ABSTRACT
Extracellular fluid (ECF) calcium is maintained under tight control within a relatively narrow range because of the importance of the calcium ion (Ca2+) as a regulator of many homeostatic processes. Fluxes of calcium across the intestine, kidney and bone are the most important determinants of ECF Ca2+. If 1000 mg of dietary Ca2+ are ingested, about 400 mg are absorbed in the intestine. Approximately 200 mg are excreted in the urine and about 200 mg are deposited in bone, to compensate for 200 mg which is resorbed from bone daily. The major hormonal regulators of Ca2+ transport across the gut, bone and kidney are the peptide hormone, parathyroid hormone (PTH) and the secosteroid 1,25-dihydroxyvitamin D (1,25(OH)2D).
