Sodium (Na) is the principal cation of extracellular fluid and is involved primarily in the maintenance of osmotic equilibrium and extracellular fluid volume.The sodium- gradient across the cellular membrane is maintained by a sodium-potassium pump, which uses considerable energy in the body at rest. The body content of the sodium and its concentrations in body fluids are under homeostatic control, kidney being the primary organ responsible for maintaining sodium balance through aldosterone action on renal tubular function.