Mechanism of Action: Calcium is essential for the functional integrity of the nervous, muscular and skeletal systems. It plays a role in normal cardiac function, renal function, respiration, blood coagulation, and cell membrane and capillary permeability. Also, calcium helps to regulate the release and storage of neurotransmitters and hormones, the uptake and binding of amino acids, absorption of vitamin B12, and gastrin secretion. The major fraction (99%) of calcium is in the skeletal structure primarily as hydroxyapatite, Ca10(PO4)6(OH)2; small amounts of calcium carbonate and amorphous calcium phosphates are also present. The calcium of bone is in a constant exchange with the calcium of plasma. Since the metabolic functions of calcium are essential for life, when there is a disturbance in the calcium balance because of dietary deficiency or other causes, the stores of calcium in bone may be depleted to fill the body's more acute needs. Therefore, on a chronic basis, normal mineralization of bone depends on adequate amounts of total body calcium.