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5 C alcium is an esse ntial element in the hum an bo dy . Although over 99% of the tota l body calci um is located in bone, calci um is a critical cation in both the extracellular and intracellular spaces. Its concentration is held in a very narrow range in both spaces. In addi- tion to its important role in the bone mineral matrix, calcium serves a vital role in nerve impulse transmission, muscular contraction, blood coagulation, hormone secretion, and intercellular adhesion. Calcium also is an important intracellular second messenger for processes such as exocytosis, chemotaxis, hormone secretion, enzymatic activity, and fertilization. Calcium balance is tightly regulated by the interplay betwee n gastrointestinal absorption, r enal excre tion, bone resorption, and t he vitamin D–parathyroid hor mone (PTH) system [1–7] . C H A P T ER Dival e nt C a t i o n Metabolism: Calcium J ames T . M c Car t hy Rajiv Kumar
