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The importance of both Ca2+ and Pi in the body is reflected in the exquisite hormonal regulation associated with each element. Calcium (Ca2+) represents approximately 1-2% of total body weight, while phosphorus (Pi) constitutes approximately half that amount. Although most Ca2+ (99%) and Pi (85%) exist as a complex within bone, each has a number of critical non-structural functions. These range from roles in phospholipid synthesis and signal transduction, to regulating nervous system activity and muscle contraction.