Sodium chloride in water dissociates to provide sodium (Na+) and chloride (Cl-) ions. These ions are normal constituents of the body fluids (principally extracellular) and are essential for maintaining electrolyte balance.
Sodium is the principal cation of extracellular fluid. It comprises more than 90% of the total cations at its normal plasma concentration of approximately 142 mEq/L. While the sodium ion can diffuse across cell membranes, the intracellular sodium is maintained at a much lower concentration than extracellular sodium through the expenditure of energy by the cell (so called "sodium cation pump"). Loss of intracellular potassium ion is usually accompanied by an increase in intracellular sodium ion. Sodium is the principal ion that determines osmotic pressure of interstitial fluids and the degree of tissue hydration.
When serum sodium concentration is low, the secretion of antidiuretic hormone (ADH) by the pituitary is inhibited, thereby preventing water reabsorption by the distal renal tubules. On the other hand, adrenal secretion of aldosterone increases renal tubular reabsorption of sodium in an effort to re-establish normal serum sodium concentration.
Chloride (Cl¯) has an integral role in buffering action when oxygen and carbon dioxide exchange occurs in the red blood cells.
The distribution and excretion of sodium (Na+) and chloride (Cl¯) are largely under the control of the kidney which maintains a balance between intake and output.