a chemical element, atomic number 11, atomic weight 22.990, symbol Na. (See Appendix 6.) Sodium is the major cation of the extracellular fluid, constituting 90 to 95 per cent of all cations in the blood plasma and interstitial fluid; it thus determines the osmolality of the extracellular fluid. The serum sodium concentration is normally about 140 mEq/L. If the sodium level and osmolality fall, osmoreceptors in the hypothalamus are stimulated and cause the release of antidiuretic hormone from the posterior lobe of the pituitary gland. This hormone increases the absorption of water in the collecting ducts of the kidneys so that water is conserved while sodium and other electrolytes are excreted in the urine. If the sodium level and osmolality rise, neurons in the thirst center of the hypothalamus are stimulated. The thirsty person then drinks enough water to restore the osmolality of the extracellular fluid to the normal level.A decrease in the serum sodium concentration (hyponatremia) can occur in a variety of conditions. It is often associated with deficient fluid volume due to diarrhea or vomiting when water is replaced faster than sodium. It can also occur in syndrome of inappropriate antidiuretic hormone, in the late stages of congestive heart failure or cirrhosis of the liver, in acute or chronic renal failure, and in diuretic therapy. An increase in the serum sodium concentration (hypernatremia) occurs when insensible water loss is not replaced by drinking, as in a comatose patient with diabetes insipidus. sodium acetate a source of sodium ions for hemodialysis and peritoneal dialysis, as well as a systemic and urinary alkalizer. sodium ascorbate an antiscorbutic vitamin and nutritional supplement for parenteral administration. It is also used as an aid to deferoxamine therapy in the treatment of chronic iron toxicity. sodium benzoate an antifungal agent also used in a test of liver function. sodium bicarbonate NaHCO3, a white powder commonly found in households. It has a wide variety of uses in chemistry, in pharmaceuticals, and in consumer products. It is sometimes taken in water as a remedy for acid indigestion but should not be used regularly since when taken in excess it tends to cause alkalosis. It can be mixed with water and applied as a paste for relief of pain in treatment of minor burns and insect bites and stings. A cupful in the bath water may help relieve itching caused by an allergic reaction. Called also baking soda and bicarbonate of soda.