Bone, Calcium, and Phosphate Metabolism

Regulation of calcium and phosphate metabolism is highly complex. Calcium and phosphate concentration in the blood is kept constant despite highly variable intake of the two ions. The two most important hormones for calcium and phosphate homeostasis are parathormone, secreted by the parathyroid glands, and calcitonin, which is formed in the medullary cells of the thyroid gland. The plasma contains only a very small fraction of the total body calcium, more than 98% of which is deposited in bones. Normal calcium concentration in the plasma ranges from 8.5 to 10 mg%. It means relatively little since the ionized calcium is actively involved in metabolic processes. About 50% of the total calcium is in the serum in ionized form, 15% is bound in complexes with bicarbonate, phosphate, and citrate, and the rest is bound to protein. The calcium concentration of the plasma falls when the protein content is low and rises when it is high. The calcium concentration is only 7 mg% in the interstitial fluid and 4–6 mg% in the cerebrospinal fluid, which contains less protein. Calcium is absorbed in the small intestine and excreted in the large intestine, urine, and sweat. The growing child and pregnant woman need up to 1 and 1.5 g calcium/day, respectively, in order to maintain a proper calcium balance, while the normal adult needs about 0.5 g. If the dietary calcium intake increases, the percentage calcium absorption in the small intestine decreases.