ABSTRACT
The principal extracellular metal ion in humans, sodium is essential for life. As the most abundant cation in the extracellular fluid, its salts account for over 90% of the osmotically active solute in the plasma and interstitial fluid. In mammals, the intracellular sodium ion concentration is 5-15 mM, whereas in the interstitial fluid a steady concentration of 145 mM Na is present. The normal body fluid molality is maintained by multiple regulatory systems. A principal regulator is renal sodium excretion, in turn controlled by a number of variables, principally glomerular filtration rate and elaboration of antidiuretic hormone(1). Together with other minerals, particularly potassium, sodium is vital to the maintenance of normal water balance, enzymatic activity, transmission of nerve impulses, and, hence, muscle contraction. Regardless of widely fluctuating dietary intake, in the healthy organism the kidneys maintain a steady level of sodium in the body.
