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Chapter: Anatomy and Physiology for Health Professionals: Fluid, Electrolyte, and Acid Base Balance

The maintenance of fluid, primarily water, and electrolyte balance requires equal quantities of these substances to enter and leave the body.


The maintenance of fluid, primarily water, and electrolyte balance requires equal quantities of these substances to enter and leave the body. When water balance is altered, the electrolyte balance is affected. The intracellular fluid compartment includes the flu-ids and electrolytes enclosed by cell membranes. The extracellular fluid compartment includes all the fluids and electrolytes outside the cell membranes. Many different types of solutes are dissolved in water, which is the universal solvent. Most extracellular fluids contain high amounts of chloride and sodium ions. Most intracellular fluids contain high amounts of magnesium, phosphate, and potassium ions. Hydro-static and osmotic pressure regulate fluid movements. Exchanges between plasma and interstitial fluid occur across capillary walls, whereas exchanges between interstitial and intracellular fluids occur across plasma membranes. Water balance requires equal intake and output. Water intake is mostly regulated by thirst. The distal convoluted tubules of the nephrons and collect-ing ducts regulate water output.

The release of ADH is proportional to the amount of water reabsorbed in the collecting ducts of the kid-neys. In the hypothalamus, osmoreceptors sense the solute concentration of the extracellular fluid, trigger-ing or inhibiting release of ADH from the posterior pituitary. The hormone aldosterone stimulates water reabsorption in the kidneys. Dehydration is the loss of water or solutes, occurring when water output exceeds water intake over time. The most important electrolytes for cellular functions dissociate in body fluids to release ions of sodium, potassium, calcium, magnesium, chloride, sulfate, phosphate, bicarbonate, and hydrogen. Most electrolytes are lost through the kidneys. Concentrations of sodium, potassium, and calcium ions are the most important of all.

Acids are electrolytes that dissociate to release hydrogen ions. Bases release ions that combine with hydrogen ions. Body fluid pH must remain within a certain range. Acids and bases may be strong or weak. Buffer systems convert strong acids into weaker acids or strong bases into weaker bases. Respiratory or met-abolic acidosis results from increases in concentra-tions of acids or loss of certain bases. Respiratory or metabolic alkalosis results from decreases in concen-trations of certain acids, loss of hydrogen ions, or gain of bases.

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