Atherosclerosis and Angina

Structure and Function of Blood Vessels

J. Gordon Betts; Kelly A. Young; James A. Wise; Eddie Johnson; Brandon Poe; Dean H. Kruse; Oksana Korol; Jody E. Johnson; Mark Womble; and Peter DeSaix

Learning Objectives

By the end of this section, you will be able to:

  • Compare and contrast the three tunics that make up the walls of most blood vessels
  • Distinguish between elastic arteries, muscular arteries, and arterioles on the basis of structure, location, and function
  • Explain the structure and function of arteries vs veins

Blood is carried through the body via blood vessels. An is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels. Eventually, the smallest arteries, vessels called , further branch into tiny capillaries, where nutrients and wastes are exchanged, and then combine with other vessels that exit capillaries to form , small blood vessels that carry blood to a , a larger blood vessel that returns blood to the heart.

Arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit (Figure 8.2). Systemic arteries provide blood rich in oxygen to the body’s 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.

This diagram shows how oxygenated and deoxygenated blood flow through the major organs in the body. On the right of the image is the arterial flow, represented as leaving the heart to the major organs capillary beds (lungs, liver, GI, kidneys, lower body).  The venous flow is on the left of the image, represented by blue.  All venous flow return the right heart which then leaves for the lungs, returning to heart via pulmonary vein (on the right of image since it's oxygenated blood)
Figure 8.2. Cardiovascular Circulation.  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 colors show the relative levels of oxygen concentration.

Shared Structures

Different types of blood vessels vary slightly in their structures, but they share the same general features. and have thicker walls than and because they are closer to the heart and receive blood that is surging at a far greater pressure (Figure 8.3). Each type of vessel has a —a 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.

The top left panel of this figure shows the ultrastructure of an artery, and the top right panel shows the ultrastructure of a vein. The bottom panel shows a micrograph with the cross sections of an artery and a vein.
Figure 8.3. Structure of Blood Vessels (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 × 160. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)

In comparison to arteries, veins and venules withstand a much lower blood pressure, which is morphologically reflected in thinner walls and larger lumens. These adaptations allow for large volumes of blood to flow with less vessel resistance. Additionally, some veins in the limbs contain valves that assist the unidirectional flow, which is challenging in the extremities due to lower pressure and gravity.

Larger arteries and veins contain small blood vessels within their walls known as the —literally “vessels of the vessel”. Vasa vasorum are located in the outer layers of the vessel (Figure 2) and provide nutrients and allow for waste exchange for the cells and tissues composing the blood vessel wall. There are also minute nerves (known as within the walls of both types of vessels that control the contraction and dilation of smooth muscle. 

Both arteries and veins have the same three distinct tissue layers, called tunics. From the most interior layer to the outer, these tunics are the , the , and the ( Figure 8.3).


Arteries Veins
General appearance Thick walls with small lumens, appear rounded Thin walls with large lumens, appear flattened
Tunica intima Endothelium appears wavy due to constriction of smooth muscle
Internal elastic membrane present in larger vessels
Endothelium appears smooth, internal elastic membrane absent
Tunica media Normally the thickest layer in arteries
Smooth muscle cells and elastic fibers predominate (the proportions of these vary with distance from the heart)
External elastic membrane present in larger vessels
Normally thinner than the tunica externa
Smooth muscle cells and collagenous fibers predominate
Nervi vasorum and vasa vasorum present
External elastic membrane absent
Tunica externa Normally thinner than the tunica media in all but the largest arteries
Collagenous and elastic fibers
Nervi vasorum and vasa vasorum present
Normally the thickest layer in veins
Collagenous and smooth fibers predominate
Some smooth muscle fibers
Nervi vasorum and vasa vasorum present

Figure 8.4 Comparison of tunics within arteries and veins

Tunica Intima

The (also called the tunica interna) is composed of epithelial () and connective tissue layers.

  • is the specialized , makes up the lining of the blood vessel lumen and is continuous throughout the vascular system, including the heart. The permeable basement membrane is located underneath endothelium and provides strength while maintaining flexibility.
  • thin layer of connective tissue that contains elastic and collagen fibers.

In larger arteries, there is also a thick, distinct layer of elastic fibers known as the (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.

Under the microscope, the lumen and the entire tunica intima of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media, the next layer of blood vessel walls.

Tunica Media

The tunica media is the thickest layer in arteries and consists of smooth muscle layers and connective tissue that is made up of elastic fibers, which appear wavy in histological slides.

Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively. Specifically in arteries, decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop. Both vasoconstriction and vasodilation are regulated in part by  .

Separating the tunica media from the outer tunica externa in larger arteries is the (also called the external elastic lamina). This structure is not usually seen in smaller arteries, nor is it seen in veins.

Tunica Externa

The outer tunic, the tunica externa (also called the tunica adventitia), is a substantial sheath of connective tissue composed primarily of collagenous fibers and some elastic fibers. The tunica externa in veins also contains groups of smooth muscle fibers. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries. The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in relative position. If you are able to palpate some of the superficial veins on your upper limbs and try to move them, you will find that the tunica externa prevents this. If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow.


An artery 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. However, those close to the heart have the thickest walls, containing a high percentage of elastic fibers in all three of their tunics. This type of artery is known as an (Figure 8.5). Vessels larger than 10 mm in diameter are typically elastic. Their abundant elastic fibers allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed.  The elastic recoil of the vascular wall helps to maintain the pressure gradient that drives the blood through the arterial system. An elastic artery is also known as a conducting artery, because the large diameter of the lumen enables it to accept a large volume of blood from the heart and conduct it to smaller branches.

The left panel shows the cross-section of an elastic artery, the middle panel shows the cross section of a muscular artery, and the right panel shows the cross-section of an arteriole.
Figure 8.5. Types of Arteries and Arterioles Comparison of the walls of an elastic artery, a muscular artery, and an arteriole is shown. In terms of scale, the diameter of an arteriole is measured in micrometers compared to millimeters for elastic and muscular arteries.

Farther from the heart, where the surge of blood has dampened, the percentage of elastic fibers in an artery’s tunica intima decreases and the amount of smooth muscle in its tunica media increases. The artery at this point is described as a . The diameter of muscular arteries typically ranges from 0.1 mm to 10 mm. Their thick tunica media allows muscular arteries to play a leading role in . In contrast, their decreased quantity of elastic fibers limits their ability to expand. Fortunately, because the blood pressure has eased by the time it reaches these more distant vessels, elasticity has become less important.

Notice that although the distinctions between elastic and muscular arteries are important, there is no “line of demarcation” where an elastic artery suddenly becomes muscular. Rather, there is a gradual transition as the vascular tree repeatedly branches. In turn, muscular arteries branch to distribute blood to the vast network of arterioles. For this reason, a muscular artery is also known as a distributing artery.


An arteriole 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 8.5).

With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and, thus, causing a substantial drop in blood pressure. Because of this, you may see them referred to as resistance vessels.


A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called . Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid (interstitial fluid). The diameter of a capillary lumen ranges from 5–10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through. Flow through capillaries is often described as .

The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers. 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.


A venule is an extremely small vein, generally 8–100 micrometers in diameter.  Multiple venules join to form veins. The walls of venules consist of endothelium, a thin middle layer with a few muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a very thin tunica externa (Figure 8.6). Venules as well as capillaries are the primary sites of emigration or diapedesis, in which the white blood cells adhere to the endothelial lining of the vessels and then squeeze through adjacent cells to enter the tissue fluid.


A vein is a blood vessel that conducts blood toward the heart. Compared to arteries, veins are thin-walled vessels with large and irregular lumens (see (Figure 8.6). 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. (Figure 8.3) compares the features of arteries and veins.

The top panel shows the cross-section of a large vein, the middle panel shows the cross-section of a medium sized vein, and the bottom panel shows the cross-section of a venule.
Figure 8.6. Comparison of Veins and Venules Many veins have valves to prevent back flow of blood, whereas venules do not. In terms of scale, the diameter of a venule is measured in micrometers compared to millimeters for veins.


Arteries Veins
Direction of blood flow Conducts blood away from the heart Conducts blood toward the heart
General appearance Rounded Irregular, often collapsed
Pressure High Low
Wall thickness Thick Thin
Relative oxygen concentration Higher in systemic arteries
Lower in pulmonary arteries
Lower in systemic veins
Higher in pulmonary veins
Valves Not present Present most commonly in limbs and in veins inferior to the heart

Figure 8.7  Comparison of Arteries and veins

Career Connection

Vascular Surgeons and Technicians Vascular surgery is a specialty in which the physician deals primarily with diseases of the vascular portion of the cardiovascular system. This includes repair and replacement of diseased or damaged vessels, removal of plaque from vessels, minimally invasive procedures including the insertion of venous catheters, and traditional surgery. Following completion of medical school, the physician generally completes a 5-year surgical residency followed by an additional 1 to 2 years of vascular specialty training. In the United States, most vascular surgeons are members of the Society of Vascular Surgery.

Vascular technicians are specialists in imaging technologies that provide information on the health of the vascular system. They may also assist physicians in treating disorders involving the arteries and veins. This profession often overlaps with cardiovascular technology, which would also include treatments involving the heart. Although recognized by the American Medical Association, there are currently no licensing requirements for vascular technicians, and licensing is voluntary. Vascular technicians typically have an Associate’s degree or certificate, involving 18 months to 2 years of training. The United States Bureau of Labor projects this profession to grow by 29 percent from 2010 to 2020.

Visit this site to learn more about vascular surgery.

Visit this site to learn more about vascular technicians.

Section Review

Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart. Arteries transport blood away from the heart and branch into smaller vessels, forming arterioles. Arterioles distribute blood to capillary beds, the sites of exchange with the body tissues. Capillaries lead back to small vessels known as venules that flow into the larger veins and eventually back to the heart.

The arterial system is a relatively high-pressure system, so arteries have thick walls that appear round in cross-section. The venous system is a lower-pressure system, containing veins that have larger lumens and thinner walls. They often appear flattened. Arteries, arterioles, venules, and veins are composed of three tunics known as the , , and . Capillaries have only a tunica intima layer. The tunica intima is a thin layer composed of a simple squamous epithelium known as and a small amount of connective tissue. The tunica media is a thicker area composed of variable amounts of smooth muscle and connective tissue. It is the thickest layer in all but the largest arteries. The tunica externa is primarily a layer of connective tissue, although in veins, it also contains some smooth muscle. Blood flow through vessels can be dramatically influenced by and in their walls.

Review Questions


This chapter is adapted from the following text:

Structure and function of blood vessels in Anatomy and Physiology by OSCRiceUniversity is licensed under a Creative Commons Attribution 4.0 International License


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Structure and Function of Blood Vessels by J. Gordon Betts; Kelly A. Young; James A. Wise; Eddie Johnson; Brandon Poe; Dean H. Kruse; Oksana Korol; Jody E. Johnson; Mark Womble; and Peter DeSaix is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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