Body composition, normal electrolyte concentrations, and the maintenance of normal volume, tonicity, and acid-base metabolism.

Because the internal environment of the body is largely a fluid medium, the preservation of the volume and composition of the body fluids is absolutely vital to circulatory status and the extraordinarily complicated functions of the human body. The fluid compartments do not exist as fixed spaces with identical compositions but rather are in constant interchange with each other and have strikingly different compositions. Methods of movement of solutes and water include diffusion along electrochemical gradients, by hydrostatic pressure, osmotic forces, bulk flow, primary and secondary active transport, capillary blood flow, and oncotic pressure. Complex feedback control mechanisms exist to ensure homeostasis or equilibrium and include participation by the kidneys, lungs, gastrointestinal tract, the circulatory system, the endocrine system, and the CNS. The maintenance of extracellular volume is centered around the control of balance of the sodium salts. Multiple afferent (or sensing) and efferent (or effector) mechanisms exist to accomplish this homeostasis. The most important determinants of the osmolality or tonicity of the body fluids is the excretion or retention of water by the kidney, thirst mechanisms, and the intake of water. The serum sodium concentration is the laboratory test most often used clinically to assess tonicity. The pH of the body fluids and the major acid-base buffer systems are also carefully regulated. The lungs are responsible for the elimination of the carbon dioxide produced by cellular metabolism, and the kidneys excrete hydrogen ions and regulate the concentration of bicarbonate in the body fluids. Urinary net acid excretion, the hydrogen ions excreted as titratable acid and ammonium ions minus any bicarbonate, equals the acid added to the ECF from the diet and metabolism plus any fecal losses of alkali.