Where is calcium found in the body?
Approximately 99% of the calcium in the body is found in the bones. The mineral phase of bone is composed of hydroxyapatite (Ca10(PO4)6(OH)2) and other minerals. Hydroxyapatite is deposited in close association with an organic phase composed primarily of collagen. Osteoblasts create new bone and osteoclasts cause bone resorption.
The remainder of the calcium is found in extracellular fluid (1%) and intracellular fluid (0.1%). Half of the extra-cellular calcium is in the free ionized form. This is the important form for physiologic processes. About 40% of extracellular calcium is bound to albumin and globulins. Binding to albumin is decreased by acidosis and increased by alkalosis. Increases or decreases in serum protein will change total serum calcium without changing ionized calcium. For this reason, the most meaningful laboratory test for calcium is serum ionized calcium, which is widely available.
Intracellular free calcium concentrations are about 0.01% of extracellular concentrations. Calcium is actively trans-ported out of the cell by ATP-dependent pumps. Muscle cells sequester calcium in the sarcoplasmic reticulum. This calcium is easily available during muscle contraction, but not present as free calcium during rest.