Calcium enters into a wide range of cellular and molecular processes. Changes of its concentration within cells regulate enzymatic activities and such fundamental cellular events as muscular contraction, secretion, and cell division. As already discussed (see Chapter 2), calcium and calmodulin also act as intracellular mediators of hormone action. In the extracellular compartment, calcium is vital for blood clotting and maintenance of normal membrane function. Calcium is the basic mineral of bones and teeth and thus plays a structural as well as a regulatory role. Not surprisingly, its concentration in extracellular fluid must be maintained within narrow limits. Deviations in either direction are not readily tolerated and, if severe, may be life threatening.
The adult human body contains approximately 1000 g of calcium, about 99% of which is sequestered in bone, primarily in the form of hydroxyapatite crystals (Ca10(PO4)6(OH)2). In addition to providing structural support, bone serves as an enormous reservoir for calcium salts. Each day about 600 mg of calcium is exchanged between bone mineral and the extracellular fluid. Much of this exchange reflects resorption and reformation of bone as the skeleton undergoes constant remodeling, but some also occurs by exchange with a labile calcium pool in bone.
Most of the calcium that is not in bone crystals is found in cells of soft tissues bound to proteins within the sarcoplasmic reticulum, mitochondria, and other organelles. Energy-dependent transport of calcium by these organelles and the cell membrane maintains the resting
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