Sodium chloride is an electrolyte replenisher. Sodium is the principal cation of extracellular fluid. With a normal plasma concentration of 142 mEq/L, sodium comprises more than 90% of the total plasma cations. While sodium can diffuse across membranes, the intracellular sodium concentration is maintained at a much lower level than extracellular concentrations-the so-called "sodium pump". Compensation for loss of intracellular potassium occurs through an increase in intracellular sodium. Sodium is the principal ion that determines osmotic pressure of interstitial fluids and the degree of tissue hydration.
Adult serum chloride values typically range from 100 to 106 mEq/L. Serum chloride levels decrease in metabolic alkalosis, as serum bicarbonate levels generally increase. In parenteral nutrition when acidosis occurs, it is common practice to reduce chloride intake by substituting acetate salts in place of chloride salts.
Normal salt intake ranges from 5 g to 15 g/day, most of which is excreted by the kidneys. Control of water and salt excretion is very intricate, involving filtration by the glomerulus and reabsorption by the tubules of approximately 99% of the filtered load. The actual quantities excreted depend upon the requirements prevailing at the moment. Finer adjustments of the tubular absorptive mechanisms are influenced by osmotic interrelationships between cell water, plasma and urine, and by certain steroid hormones influencing electrolyte excretion and the posterior pituitary hormone regulating water excretion. When food intake ceases or salt is withheld, the urine content of sodium chloride diminishes rapidly so that body stores are conserved. Similar renal retention of electrolytes occurs when salt is lost via vomiting. However, in other conditions, the kidneys fail to eliminate sufficient sodium. Examples include congestive heart failure, cirrhosis, nephritis, or hypersecretion of the adrenal cortical hormones. The result is retention of both salt and water, producing an excessive accumulation of extracellular fluid, which may be effectively treated by restricting salt intake and use of a diuretic.
Symptoms of sodium chloride deficiency include nausea, vomiting, and increased muscle irritability manifested by cramps, and possibly convulsions. It is recognized that excessive sweating can cause "heat cramps"–muscle cramps in the abdomen and extremities, which can be relieved completely by ingestion of a weak salt solution. Other causes of salt depletion include overzealous treatment of fluid and sodium retention, diabetic acidosis, burns, excessive sweating with free drinking of water, repeated paracentesis for removal of ascitic fluid, adrenal cortical hypofunction and certain forms of nephritis, as well as abnormal loss of gastrointestinal secretions. Reduction in the osmotic pressure of extracellular fluid accompanies salt loss. Urine volume may be maintained, but urine is hypo-osmolar secondary to salt conservation by the renal tubules.