Calcium (Ca), phosphorus (P) and magnesium (Mg) are considered together as 98%, 80% and 65% of their body content within the skeleton. The majority of Ca and P are found together as components of microcrystalline apatite [Ca5(PO4)3(OH)], the bone mineral which is forming in bone only if Ca and P are simultaneously available in optimal proportions.
Of the total body phosphorus 20% is found in tissue. The molar Ca:P ratio is 1.67 in apatite and 1.3 in the whole body.
In addition to their function as skeleton component, Ca, P, and Mg also play major roles in many physiologic processes.