1026

PART 4 Regulation and Maintenance

27.1 Body Fluids (p. 1002)

1. Intracellular fluid is inside cells.2. Extracellular fluid is outside cells and includes interstitial fluid andplasma.

Summary

27.4 Regulation of Specific Electrolytesin the Extracellular Fluid (p. 1010)

The intake and elimination of substances from the body and the exchangeof substances between the extracellular and intracellular fluids determineextracellular fluid composition.

27.2 Regulation of Body FluidConcentration and Volume (p. 1003)

Regulation of Water Content

1. Water crosses the wall of the digestive tract through osmosis.2. An increase in extracellular osmolality or a decrease in blood pressurestimulates the sense of thirst.3. Wetting of the oral mucosa or stretch of the digestive tract inhibits thirst.4. Learned behavior plays a role in the amount of fluid ingested.5. Water leaves the body by the following routes: ■ Through evaporation from the respiratory system and the skin (insen-sible perspiration and sweat) ■ Through the digestive tract (this amount is normally small, butvomiting or diarrhea can significantly increase it) ■ Through the kidneys, the primary regulator of water excretion;urine output can vary from a small amount of concentrated urineto a large amount of dilute urine

Regulation of Sodium Ions

1. Sodium is responsible for 90–95% of extracellular osmotic pressure.2. The amount of Na + excreted in the kidneys is the difference betweenthe amount of Na + that enters the renal tubule and the amount that isreabsorbed from the renal tubule. ■ The glomerular filtration rate determines the amount of Na + enteringthe renal tubule. ■ Aldosterone determines the amount of Na + reabsorbed.3. Small quantities of Na + are excreted in sweat.4. Increased blood osmolality leads to the production of a small volumeof concentrated urine and to thirst. Decreased blood osmolality leadsto the production of a large volume of dilute urine and to decreasedthirst.5. Increased blood pressure increases water and salt loss. ■ Elevated extracellular fluid volume reduces ADH secretion. ■ Renin secretion is inhibited, leading to reduced aldosterone production.

Regulation of Extracellular Fluid Osmolality

1. Increased water consumption and ADH secretion occur in responseto increases in extracellular fluid osmolality. Decreased water consump-tion and ADH secretion occur in response to decreases in extracellularfluid osmolality.2. Increased water consumption and ADH decrease extracellular fluidosmolality by increasing water absorption from the intestines and waterreabsorption from the renal tubules. Decreased water consumption andADH increase extracellular fluid osmolality by decreasing absorptionfrom the intestines and water reabsorption from the renal tubules.

Regulation of Chloride Ions

Chloride ions are the dominant negatively charged ions in extracellular fluid.

Regulation of Potassium Ions

1. The extracellular concentration of K + affects resting membranepotentials.2. The amount of K + excreted depends on the amount that enters withthe glomerular filtrate, the amount actively reabsorbed by the renaltubule, and the amount secreted into the distal convoluted tubule.3. Aldosterone increases the amount of K + secreted.

Regulation of Extracellular Fluid Volume

1. Increased extracellular fluid volume results in decreased aldosteronesecretion, increased ANH secretion, decreased ADH secretion, anddecreased sympathetic stimulation of afferent arterioles. These changesdecrease Na + reabsorption and increase urine volume so as to decreaseextracellular fluid volume.2. Decreased extracellular fluid volume results in increased aldosteronesecretion, decreased ANH secretion, increased ADH secretion, andincreased sympathetic stimulation of the afferent arterioles. Thesechanges increase Na + reabsorption and decrease urine volume so asto increase extracellular fluid volume.

Regulation of Calcium Ions

1. Elevated extracellular Ca 2+ levels prevent membrane depolarization.Decreased levels lead to spontaneous action potential generation.2. PTH increases extracellular Ca 2+ levels and decreases extracellularphosphate levels. It stimulates osteoclast activity, increases Ca 2+ reab-sorption from the kidneys, and stimulates active vitamin D production.3. Vitamin D stimulates Ca 2+ uptake in the intestines.4. Calcitonin decreases extracellular Ca 2+ levels.

Regulation of Magnesium Ions

The kidneys’ capacity to reabsorb Mg 2+ is limited, so excess Mg 2+ isexcreted in the urine, and decreased extracellular Mg 2+ leads to increasedMg 2+ reabsorption.

27.3 Regulation of Intracellular FluidComposition (p. 1009)

1. Substances used or produced inside the cell and substances exchangedwith the extracellular fluid determine the composition of intracellularfluid.2. Intracellular fluid is different from extracellular fluid because theplasma membrane regulates the movement of materials.3. The difference between intracellular and extracellular fluid concen-trations determines water movement.

Regulation of Phosphate Ions

1. Under normal conditions, reabsorption of phosphate occurs at amaximum rate in the renal tubule.2. An increase in plasma phosphate increases the amount of phosphatein the renal tubule beyond that which can be reabsorbed, and theexcess is excreted in the urine.