A. Background Information

Calcium and phosphorus are the most abundant of the inorganic elements in humans; together they are the key structural minerals of the human body. A 70-kg man contains about 1200 g of calcium and 770 mg of phosphorus present as phosphate. Calcium and phosphorus are also essential to a great number of cellular processes (see Table 9-1).

For optimal growth and function, living organisms require an adequate supply of calcium and phosphate to meet their metabolic and structural needs. Higher organisms must be dietarily supplied with calcium and phosphorus on a continual basis. The plasma concentrations of these substances are maintained within a surprisingly narrow limit by an endogenous control mechanism, often of great subtlety and elegant precision.

The three primary target tissues involved in calcium and phosphorus homeostasis in humans and higher animals are the intestine, where these ions enter into the physiological milieu, bone, where they are stored and made available for minute-by-minute regulation of the serum levels of these ions, and kidney, where their extent of excretion can be monitored and regulated. The maintenance of calcium and phosphorus homeostasis thus involves the delicate and coordinated


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