Calcium.

Calcium is the fifth most abundant element in the body with >99% residing in the skeleton as hydroxyapatite, a complex calcium phosphate molecule. This mineral supplies the strength to bones that support locomotion, but it also serves as a reservoir to maintain serum calcium concentrations. Calcium plays a central role in a wide range of essential functions. Its metabolism is regulated by 3 major transport systems: intestinal absorption, renal reabsorption, and bone turnover. Calcium transport in these tissues is regulated by a sophisticated homeostatic hormonal system that involves parathyroid hormone, and 1,25-dihydroxyvitamin D in response to decreased serum ionized calcium, detected by the calcium-sensing receptor (1).