Calcium basics

Calcium is the most abundant mineral in the body

  • 1.5% of body weight; 99% is in bone matrix; 1100g (27mol)
  • Plasma Calcium is 2.5mmol/L
    • 50% of Ca in ECF is bound to albumin
    • 50% is active in ionised form and freely diffusible

[Ionised Calcium] is pH dependent

  • Alkalosis
    • Increase Calcium binding to albumin
    • Reduces active ionised Calcium in serum
  • Acidosis
    • Increase the active ionised calcium

Relation of calcium to albumin

  • Ionised Ca is inversely related to serum albumin
  • Total serum Ca is directly related to serum albumin
  • Total serum Ca is adjusted by 0.8 mg/dL for every 1G/dL change in serum albumin)
  • So in hypoalbuminaemia the serum Ca may measure low but the ionised Ca may be normal

Calcium control

Plasma Ca is under tight control and input usually equals output

  • PTH release stimulated by low ionised calcium levels
    • Increases resorption of bone by osteoclast stimulation,  increases GIT Calcium absorption (via Vitamin D), increases renal tubular reabsorption of Ca and increases Vitamin D synthesis
    • Essential to renal excretion of phosphate
      • ↑PTH causes a rise in serum Ca and fall in phosphate
      • ↓PTH causes increase in phosphate and decrease Calcium
  • Vitamin D increase GIT and kidney resorption
    • Made in liver converted to active form by the kidney
    • Promotes gut absorption of Ca and bone resorption
    • Causes rise in serum Ca and fall in PO4
  • Calcitonin decrease bone release and increase renal loss
    • Made in parafollicular thyroid cells
    • Increased renal excretion of Ca, inhibition of bone resoprtion
    • Causes fall in Calcium and Phosphate
    • Marker for medullary cell carcinoma

Importance of Calcium

  • Neutrophil chemotaxis, lymphocyte activation
  • Membrane stability (antagonises the effect of Mg and K at the cell membrane so is useful in treating toxic effects of these two cations)
  • Clotting factor cascade
  • Neurotransmitter release
  • Muscle cell depolarisation, myocardial contraction and bone strength
  • Bone formation

CCC 700 6

Critical Care


BA MA (Oxon) MBChB (Edin) FACEM FFSEM. Emergency physician, Sir Charles Gairdner Hospital.  Passion for rugby; medical history; medical education; and asynchronous learning #FOAMed evangelist. Co-founder and CTO of Life in the Fast lane | Eponyms | Books | Twitter |

One comment

  1. Hello. There is a slight error on this page. Under the actions of ‘Vitamin D’, it has been listed that VitD causes an increase in serum Ca and a fall in PO4, however VitD causes both Ca and PO4 to increase. Great summary of information though, such a useful page!

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