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Ca^2+ and PO4^3+ form hydroxyapatite
Diffusible pool
~50% of plasma calcium is free/unbound ionic Ca2+ (physiologically important form)
~10% of plasma calcium is complexed with small molecular weight compounds (citrate, phosphate)
Non-diffusible pool
~40% of plasma calcium is bound to calcium-binding proteins and plasma proteins (albumin)
The body utilises plasma albumin-bound calcium as a circulating reserve