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  • Created by: jo bill
  • Created on: 23-12-14 11:27

calcium in the body

  • 1000g in total
  • 99% in bone mainly as the calcium phosphate compound hydroxyapatite
  • calcium means total; Ca2+ means ionized calcium
  • cytosolic Ca2+ < 100 nM
  • calcium in interstitial fluid 1.5 mM
  • calcium in blood 2.5 mM

calcium in blood

  • usually total calcium and albumin are measured and an adjusted value can be calculated if albumin deviates from 40g/l
  • ionised Ca2+ can be measured with an ion-selective electrode
  • acidosis increases plasma Ca2+ because H+ binds to albumin displacing Ca2+
  • alkalosis has the opposite effect
  • increases in blood phosphate will decrease Ca2+

calcium balance

  • intestine : carrier mediated, regulated by 1,25 (OH)2D3
  • formation and reabsorption of bone controlled by parathyroid hormone
  • Renal : 90% reabsorbed in proximal tubule coupled to Na+; active transport in distal tubule/collecting duct regulated by 1,25(OH)2D3 and parathyroid hormone

parathyroid hormone

  • single polypeptide chain of 84 amino acids
  • secreted by chief cells of the parathyroid glands
  • 1-5 pmol/l in blood
  • metabolised by liver and kidney to inactive fragments which can accumulate to higher concentrations than the intact peptide
  • presensce of amino terminus critical for activity


  • low extracellular Ca2+ stimulates parathyroid hormone secretion
  • high extracellular Ca2+ inhibits parathyroid hormone secretion
  • regulation of secretion is mediated by a calcium sensing G-protein- linked receptor on the chief cells
  • bimnding of Ca2+ lowers cyclic AMP


  • parathyroid hormone raises blood calcium
  • the receptor for parathyroid hormone is 7-transmemrane G protein receptor linked to both Gas and Gaq

acts on bone

  • osteoblasts cause bone formation
  • during growth some osteoblasts get trapped and differentiate into osteocytes
  • osteoclasts are large multinuclear cells that reabsorb bone

cause bone reabsorption

  • parathyroid hormone receptor is on osteoblasts
  • osteoblasts release M-CSF and form…


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