Regulation of blood calcium in humans.

Serum calcium in humans is closely regulated, with a maximum excursion of values throughout the day of less than 6%. A number of hormonal controls serve to maintain the stability of the serum calcium value. The three most important hormones--PTH, 1,25-dihydroxyvitamin D, and calcitonin--act chiefly on three organs--intestine, bone, and kidney. A set of simple diagrams that summarize these control mechanisms is presented. Organ diagrams are used to indicate the normal fluxes of calcium (or other ions) and the sites of hormone action, while a feedback loop diagram summarizes the interactions among the various factors. The feedback loop diagram predicts the direction of changes observed in each parameter in various disorders of calcium metabolism and may be useful for constructing experimental models or for teaching purposes.