y 25 percent of total body mass. Moreover, ECF may be further subdivided into the plasma or intravascular fluid, and the interstitial or extravascular fluid (Watkins, 1995, p. 16). In addition, the ECF also includes various transcellular fluids, or epithelial cell secretions (e.g., the cerebrospinal, pleural, peritoneal, and synovial fluids).
The composition of the different body fluid compartments are distinct: The electrolyte content of the ICF, for example, differs markedly from that of the ECF. The major ICF cations consist of potassium and magnesium, and the primary ICF anions include the phosphates and sulfates. The main ECF electrolytes, in contrast, are sodium and chloride. Roughly 90 percent of the ECF electrolyte concentration, or osmolality, is caused by sodium. Moreover, sodium is of primary importance in regulating body fluid volume: The retention of sodium is closely correlated with the retention of fluid. When large quantities of sodium are lost, body fluid volume typically decreases (Metheny, 1992, pp. 3-10). In addition to sodium chloride though, the plasma fluid also contains bicarbonate, potassium, calcium, magnesium, phosphates, sulfates, organic acids, and protein (i.e. albumin). While the interstitial fluid composition is similar to that of the plasma, it generally lacks the plasma's proteinaceous component (Watkins, 1995, p. 16).
Maintaining the chemical compositions of the body's separate fluid compartments requires considerable energy. The tendency for sodium to diffuse from the ECF to the ICF is countered by cell membrane sodium-potassium pumps. In the presence of adenosine triphosphate (
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