Physiology, Calcium

Excerpt

Calcium is an essential element that serves an important role in skeletal mineralization. More than 99% of the calcium in the body is stored in bone as hydroxyapatite. Calcium in this form provides skeletal strength as well as a reservoir for calcium to be released into the serum.

In serum, calcium exists in 3 forms: protein-bound, ionized (free), and complexed (chelated). Protein-bound calcium, which accounts for 40% of the serum calcium, cannot be used by tissues. Albumin and globulin are the primary calcium-binding proteins in the serum whereas calmodulin is the primary calcium-binding protein in the cell. Chelated calcium, which accounts for 9% of the serum calcium, allows calcium to be absorbed by various tissues or carried between parts of the body. Serum calcium is often chelated into the ionic complexes of calcium phosphate, calcium carbonate, and calcium oxalate. Finally, free calcium, which makes up 51% of the serum calcium, is utilized by the body to maintain physiologic functions. If the serum calcium concentration exceeds the 8.8 to 10.4 mg/dL, then the body is considered in a state of calcium toxicity.