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Calcium and Phosphate Balance

  • The regulation of calcium and phosphate is more complex than that of other molecules because of the presence of bone, which represents an enormous internal reservoir of both calcium and phosphate. Therefore, changes in plasma calcium and phosphate concentration is achieved by regulated exchange of these molecules with the outside world, termed External Balance, and the bone, termed Internal Balance. Of the two molecules, precise regulation of extracellular calcium concentrations is vastly more important as calcium levels are critical for proper nerve conduction, muscle contraction, and blood clotting. Consequently, in a normal individual blood calcium ranges only a few percent from its average value and large shifts can result in the clinical syndromes of hypercalcemia and hypocalcemia. In contrast, extracellular phosphate concentration is not precisely controlled as even large swings in blood phosphate do not cause major physiological consequences. However, it should be pointed out that at very high levels, phosphate can begin to chemically bind calcium and begin to reduce levels of free blood calcium.
External Calcium and Phosphate Balance
  • Overview
    • Exchange of calcium and phosphate with the outside world occurs in the alimentary tract and via renal excretion.
  • GI Exchange
    • GI absorption of calcium is fairly inefficient and only a third of ingested calcium is actually absorbed. Unabsorbed calcium is excreted in the feces along with any calcium lost in digestive secretions. Although inefficient, the quantity of GI calcium absorption is under hormonal regulation by Vitamin D. In contrast to calcium, GI phosphate resorption is highly efficient and nearly all ingested phosphate is absorbed by the alimentary tract in the absence of over regulation.
  • Renal Exchange
Internal Calcium and Phosphate Balance
  • Overview
    • Calcium and phosphate are present in bone in two basic structural forms which are utilized to buffer changes in extracellular calcium and phosphate concentrations on different timescales.
  • Hydroxyapatite:
    • This crystalline form of calcium and phosphate is a major element providing the structural strength of bone and composes ~70% of total bone mass. Resorption and deposition of calcium and phosphate in this crystalline form is a slow process that requires the action of osteoblasts and osteoclasts. Therefore, hydroxyapatite is used as a longer-term storage depot for the body's calcium and phosphate which can be used for extended changes in extracellular calcium levels.
  • Bone Salts
    • These are simple, amorphous, non-crystalline forms of calcium and phosphate which compose <1% of total bone mass. Resorption and deposition of calcium and phosphate in this salt form is extremely rapid and can occur within seconds to minutes. Therefore, Bone Salts are used as a rapid mechanism for buffering short-term changes in ECF calcium and phosphate concentrations.