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Figure

Extracellular fluid has relatively high concentrations of sodium, calcium, chloride, and bicarbonate ions. Intracellular fluid has relatively high concentrations of potassium, magnesium, phosphate, and sulfate ions.

Recall that osmotic pressure is due to the presence of im-permeant solutes on one side of a cell membrane. Because of the Na+/K+ pump, sodium (extracellular) and potassium (intracellular) ions behave as impermeant solutes, and create an osmotic pressure. For example, since most cell membranes in the body are freely permeable to water, a decrease in extracellular sodium ion concentration will cause a net movement of water from the extracellular compartment into the intracellular compartment by osmosis. The cell swells. Conversely, if the extracellular sodium ion concentration increases, cells will shrink as they lose water. Although the solute composition of body fluids varies between intracellular and extracellular compartments, water will "follow salt" and distribute by osmosis such that the water concentration (and total solute concentration) is essentially equal inside and outside cells.

Different substances may distribute to different compartments. For example, an infusion of 1 liter of isotonic sodium chloride solution is restricted largely to the extracellular fluid due to the active transport sodium pumps in cell membranes. In contrast, a liter of isotonic glucose solution may be given intravenously without damaging red blood cells, but as the glucose is metabolized aerobically, it is converted to carbon dioxide and water. Thus, the liter of isotonic glucose ends up as a liter of water that can distribute throughout in-tracellular and extracellular compartments. Depending on the reason for the infusion, one may be preferable over the other.

Which factors control the movement of water and electrolytes from one fluid compartment to another?

How does the sodium ion concentration within body fluids affect the net movement of water between the compartments?

Water Balance

Water balance exists when water intake equals water output. Homeostasis requires that both water intake and water output be controlled to maintain the constancy of the internal environment. Ultimately, maintenance of the internal environment depends on the kidneys' ability to vary water output, and thirst centers in the brain to vary water intake.

Water Intake

The volume of water gained each day varies from individual to individual. An average adult living in a moderate environment takes in about 2,500 milliliters. Probably 60% is obtained from drinking water or beverages, and another 30% comes from moist foods. The remaining 10% is a by-product of the oxidative metabolism of nutrients, which is called water of metabolism (fig. 21.5).

Regulation of Water Intake

The primary regulator of water intake is thirst. The intense feeling of thirst derives from the osmotic pressure

Serous membrane

Blood flow

Cell membrane v

Serous membrane

Blood flow

Figure 21.4

Net movements of fluids between compartments result from differences in hydrostatic and osmotic pressures.

Average daily intake of water

Total intake (2,500 mL)

Water of

— metabolism

Water in

Total intake (2,500 mL)

Water in beverages (1,500 mL or 60%)

Figure 21.5

Major sources of body water.

Water in beverages (1,500 mL or 60%)

Figure 21.5

Major sources of body water.

Figure 21.4

Net movements of fluids between compartments result from differences in hydrostatic and osmotic pressures.

Regulation of Water Intake

Regulation of Water Intake

1. The body loses as little as 1% of its water.

2. An increase in the osmotic pressure of extracellular fluid due to water loss stimulates osmoreceptors in the thirst center.

3. Activity in the hypothalamus causes the person to feel thirsty and to seek water.

4. Drinking and the resulting distension of the stomach by water stimulate nerve impulses that inhibit the thirst center.

5. Water is absorbed through the walls of the stomach and small intestine.

6. The osmotic pressure of extracellular fluid returns to normal.

of extracellular fluids and a thirst center in the hypothalamus of the brain.

As the body loses water, the osmotic pressure of the extracellular fluids increases. Such a change stimulates osmoreceptors in the thirst center, and as a result, the hypothalamus causes the person to feel thirsty and to seek water. A thirsty person usually has a dry mouth, which is a consequence of the loss of extracellular water and resulting decreased flow of saliva.

The thirst mechanism is normally triggered whenever the total body water decreases by as little as 1%. The act of drinking and the resulting distension of the stomach wall trigger nerve impulses that inhibit the thirst mechanism. Thus, drinking stops long before the swallowed water is absorbed. This inhibition helps prevent the person from drinking more than is required to replace the volume lost, avoiding development of an imbalance. Table 21.1 summarizes the steps in this mechanism.

H What is water balance?

Where is the thirst center located? B What stimulates fluid intake? What inhibits it?

Water Output

Water normally enters the body only through the mouth, but it can be lost by a variety of routes. These include obvious losses in urine, feces, and sweat (sensible perspiration), as well as evaporation of water from the skin (insensible perspiration) and from the lungs during breathing.

If an average adult takes in 2,500 milliliters of water each day, then 2,500 milliliters must be eliminated to maintain water balance. Of this volume, perhaps 60% will be lost in urine, 6% in feces, and 6% in sweat. About 28% will be lost by evaporation from the skin and lungs (fig. 21.6). These percentages vary with such environmental factors as temperature and relative humidity and with physical exercise.

If insufficient water is taken in, water output must be reduced to maintain balance. Water lost by sweating is a necessary part of the body's temperature control mechanism; water lost in feces accompanies the elimination of undigested food materials; and water lost by evaporation is largely unavoidable. Therefore, the primary means of regulating water output is control of urine production.

Proteins called aquaporins form water-selective membrane channels that enable body cells, including red blood cells and cells in the proximal convoluted tubules and descending limbs of the nephron loops, to admit water. A mutation in one aquaporin gene (which instructs cells to manufacture a type of aquaporin protein) causes a form of diabetes insipidus, in which the renal tubules fail to reabsorb water. Interestingly, rare individuals have been identified who lack certain other aquaporin genes, and they apparently have no symptoms. This suggests that cells have more than one way to admit water.

Regulation of Water Output

The distal convoluted tubules and collecting ducts of the nephrons regulate the volume of water excreted in the urine. The epithelial linings of these segments of the renal tubule remain relatively impermeable to water unless antidiuretic hormone (ADH) is present.

Recall from chapter 13 (p. 519) that osmoreceptors in the hypothalamus help control release of ADH. If the blood plasma becomes more concentrated because of excessive water loss, the osmoreceptors lose water by osmosis and shrink. This change triggers impulses that signal the posterior pituitary gland to release ADH. The ADH re-

Average daily output of water

Water lost

Water lost in feces (150 mL or 6%)

Water lost

— through skin and lungs

Total output (2,500 mL)

Total output (2,500 mL)

Water lost in urine

Water lost in urine

Figure

Routes by which the body loses water. Regulation of urinary water loss is most important in the control of water balance.

leased into the bloodstream reaches the kidneys, where it increases the permeability of the distal convoluted tubules and collecting ducts. Consequently, water reabsorption increases, and water is conserved. This action resists further osmotic change in the plasma. In fact, the osmoreceptor (oz"mo-re-sep'tor)-^DH mechanism can reduce a normal urine production of 1,500 milliliters per day to about 500 milliliters per day when the body is dehydrating.

On the other hand, if a person drinks too much water, the plasma becomes less concentrated, and the os-moreceptors swell as they receive extra water by osmosis. In this instance, ADH release is inhibited, and the distal tubules and collecting ducts remain impermeable to water. Consequently, less water is reabsorbed and more urine is produced. Table 21.2 summarizes the steps in this mechanism. Clinical Application 21.1 discusses disorders resulting from water imbalance.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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