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Calcium and Phosphate Physiologic Forms

  • The vast majority of total-body calcium and phosphate are stored within the bones and only a fraction circulates within the extracellular fluid. This extracellular supply of calcium and phosphate can in different physiological forms which exhibit distinct physiological activities.
Extracellular Calcium
  • Protein Bound: 40%
    • Roughly 40% of extracellular calcium is bound to plasma proteins such as albumin. Importantly, this protein-bound supply cannot diffuse through capillaries and remains within the vasculature.
  • Chemically Bound: 10%
    • Roughly 10% of the extracellular calcium is complexed with anions such as phosphate and citrate. Although this chemically-bound supply of calcium can diffuse through capillaries it is not physiologically active.
  • Free Calcium: 50%
    • Roughly half of the extracellular calcium is in its free ionized form as Ca++). Ionized calcium is freely diffusible through the capillaries and is the only physiologically active form of the calcium. Thus, discussions of regulatory mechanisms of calcium concentration are in reality only referring to this free, ionized form of the atom.
Extracellular Phosphate
  • Extracellular phosphate is present in inorganic form as either HPO42- or H2PO4-. The differences between these two ionic forms of phosphate are physiologically negligible and biochemically difficult to distinguish; consequently, most discussions of extracellular phosphate will conflate these two pools as total phosphorous.