Homeostasis Module

Water Balance and Electrolytes

Learning Objectives

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

  • Explain the importance of water in the body
  • Contrast the composition of the intracellular fluid with that of the extracellular fluid
  • Correctly use medical terminology to describe abnormal electrolyte concentrations
  • Explain how water balance is maintained, focusing on the role of the hypothalamus, the kidneys, and anti-diuretic hormone
  • Describe the causes and consequences of water imbalance

The chemical reactions of life take place in aqueous solutions. The dissolved substances in a solution are called solutes. In the human body, solutes vary in different parts of the body, but may include proteins—including those that transport lipids, carbohydrates, and, very importantly, electrolytes. Often in medicine, a mineral dissociated from a salt that carries an electrical charge (an ion) is called and electrolyte. For instance, sodium ions (Na+) and chloride ions (Cl–) are often referred to as electrolytes.

In the body, water moves through semi-permeable membranes of cells and from one compartment of the body to another by a process called osmosis. Osmosis is basically the diffusion of water from regions of higher concentration to regions of lower concentration, along an osmotic gradient across a semi-permeable membrane. As a result, water will move into and out of cells and tissues, depending on the relative concentrations of the water and solutes found there. An appropriate balance of solutes inside and outside of cells must be maintained to ensure normal function.

Body Water Content

Human beings are mostly water, ranging from about 75 percent of body mass in infants to about 50–60 percent in adult men and women, to as low as 45 percent in old age. The percent of body water changes with development, because the proportions of the body given over to each organ and to muscles, fat, bone, and other tissues change from infancy to adulthood (Figure 1). Your brain and kidneys have the highest proportions of water, which composes 80–85 percent of their masses. In contrast, teeth have the lowest proportion of water, at 8–10 percent.

This illustration shows a silhouette of a human body with various organs highlighted. The percent of water contained in each organ is given. The brain typically contains 80% to 85% water, teeth contain 8% to 10% water, a single lung contains 75% to 80% water, the heart contains 75% to 80% water, the bones contain 20% to 25% water, the liver contains 70% to 75% water, the kidneys contain 80% to 85% water, the skin contains 70% to 75% water and the muscles also contain 70% to 75% water.
Figure 1. Water Content of the Body’s Organs and Tissues. Water content varies in different body organs and tissues, from as little as 8 percent in the teeth to as much as 85 percent in the brain.

Fluid Compartments

Body fluids can be discussed in terms of their specific fluid compartment, a location that is largely separate from another compartment by some form of a physical barrier. The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes. Extracellular fluid (ECF) surrounds all cells in the body. Extracellular fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not in the blood (Figure 2).

This diagram shows a small blood vessel surrounded by several body cells. The fluid between the body cells is the interstitial fluid (IF), which is a type of extracellular fluid (ECF). The fluid in the blood vessel is also an example of extracellular fluid. The fluid in the cytoplasm of each body cell is intracellular fluid, or ICF.
Figure 2. Fluid Compartments in the Human Body. The intracellular fluid (ICF) is the fluid within cells. The interstitial fluid (IF) is part of the extracellular fluid (ECF) between the cells. Blood plasma is the second part of the ECF. Materials travel between cells and the plasma in capillaries through the IF.

Intracellular Fluid (ICF)

The ICF lies within cells and is the principal component of the cytosol/cytoplasm. The ICF makes up about 60 percent of the total water in the human body (Figure 3). This fluid volume tends to be very stable, because the amount of water in living cells is closely regulated.

This pie chart shows that about 55% of water in the human body is intracellular fluid. About 30% of the water in the human body is interstitial fluid. Most of the remaining 15% of water is plasma, along with a small percentage labeled “other fluid”.
Figure 3. A Pie Graph Showing the Proportion of Total Body Fluid in Each of the Body’s Fluid Compartments. Most of the water in the body is intracellular fluid. The second largest volume is the interstitial fluid, which surrounds cells that are not blood cells.

Extracellular Fluid (ECF)

The ECF accounts for the other one-third of the body’s water content. Approximately 20 percent of the ECF is found in plasma. Plasma travels through the body in blood vessels and transports a range of materials, including blood cells, proteins (including clotting factors and antibodies), electrolytes, nutrients, gases, and wastes. Gases, nutrients, and waste materials travel between capillaries and cells through the interstitial fluid (IF). Cells are separated from the IF by a selectively permeable cell membrane that helps regulate the passage of materials between the IF and the interior of the cell.

Composition of Body Fluids

The compositions of the two components of the ECF—plasma and IF—are more similar to each other than either is to the ICF (Figure 4). Blood plasma has high concentrations of sodium, chloride, bicarbonate, and protein. The IF has high concentrations of sodium, chloride, and bicarbonate, but a relatively lower concentration of protein. In contrast, the ICF has elevated amounts of potassium, phosphate, magnesium, and protein. Overall, the ICF contains high concentrations of potassium and phosphate (HPO42−HPO42−), whereas both plasma and the ECF contain high concentrations of sodium and chloride.

This bar graph shows the concentration of several ions and proteins in intracellular fluid, interstitial fluid and plasma. The ions and proteins are categories on the X axis . The Y axis shows concentration, in milliequivalents per liter, ranging from zero to 160. Three different colored bars are shown above each compound on the X axis. One bar represents intracellular fluid (ICF), a second bar represents interstitial fluid (IF, which is part of ECF) and the third bar represents plasma (ECF). Intracellular fluid contains high concentrations of K plus and HPO four two minus. It has lower concentrations of MG two plus and protein, and negligible amounts of the other compounds. Interstitial fluid contains high concentrations of NA plus and CL minus, along with a smaller amount of HCO 3 minus, and negligible amounts of the other compounds. Plasma contains large concentrations of NA plus and CL minus, with smaller concentrations of HCO 3 minus and protein, and negligible amounts of the other compounds.
Figure 4. The Concentrations of Different Elements in Key Bodily Fluids. The graph shows the composition of the ICF, IF, and plasma. The compositions of plasma and IF are similar to one another but are quite different from the composition of the ICF.

The ions in plasma also contribute to the osmotic balance that controls the movement of water between cells and their environment. Imbalances of these ions can result in various problems in the body. Adjustments in respiratory and renal functions allow the body to regulate the levels of these ions in the ECF.

Water Balance

On a typical day, the average adult will take in about 2500 mL of aqueous fluids. Although most of the intake comes through the digestive tract, about 300 mL per day is generated metabolically, in the last steps of aerobic respiration. Additionally, each day about the same volume (2500 mL) of water leaves the body by different routes; most of this lost water is removed as urine. The kidneys also can adjust blood volume though mechanisms that draw water out of the filtrate and urine. Some water is lost through sweat and stool. Water is also lost through the skin through evaporation from the skin surface without overt sweating and from air expelled from the lungs. This type of water loss is called insensible water loss because a person is usually unaware of it.

Regulation of Water Content

Osmolality is the ratio of solutes in a solution to a volume of solvent in a solution. Plasma osmolality is thus the ratio of solutes to water in blood plasma. A person’s plasma osmolality value reflects his or her state of hydration. A healthy body maintains plasma osmolality within a narrow range, by employing several mechanisms that regulate both water intake and output.

Drinking water is considered voluntary. So how is water intake regulated by the body? Consider someone who is experiencing dehydration, a net loss of water that results in insufficient water in blood and other tissues. The water that leaves the body, as exhaled air, sweat, or urine, is ultimately extracted from blood plasma. As the blood becomes more concentrated, the thirst response—a sequence of physiological processes—is triggered (Figure 5).

This figure is a top-to bottom flowchart describing the thirst response. The topmost box of the chart states that there is insufficient water in the body, which has two effects. The left branch of the chart leads to decreased blood volume, which leads to decreased blood pressure. This triggers an increase in angiotensin two. Angiotensin two stimulates the thirst center in the hypothalamus. On the right branch, insufficient water in the body leads to increased blood osmolality, which causes dry mouth. Increased blood osmolality and dry mouth is sensed by osmoreceptors in the hypothalamus. This stimulates the thirst center in the hypothalamus to increase thirst, giving a person the urge to drink. Drinking decreases blood osmolality back to homeostatic levels.
Figure 5. A Flowchart Showing the Thirst Response. The thirst response begins when osmoreceptors detect a decrease in water levels in the blood.

Osmoreceptors are sensory receptors in the thirst center in the hypothalamus that monitor the concentration of solutes (osmolality) of the blood. If blood osmolality increases above its ideal value, the hypothalamus transmits signals that result in a conscious awareness of thirst. To conserve water, the hypothalamus of a dehydrated person also sends signals via the sympathetic nervous system to the salivary glands in the mouth. The signals result in a decrease in watery, serous output (and an increase in stickier, thicker mucus output). These changes in secretions result in a “dry mouth” and the sensation of thirst.

The hypothalamus of a dehydrated person also releases antidiuretic hormone (ADH) through the posterior pituitary gland (Figure 7).

This set of diagrams shows the effects of ADH on various structures within the body. In the brain, ADH affects the cerebrum by influencing social behavior in some mammals. ADH is also produced in the brain by the hypothalamus and released in the posterior pituitary. ADH also constricts arterioles in the body, which are the small arteries that enter into capillary beds. Finally, a kidney is shown because ADH increases the reabsorption of water in the kidneys.
Figure 7. Antidiuretic Hormone (ADH). ADH is produced in the hypothalamus and released by the posterior pituitary gland. It causes the kidneys to retain water, constricts arterioles in the peripheral circulation, and affects some social behaviors in mammals.

ADH constricts the arterioles in the peripheral circulation, which reduces the flow of blood to the extremities and thereby increases the blood supply to the core of the body. ADH also increases the water permeability of the renal collecting tubules to move water from the urine through the walls of the collecting tubules, leading to more reabsorption of water into the bloodstream.

Decreased blood volume resulting from water loss also impacts the kidney’s renin-angiotensin hormonal system (Figure 6). It increases the production of the active form of the hormone angiotensin II, which helps stimulate thirst, but also stimulates the release of the hormone aldosterone from the adrenal glands. Aldosterone increases the reabsorption of sodium and excetion of potassium in the distal tubules of the nephrons in the kidneys, and water follows this reabsorbed sodium back into the blood.

This figure shows the hormone cascade that that increases kidney reabsorption of NA plus and water. In the first step, the kidneys release renin into the blood stream. The blood stream is depicted with a red arrow pointing from left to right. At the same time, the liver releases angiotensinogen into the blood, which combines with the renin, yielding angiotensin one. The blood flow then leads to the lungs. Within the pulmonary blood, angiotensin-converting enzyme (ACE) converts angiotensin one to angiotensin two. The blood then flows to the adrenal cortex, where angiotensin two stimulates the adrenal cortex to secrete aldosterone. Aldosterone causes the kidney tubules to increase reabsorption of NA plus and water into the blood.
Figure 6. The Renin-Angiotensin System. Angiotensin II stimulates the release of aldosterone from the adrenal cortex.

Water Imbalance

If adequate fluids are not consumed, dehydration results and a person’s body contains too little water to function correctly. A person who repeatedly vomits,  who has diarrhea may become dehydrated, or who with burn wound exudates, can become dangerously dehydrated very quickly. Endurance athletes such as distance runners often become dehydrated during long races with increased sweating. Dehydration can be a medical emergency, and a dehydrated person may lose consciousness, become comatose, or die, if his or her body is not rehydrated quickly.

If a person’s body contains too much water, through either excess intake or inadequate output, water intoxication can occur. Excess body water content may be observed in a person who drinks too much water (e.g., polydipsia) or those with a failure in water regulatory mechanisms (e.g., kidney failure). Water intoxication can also be a result of medical errors, such as IV fluid overload. The first observable symptoms of water intoxication includes headaches, confusion, changes in behaviour, and drowsiness. Water intoxication can lead to vomitting, nausea, muscle weakness, bradycardia, cerebral edema, seizures, coma or death.

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Path 300 - Clinical Chemistry Copyright © by Deb Chen is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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