99% of calcium in the body is found within bone where it constitutes roughly 2% of total body weight. It forms the mineral hydroxyapatite.
Calcium within extracellular body fluids is distributed as detailed:
Biological activity is dependent upon the ionised fraction. The various forms are in dynamic equilibrium: various factors such as alkaline pH, and increased concentrations of citrate or phosphate, all decrease the ionised fraction. Calcium and phosphate have an inverse relationship governed by their solubility product.
The majority of intracellular calcium is protein-bound and sequestered within organelles such as mitochondria. Typical cytosolic free calcium concentration is 0.1micromol/l whereas it is 1000 times this level extracellularly. The gradient is maintained by active transport ionic pumps.
The concentration gradients for calcium which exist within a cell and across the plasma membrane are beneficial as the basis for secondary messenger systems involving the ion.
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