ABSTRACT
Maintaining the composition and volume of the extracellular uid (ECF) and intracellular uid (ICF) within a restricted range of variation is critical for normal tissue perfusion and cellular function (Strange 1993). The ECF is primarily composed by the vascular and extravascular (interstitial) uid, and has sodium as a determinant ion of its osmolarity and volume. The ECF/plasma volume ratio is of utmost importance for maintaining vascular capacitance and, consequently, venous return, cardiac output, and arterial pressure (Ramsay 1991). Maintaining the variability of the ECF volume within a strict range is a central goal of homeostatic mechanisms.
