Significance of Calcium

SIGNIFICANCE OF CALCIUM

More than 99% of the body’s calcium is located in the skeletal sys-tem; it is a major component of bones and teeth. About 1% of skeletal calcium is rapidly exchangeable with blood calcium; the rest is more stable and only slowly exchanged. The small amount of cal-cium located outside the bone circulates in the serum, partly bound to protein and partly ionized. Calcium plays a major role in trans-mitting nerve impulses and helps to regulate muscle contraction and relaxation, including cardiac muscle. Calcium is instrumental in activating enzymes that stimulate many essential chemical re-actions in the body, and it also plays a role in blood coagulation. Because many factors affect calcium regulation, both hypocalcemia and hypercalcemia are relatively common disturbances.

The normal total serum calcium level is 8.5 to 10.5 mg/dL (2.1–2.6 mmol/L). It exists in plasma in three forms: ionized, bound, and complexed. About 50% of the serum calcium exists in an ionized form that is physiologically active and important for neuromuscular activity and blood coagulation. The normal ionized serum calcium level is 4.5 to 5.1 mg/dL (1.1–1.3 mmol/L) and is the only form that is physiologically and clinically significant. Less than half of the plasma calcium is bound to serum proteins, pri-marily albumin. The remainder is combined with nonprotein anions: phosphate, citrate, and carbonate.

Calcium is absorbed from foods in the presence of normal gas-tric acidity and vitamin D. Calcium is excreted primarily in the feces, the remainder in urine. The serum calcium level is controlled by PTH and calcitonin. As ionized serum calcium decreases, the parathyroid glands secrete PTH. This event then increases calcium absorption from the GI tract, increases calcium reabsorption from the renal tubule, and releases calcium from the bone. The increase in calcium ion concentration suppresses PTH secretion. When calcium increases excessively, the thyroid gland secretes calcitonin. It briefly inhibits calcium reabsorption from bone and decreases the serum calcium concentration.