Disorders of Calcium Metabolism

The adult human body contains 1.3 kg of calcium of which 99% is contained in bones and teeth, 1% in cells of soft tissue, and 0.15% in the extracellular fluid (1). The bone calcium is in the form of hydroxyapatite crystals, and the exchange of calcium between bone and extracellular fluid occurs by two mechanisms: bone turnover, which is concerned with replacement and remodeling of bone and involves coupled osteoclastic resorption and osteblastic deposition, and homeostatic equilibration, which is concerned with the regulation of the plasma calcium level (2, 3). The normal total plasma calcium concentration is 4.5–5.1 mEq/1 (9–10.2mg/dl). Of the total calcium 40–43% is bound to plasma protens (mainly albumin), 5–10% is combined with anions such as citrate and phosphate to form nonionized complexes, and the remaining 40–50% is free ionized calcium. Only this last fraction is physiologically active. In addition, a small amount of calcium is also bound to serum globulins.