Adequate amounts of calcium in its ionized form, Ca2+, are needed for normal function of all cells. Calcium ion regulates a wide range of biological processes and is one of the principal constituents of bone. In terrestrial vertebrates, including humans, maintenance of adequate concentrations calcium ion1 in the extracellular fluid requires the activity of two hormones, parathyroid hormone (PTH) and a derivative of vitamin D called 1a,25-dihydroxycholecalciferol (1,25(OH)2D3) or calcitriol. In more primitive vertebrates living in a marine environment, guarding against excessively high concentrations of calcium requires another hormone, calcitonin, which appears to have only vestigial activity in humans.

Body calcium ultimately is derived from the diet, and daily intake is usually offset by urinary loss. The skeleton acts as a major reservoir of calcium and can buffer the concentration of calcium in extracellular fluid by taking up or releasing calcium phosphate. PTH promotes the transfer of calcium from bone, the glomerular filtrate, and intestinal contents into the extracellular fluid. It acts on bone cells to promote calcium mobilization and on renal tubules to reabsorb calcium and excrete phosphate. It promotes intestinal transport of calcium and phosphate indirectly by increasing the formation of 1,25(OH)2D3 required for calcium uptake by intestinal cells. This vitamin D metabolite also promotes calcium mobilization from bone and reinforces the actions of PTH on this process. In addition, 1,25(OH)2D3 promotes reabsorption of calcium and phosphate by renal tubules. The rate of PTH secretion is inversely related to the concentration of blood calcium, which directly inhibits secretion by the chief cells of the parathyroid glands. Calcitonin inhibits the activity of bone-resorbing cells, and thus blocks inflow of calcium to the extracellular

1Calcium is present in several forms within the body, but only the ionized form, Ca2+, is monitored and regulated. In this discussion, calcium refers to the ionized form except when otherwise specified.

fluid compartment. Its secretion is stimulated by high concentrations of blood calcium.

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