferior to the diaphragm, it is called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1029\">abdominal aorta<\/a><\/strong>. The abdominal aorta terminates when it bifurcates into the two common iliac arteries at the level of the fourth lumbar vertebra. See Figure 55 for an illustration of the ascending aorta, the aortic arch, and the initial segment of the descending aorta plus major branches.<\/p>\n<h5 style=\"text-align: justify\"><strong>Coronary Circulation<\/strong><\/h5>\n<p style=\"text-align: justify\">The first vessels that branch from the ascending aorta are the paired <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_513\">coronary arteries<\/a> (see Figure 42), which arise from two of the three sinuses in the ascending aorta just superior to the aortic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_508\">semilunar valve<\/a>. These sinuses contain the aortic baroreceptors and chemoreceptors critical to maintain cardiac function. The left coronary artery arises from the left posterior <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1009\">aortic sinus<\/a>. The right coronary artery arises from the anterior aortic sinus. Normally, the right posterior aortic sinus does not give rise to a vessel.<\/p>\n<p style=\"text-align: justify\">The coronary arteries encircle the heart, forming a ring-like structure that divides into the next level of branches that supplies blood to the heart tissues.<\/p>\n<h5 style=\"text-align: justify\"><strong>Aortic Arch Branches<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0There are three major branches of the aortic arch: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1034\"><strong>brachiocephalic<\/strong> artery<\/a>, the <strong>left common carotid artery<\/strong>, and the <strong>left <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1019\">subclavian<\/a><\/strong>\u00a0(literally \u201cunder the clavicle\u201d) <strong>artery<\/strong>. As you would expect based upon proximity to the heart, each of these vessels is classified as an elastic artery.<\/p>\n<p style=\"text-align: justify\">The brachiocephalic artery is located only on the right side of the body; there is no corresponding artery on the left. The brachiocephalic artery branches into the <strong>right subclavian artery<\/strong> and the <strong>right common carotid artery<\/strong>. The left subclavian and left common carotid arteries arise independently from the aortic arch but otherwise follow a similar pattern and distribution to the corresponding arteries on the right side (see Figure 14).<\/p>\n<p style=\"text-align: justify\">Each <strong>subclavian artery<\/strong> supplies blood to the arms, chest, shoulders, back, and central nervous system.<\/p>\n<p style=\"text-align: justify\">The <strong>common carotid<\/strong> artery divides into internal and external carotid arteries. The right common carotid artery arises from the brachiocephalic artery and the left common carotid artery arises directly from the aortic arch. The <strong>branches of the carotid arteries<\/strong> supply blood to numerous structures within the head and neck. Each internal carotid artery initially forms an expansion known as the carotid sinus, containing the carotid baroreceptors and chemoreceptors. Like their counterparts in the aortic sinuses, the information provided by these receptors is critical to maintaining cardiovascular homeostasis (see Figure 13).<\/p>\n<p>&nbsp;<\/p>\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 972px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-87 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image55-OpenStax-aorta-arch-and-arteries-972x1024.png\" alt=\"\" width=\"972\" height=\"1024\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image55-OpenStax-aorta-arch-and-arteries-972x1024.png 972w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image55-OpenStax-aorta-arch-and-arteries-285x300.png 285w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image55-OpenStax-aorta-arch-and-arteries-768x809.png 768w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image55-OpenStax-aorta-arch-and-arteries-65x68.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image55-OpenStax-aorta-arch-and-arteries-225x237.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image55-OpenStax-aorta-arch-and-arteries-350x369.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image55-OpenStax-aorta-arch-and-arteries.png 981w\" sizes=\"auto, (max-width: 972px) 100vw, 972px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong>Figure 14. Aorta.<\/strong> The aorta has distinct regions, including the ascending aorta, aortic arch, and the descending aorta, which includes the thoracic and abdominal regions.<\/figcaption><\/figure>\n<table style=\"border-collapse: collapse;width: 100%\">\n<caption>Table 5: Components of the aorta<\/caption>\n<tbody>\n<tr>\n<th style=\"width: 13.7427%\" scope=\"col\"><strong>Vessel<\/strong><\/th>\n<th style=\"width: 86.2573%\" scope=\"col\"><strong>Description<\/strong><\/th>\n<\/tr>\n<tr>\n<td style=\"width: 13.7427%\" scope=\"row\">Aorta<\/td>\n<td style=\"width: 86.2573%\">Largest artery in the body; originates from the left ventricle and descends to the abdominal region then bifurcates into the left and right common iliac arteries at the level of the fourth lumbar vertebra<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 13.7427%\" scope=\"row\">Ascending aorta<\/td>\n<td style=\"width: 86.2573%\">Initial portion of the aorta; rises superiorly from the left ventricle for a distance of approximately 5 cm<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 13.7427%\" scope=\"row\">Aortic arch<\/td>\n<td style=\"width: 86.2573%\">Graceful arc to the left that connects the ascending aorta to the descending aorta; ends at the intervertebral disk between the fourth and fifth thoracic vertebrae<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 13.7427%\" scope=\"row\">Descending aorta<\/td>\n<td style=\"width: 86.2573%\">Continues inferiorly from the end of the aortic arch; subdivided into the thoracic aorta and the abdominal aorta<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 13.7427%\" scope=\"row\">Thoracic aorta<\/td>\n<td style=\"width: 86.2573%\">Portion of the descending aorta superior to the aortic hiatus<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 13.7427%\" scope=\"row\">Abdominal aorta<\/td>\n<td style=\"width: 86.2573%\">Portion of the aorta inferior to the aortic hiatus; ends at its bifurcation into the left common iliac artery and the right common iliac artery<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h5 style=\"text-align: justify\"><strong>Thoracic Aorta and Major Branches<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1030\">thoracic aorta<\/a> begins at the level of vertebra T5 and continues through to the diaphragm at the level of T12, initially traveling within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_494\">mediastinum<\/a> to the left of the vertebral column. As it passes through the thoracic region, the thoracic aorta gives rise to several branches (Figure 15).<\/p>\n<figure style=\"width: 1100px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image57.png\" alt=\"image\" width=\"1100\" height=\"1037\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 15. Arteries of the Thoracic and Abdominal Regions.<\/strong> The thoracic aorta gives rise to the arteries of the visceral and parietal branches.<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong>Abdominal Aorta and Major Branches<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0After crossing through the diaphragm, the thoracic aorta is called the abdominal aorta. This vessel remains to the left of the vertebral column and is embedded in adipose tissue behind the peritoneal cavity. It formally ends at approximately the level of vertebra L4, where it bifurcates to form the two (left and right)\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1036\">common iliac arteries<\/a>.<\/strong> Before this division, the abdominal aorta gives rise to several important branches.\u00a0 The common iliac arteries provide blood to the pelvic region and ultimately to the lower limbs.<\/p>\n<h5 style=\"text-align: justify\"><strong>Arteries Serving the Upper and Lower Limbs<\/strong><\/h5>\n<p style=\"text-align: justify\"><strong>Arteries Serving the Upper Limbs:<\/strong> As each subclavian artery exits the thorax into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1025\">axillary region<\/a>, it is renamed the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1020\">axillary artery<\/a><\/strong>. Although each axillary artery does branch and supply blood to the region near the head of the humerus (via the humeral circumflex arteries), the majority of the vessel continues into the upper arm, or brachium, and becomes the brachial artery.<\/p>\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 676px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-89 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image58-OpenStax-systemic-veins-676x1024.png\" alt=\"\" width=\"676\" height=\"1024\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image58-OpenStax-systemic-veins-676x1024.png 676w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image58-OpenStax-systemic-veins-198x300.png 198w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image58-OpenStax-systemic-veins-65x98.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image58-OpenStax-systemic-veins-225x341.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image58-OpenStax-systemic-veins-350x530.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image58-OpenStax-systemic-veins.png 686w\" sizes=\"auto, (max-width: 676px) 100vw, 676px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong>Figure 16. Major Systemic Veins of the Body.<\/strong> The major systemic veins of the body are shown here in anterior view.<\/figcaption><\/figure>\n<p style=\"text-align: justify\"><strong>Arteries Serving the Lower Limbs:<\/strong> Each external iliac artery exits the body cavity and enters the femoral region of the lower leg. As it passes through the body wall, it is renamed the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1037\">femoral artery<\/a><\/strong>. Each femoral artery gives rise to the genicular artery, which provides blood to the region of the knee. As each femoral artery passes posterior to the knee near the popliteal fossa, it is called the popliteal artery. Each popliteal artery branches into anterior and posterior tibial arteries.<\/p>\n<h5 style=\"text-align: justify\"><strong>Overview of Systemic Veins<\/strong><\/h5>\n<p style=\"text-align: justify\">\u00a0Systemic veins return blood to the right atrium. Since the blood has already passed through the systemic capillaries, it will be relatively low in oxygen concentration (Figure 16).<\/p>\n<p style=\"text-align: justify\">The right atrium receives all of the systemic venous return. Most of the blood flows into either the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_422\">superior vena cava<\/a> <\/strong>or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_423\">inferior vena cava<\/a>.<\/strong> If you draw an imaginary line at the level of the diaphragm, systemic venous circulation from above that line will generally flow into the superior vena cava; this includes blood from the head, neck, chest, shoulders, and upper limbs. The exception to this is that most venous blood flow from the coronary veins flows directly into the coronary sinus and from there directly into the right atrium. Beneath the diaphragm, systemic venous flow enters the inferior vena cava, that is, blood from the abdominal and pelvic regions and the lower limbs.<\/p>\n<h5 style=\"text-align: justify\"><strong>The Superior and Inferior Vena Cavae<\/strong><\/h5>\n<p style=\"text-align: justify\"><strong>The Superior Vena Cava:<\/strong> The <strong>superior vena cava<\/strong> drains most of the body superior to the diaphragm (Figure 17). On both the left and right sides, the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_602\">subclavian vein<\/a><\/strong> forms when the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1038\">axillary vein<\/a><\/strong> passes through the body wall from the axillary region. Each subclavian vein joins with the external and internal jugular veins from the head and neck to form the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1039\">brachiocephalic vein<\/a><\/strong>.<\/p>\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 1024px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-90 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image59-OpenStax-systemic-veins-thoracic-abdominal-region-1024x923.png\" alt=\"\" width=\"1024\" height=\"923\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image59-OpenStax-systemic-veins-thoracic-abdominal-region-1024x923.png 1024w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image59-OpenStax-systemic-veins-thoracic-abdominal-region-300x270.png 300w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image59-OpenStax-systemic-veins-thoracic-abdominal-region-768x692.png 768w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image59-OpenStax-systemic-veins-thoracic-abdominal-region-65x59.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image59-OpenStax-systemic-veins-thoracic-abdominal-region-225x203.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image59-OpenStax-systemic-veins-thoracic-abdominal-region-350x315.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image59-OpenStax-systemic-veins-thoracic-abdominal-region.png 1145w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong>Figure 17. Veins of the Thoracic and Abdominal Regions.<\/strong> Veins of the thoracic and abdominal regions drain blood from the area above the diaphragm, returning it to the right atrium via the superior vena cava.<\/figcaption><\/figure>\n<p style=\"text-align: justify\"><strong>The Inferior Vena Cava:<\/strong> Most of the blood inferior to the diaphragm drains into the <strong>inferior vena cava<\/strong> before it is returned to the heart (see Figure 17). Lying just beneath the parietal peritoneum in the abdominal cavity, the inferior vena cava parallels the abdominal aorta, where it can receive blood from abdominal veins.<\/p>\n<h5 style=\"text-align: justify\"><strong>Veins Draining the Lower Limbs<\/strong><\/h5>\n<p style=\"text-align: justify\">As each <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1040\">femoral vein<\/a><\/strong> penetrates the body wall from the femoral portion of the upper limb, it becomes the external iliac vein, a large vein that drains blood from the leg to the common iliac vein (Figure 18). The pelvic organs and integument drain into the internal iliac vein on either side of the body, which forms from several smaller veins in the region, including the umbilical veins that run on either side of the bladder. The external and internal iliac veins combine near the inferior portion of the sacroiliac joint on either side to form the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_92_1041\">common iliac vein<\/a><\/strong>. In addition to blood supply from the external and internal iliac veins, the middle sacral vein drains the sacral region into the common iliac vein. Similar to the common iliac arteries, the two common iliac veins come together at the level of L5 to form the <strong>inferior vena cava<\/strong>.<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<div class=\"unit-2:-the-cardiovascular-system-\">\n<figure id=\"attachment_91\" aria-describedby=\"caption-attachment-91\" style=\"width: 1024px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-91 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image60-OpenStax-systemic-veins-lower-limbs-1024x946.png\" alt=\"\" width=\"1024\" height=\"946\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image60-OpenStax-systemic-veins-lower-limbs-1024x946.png 1024w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image60-OpenStax-systemic-veins-lower-limbs-300x277.png 300w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image60-OpenStax-systemic-veins-lower-limbs-768x710.png 768w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image60-OpenStax-systemic-veins-lower-limbs-65x60.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image60-OpenStax-systemic-veins-lower-limbs-225x208.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image60-OpenStax-systemic-veins-lower-limbs-350x324.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image60-OpenStax-systemic-veins-lower-limbs.png 1123w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption id=\"caption-attachment-91\" class=\"wp-caption-text\"><strong>Figure 18. The Major Veins of the Lower Limbs.<\/strong><\/figcaption><\/figure>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\"><a id=\"P\"><\/a>Practice Questions<\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p><strong>Part 1:<\/strong> Structure and function of blood vessels<\/p>\n<div id=\"h5p-102\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-102\" class=\"h5p-iframe\" data-content-id=\"102\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4-1\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-101\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-101\" class=\"h5p-iframe\" data-content-id=\"101\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4-2\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 2:<\/strong> Capillary Exchange<\/p>\n<div id=\"h5p-103\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-103\" class=\"h5p-iframe\" data-content-id=\"103\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4-3\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 3:<\/strong>\u00a0Blood flow, blood pressure, and resistance<\/p>\n<div id=\"h5p-104\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-104\" class=\"h5p-iframe\" data-content-id=\"104\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4-4\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 4:<\/strong> Hemostatic Regulation of the Vascular System<\/p>\n<div id=\"h5p-106\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-106\" class=\"h5p-iframe\" data-content-id=\"106\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4-5\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 5:<\/strong> Circulatory Pathways<\/p>\n<div id=\"h5p-107\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-107\" class=\"h5p-iframe\" data-content-id=\"107\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4-6\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-105\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-105\" class=\"h5p-iframe\" data-content-id=\"105\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4-7\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-108\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-108\" class=\"h5p-iframe\" data-content-id=\"108\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"4-8\"><\/iframe><\/div>\n<\/div>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_92_976\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_976\"><div tabindex=\"-1\"><p>Blood vessel that conducts blood away from the heart; may be a conducting or distributing vessel.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_598\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_598\"><div tabindex=\"-1\"><p>Very small artery that leads to a capillary.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_977\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_977\"><div tabindex=\"-1\"><p>Smallest of the blood vessels where physical exchange occurs between the blood and tissue cells surrounded by interstitial fluid.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_599\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_599\"><div tabindex=\"-1\"><p>Small vessel leading from the capillaries to veins.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_978\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_978\"><div tabindex=\"-1\"><p>Blood vessel that conducts blood toward the heart.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_421\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_421\"><div tabindex=\"-1\"><p>Parts of the circulatory system involving blood flow to and from almost all the tissues in the body (other than the pulmonary circuit)<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_420\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_420\"><div tabindex=\"-1\"><p>Parts of the circulatory system involving blood flow to and from the lungs.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_777\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_777\"><div tabindex=\"-1\"><p>Space inside of a tube, hollow organ or blood vessel.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_982\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_982\"><div tabindex=\"-1\"><p>(Also, tunica interna) innermost lining or tunic of a vessel.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_983\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_983\"><div tabindex=\"-1\"><p>Middle layer or tunic of a vessel (except capillaries).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_984\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_984\"><div tabindex=\"-1\"><p>(Also, tunica adventitia) outermost layer or tunic of a vessel (except capillaries).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_783\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_783\"><div tabindex=\"-1\"><p>The most abundant of three protein fibres found in the extracellular matrix of connective tissues.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_580\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_580\"><div tabindex=\"-1\"><p>Hormones that cause vasoconstriction or release of NO.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_985\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_985\"><div tabindex=\"-1\"><p>A type of loose connective tissue proper that shows little specialization with cells dispersed in the matrix.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_986\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_986\"><div tabindex=\"-1\"><p>Constriction of blood vessels<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_755\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_755\"><div tabindex=\"-1\"><p>Dilation (increased internal diameter) of blood vessels.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_987\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_987\"><div tabindex=\"-1\"><p>Delivery of blood through a capillary bed.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_595\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_595\"><div tabindex=\"-1\"><p>Extracellular fluid in the small spaces between cells not contained within blood vessels.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_543\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_543\"><div tabindex=\"-1\"><p>(Also, red blood cell) mature myeloid blood cell that is composed mostly of hemoglobin and functions primarily in the transportation of oxygen and carbon dioxide.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_567\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_567\"><div tabindex=\"-1\"><p>(Also, emigration) process by which leukocytes squeeze through adjacent cells in a blood vessel wall to enter tissues.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_544\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_544\"><div tabindex=\"-1\"><p>(also, white blood cell) colorless, nucleated blood cell, the chief function of which is to protect the body from disease.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_988\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_988\"><div tabindex=\"-1\"><p>Passive diffusion of a substance with the aid of a membrane protein.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_990\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_990\"><div tabindex=\"-1\"><p>Type of capillary with pores or fenestrations in the endothelium that allow for rapid passage of certain small materials.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_989\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_989\"><div tabindex=\"-1\"><p>Rarest type of capillary, which has extremely large intercellular gaps in the basement membrane in addition to clefts and fenestrations; found in areas such as the bone marrow and liver where passage of large molecules occurs.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_991\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_991\"><div tabindex=\"-1\"><p>Import of material into the cell by formation of a membrane-bound vesicle.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_992\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_992\"><div tabindex=\"-1\"><p>Export of a substance out of a cell by formation of a membrane-bound vesicle.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_903\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_903\"><div tabindex=\"-1\"><p>Diffusion of water molecules down their concentration gradient across a selectively permeable membrane.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_994\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_994\"><div tabindex=\"-1\"><p>Larger number recorded when measuring arterial blood pressure; represents the maximum value following ventricular contraction.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_993\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_993\"><div tabindex=\"-1\"><p>Lower number recorded when measuring arterial blood pressure; represents the minimal value corresponding to the pressure that remains during ventricular relaxation.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_995\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_995\"><div tabindex=\"-1\"><p>Average driving force of blood to the tissues; approximated by taking diastolic pressure and adding 1\/3 of pulse pressure.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_996\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_996\"><div tabindex=\"-1\"><p>Insufficient blood flow to the tissues.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_997\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_997\"><div tabindex=\"-1\"><p>Lack of oxygen supply to the tissues.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_998\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_998\"><div tabindex=\"-1\"><p>Noises created by turbulent blood flow through the vessels.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_999\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_999\"><div tabindex=\"-1\"><p>Right common carotid artery arises from the brachiocephalic artery, and the left common carotid arises from the aortic arch; gives rise to the external and internal carotid arteries; supplies the respective sides of the head and neck.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1000\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1000\"><div tabindex=\"-1\"><p>continuation of the axillary artery in the brachium; supplies blood to much of the brachial region; gives off several smaller branches that provide blood to the posterior surface of the arm in the region of the elbow; bifurcates into the radial and ulnar arteries at the coronoid fossa<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1001\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1001\"><div tabindex=\"-1\"><p>Continuation of the external iliac artery after it passes through the body cavity; divides into several smaller branches, the lateral deep femoral artery, and the genicular artery; becomes the popliteal artery as it passes posterior to the knee.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1002\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1002\"><div tabindex=\"-1\"><p>Continuation of the femoral artery posterior to the knee; branches into the anterior and posterior tibial arteries.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1003\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1003\"><div tabindex=\"-1\"><p>Branch from the popliteal artery that gives rise to the fibular or peroneal artery; supplies blood to the posterior tibial region.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1004\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1004\"><div tabindex=\"-1\"><p>One of the seven bones that make up the posterior foot; includes the calcaneus, talus, navicular, cuboid, medial cuneiform, intermediate cuneiform, and lateral cuneiform bones.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1005\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1005\"><div tabindex=\"-1\"><p>Forms from the anterior tibial artery; branches repeatedly to supply blood to the tarsal and dorsal regions of the foot.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_449\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_449\"><div tabindex=\"-1\"><p>Primary and most potent catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called adrenaline.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_454\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_454\"><div tabindex=\"-1\"><p>Secondary catecholamine hormone secreted by the adrenal medulla in response to short-term stress; also called noradrenaline.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1006\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1006\"><div tabindex=\"-1\"><p>Collection of hormones secreted by the thyroid gland with wide-ranging metabolic affects.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_536\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_536\"><div tabindex=\"-1\"><p>Branch of the autonomic nervous system associated with resting systems (\"rest and digest\").<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_546\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_546\"><div tabindex=\"-1\"><p>In blood, the liquid extracellular matrix composed mostly of water that circulates the formed elements and dissolved materials throughout the cardiovascular system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1007\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1007\"><div tabindex=\"-1\"><p>Process of producing red blood cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_565\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_565\"><div tabindex=\"-1\"><p>Elevated level of hemoglobin, whether adaptive or pathological.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_564\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_564\"><div tabindex=\"-1\"><p>Deficiency of red blood cells or hemoglobin, often linked to iron deficiency.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_545\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_545\"><div tabindex=\"-1\"><p>(Also, thrombocytes) one of the formed elements of blood that consists of cell fragments broken off from megakaryocytes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_529\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_529\"><div tabindex=\"-1\"><p>Period of time when the heart muscle is relaxed and the chambers fill with blood.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_725\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_725\"><div tabindex=\"-1\"><p>Lowest (most inferior) part of the brain, controlling many autonomic functions including heart rate, breathing, and digestion.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_538\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_538\"><div tabindex=\"-1\"><p>Tenth cranial nerve; responsible for the autonomic control of organs in the thoracic and upper abdominal cavities.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1016\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1016\"><div tabindex=\"-1\"><p>Gas produced by the blood vessel endothelium that acts as a powerful vasodilator, active over short distances (between nearby cells) for very short times (seconds).Not to be confused with the anesthetic nitrous oxide (N2O).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1009\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1009\"><div tabindex=\"-1\"><p>Small pocket in the ascending aorta near the aortic valve that are the locations of the baroreceptors (stretch receptors) and chemoreceptors that trigger a reflex that aids in the regulation of vascular homeostasis.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1010\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1010\"><div tabindex=\"-1\"><p>Small pocket near the base of the internal carotid arteries that are the locations of the baroreceptors and chemoreceptors that trigger a reflex that aids in the regulation of vascular homeostasis.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1011\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1011\"><div tabindex=\"-1\"><p>Arises from the common carotid artery and begins with the carotid sinus; goes through the carotid canal of the temporal bone to the base of the brain; combines with branches of the vertebral artery forming the arterial circle; supplies blood to the brain.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1012\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1012\"><div tabindex=\"-1\"><p>Change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibres.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_535\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_535\"><div tabindex=\"-1\"><p>Branch of the autonomic nervous system associated with emergency systems (\"fight of flight\").<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1013\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1013\"><div tabindex=\"-1\"><p>Sensory receptor (stretch receptor) sensitive to changes in pressure.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_734\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_734\"><div tabindex=\"-1\"><p>Sensory receptor that senses chemical concentrations.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_473\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_473\"><div tabindex=\"-1\"><p>(Also, vasopressin) hypothalamic hormone that is stored by the posterior pituitary and that signals the kidneys to reabsorb water.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_464\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_464\"><div tabindex=\"-1\"><p>Molecule (usually a protein) that catalyzes chemical reactions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1014\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1014\"><div tabindex=\"-1\"><p>Located at the juncture of the distal convoluted tubule and the afferent and efferent arterioles of the glomerulus; plays a role in the regulation of renal blood flow and glomerular filtration rate.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1015\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1015\"><div tabindex=\"-1\"><p>Hormone produced and secreted by the adrenal cortex that stimulates sodium and fluid retention and increases blood volume and blood pressure.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_392\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_392\"><div tabindex=\"-1\"><p>Region of the diencephalon inferior to the thalamus that functions in neural and endocrine signaling.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_562\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_562\"><div tabindex=\"-1\"><p>Glycoprotein that triggers the bone marrow to produce RBCs; secreted by the kidney in response to low oxygen levels.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_866\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_866\"><div tabindex=\"-1\"><p>A ring-shaped smooth muscle that can open or close a passage in the body.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1017\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1017\"><div tabindex=\"-1\"><p>Similar to hormones, lipids produced by various cells (not glands), usually at the site of an injury or other issue, that are active over a short distance (targeting other cells in the same tissue).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_633\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_633\"><div tabindex=\"-1\"><p>(Also, immunoglobulin) antigen-specific protein secreted by plasma cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1018\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1018\"><div tabindex=\"-1\"><p>Physiological process by which bleeding ceases.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1019\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1019\"><div tabindex=\"-1\"><p>Right subclavian arises from the brachiocephalic artery, whereas the left subclavian artery arises from the aortic arch; gives rise to the internal thoracic, vertebral, and thyrocervical arteries; supplies blood to the arms, chest, shoulders, back, and central nervous system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1020\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1020\"><div tabindex=\"-1\"><p>Continuation of the subclavian artery as it penetrates the body wall and enters the axillary region; supplies blood to the region near the head of the humerus (humeral circumflex arteries); the majority of the vessel continues into the brachium and becomes the brachial artery.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1025\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1025\"><div tabindex=\"-1\"><p>Area inferior to the shoulder joint (armpit, or underarm).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_422\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_422\"><div tabindex=\"-1\"><p>Large systemic vein that returns blood to the heart from the superior portion of the body.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_423\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_423\"><div tabindex=\"-1\"><p>Large systemic vein that returns blood to the heart from the inferior portion of the body.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_504\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_504\"><div tabindex=\"-1\"><p>Large, thin-walled vein on the posterior surface of the heart that lies within the atrioventricular sulcus and drains the heart myocardium directly into the right atrium.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_497\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_497\"><div tabindex=\"-1\"><p>The single large vessel exiting the right ventricle that divides to form the right and left pulmonary arteries.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_508\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_508\"><div tabindex=\"-1\"><p>Valves located at the base of the pulmonary trunk and at the base of the aorta.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1026\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1026\"><div tabindex=\"-1\"><p>Single large vessel exiting the right ventricle that divides to form the right and left pulmonary arteries.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_663\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_663\"><div tabindex=\"-1\"><p>Small, grape-like sac that performs gas exchange in the lungs (pl.= alveoli).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1027\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1027\"><div tabindex=\"-1\"><p>Two sets of paired vessels, one pair on each side, that are formed from the small venules leading away from the pulmonary capillaries that flow into the left atrium.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1028\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1028\"><div tabindex=\"-1\"><p>Largest artery in the body, originating from the left ventricle and descending to the abdominal region where it bifurcates into the common iliac arteries at the level of the fourth lumbar vertebra; arteries originating from the aorta distribute blood to virtually all tissues of the body.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1032\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1032\"><div tabindex=\"-1\"><p>Portion of the aorta that continues downward past the end of the aortic arch; subdivided into the thoracic aorta and the abdominal aorta.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1033\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1033\"><div tabindex=\"-1\"><p>The direction towards the head.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_731\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_731\"><div tabindex=\"-1\"><p>Arc that connects the ascending aorta to the descending aorta; ends at the intervertebral disk between the fourth and fifth thoracic vertebrae.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1031\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1031\"><div tabindex=\"-1\"><p>Portion of the aorta that continues downward past the end of the aortic arch; subdivided into the thoracic aorta and the abdominal aorta.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1030\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1030\"><div tabindex=\"-1\"><p>Portion of the descending aorta superior to the aortic hiatus.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1029\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1029\"><div tabindex=\"-1\"><p>Portion of the aorta inferior to the aortic hiatus and superior to the common iliac arteries.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_513\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_513\"><div tabindex=\"-1\"><p>Branches of the ascending aorta that supply blood to the heart; the left coronary artery feeds the left side of the heart, the left atrium and ventricle, and the interventricular septum; the right coronary artery feeds the right atrium, portions of both ventricles, and the heart conduction system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1034\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1034\"><div tabindex=\"-1\"><p>Single vessel located on the right side of the body; the first vessel branching from the aortic arch; gives rise to the right subclavian artery and the right common carotid artery; supplies blood to the head, neck, upper limb, and wall of the thoracic region.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_494\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_494\"><div tabindex=\"-1\"><p>A central compartment in the thoracic cavity located intermediate to the left and right pleural cavities.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1036\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1036\"><div tabindex=\"-1\"><p>Branch of the aorta that leads to the internal and external iliac arteries.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1037\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1037\"><div tabindex=\"-1\"><p>Continuation of the external iliac artery after it passes through the body cavity; divides into several smaller branches, the lateral deep femoral artery, and the genicular artery; becomes the popliteal artery as it passes posterior to the knee.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_602\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_602\"><div tabindex=\"-1\"><p>(Left and right) located deep in the thoracic cavity; becomes the axillary vein as it enters the axillary region; drains the axillary and smaller local veins near the scapular region; leads to the brachiocephalic vein.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1038\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1038\"><div tabindex=\"-1\"><p>Major vein in the axillary region; drains the upper limb and becomes the subclavian vein.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1039\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1039\"><div tabindex=\"-1\"><p>One of a pair of veins that form from a fusion of the external and internal jugular veins and the subclavian vein; subclavian, external and internal jugulars, vertebral, and internal thoracic veins lead to it; drains the upper thoracic region and flows into the superior vena cava.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1040\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1040\"><div tabindex=\"-1\"><p>Drains the upper leg; receives blood from the great saphenous vein, the deep femoral vein, and the femoral circumflex vein; becomes the external iliac vein when it crosses the body wall.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_92_1041\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_92_1041\"><div tabindex=\"-1\"><p>One of a pair of veins that flows into the inferior vena cava at the level of L5; the left common iliac vein drains the sacral region; divides into external and internal iliac veins near the inferior portion of the sacroiliac joint.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><\/div>","protected":false},"author":1440,"menu_order":3,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-92","chapter","type-chapter","status-publish","hentry"],"part":41,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters\/92","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/users\/1440"}],"version-history":[{"count":18,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters\/92\/revisions"}],"predecessor-version":[{"id":1617,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters\/92\/revisions\/1617"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/parts\/41"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters\/92\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/media?parent=92"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapter-type?post=92"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/contributor?post=92"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/license?post=92"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}