With an average of 1 kg of calcium in the body, calcium is one of the most abundant cations in the body. It has several functions:
- mineral component of bone
- clotting factor in clotting cascade (Factor IV)
- second messenger in intra-cellular signalling
- required in excitable tissues, particularly nerve and muscle
- enzymatic co-factor
99% of calcium is stored in a mineral form as hydroxyapatite in bone. The remaining 1% is found mainly in plasma, with small amounts in extra- and intra-cellular fluid.
Within the plasma, calcium exists in equilibrium between the 3 pools:
- ionised, 'free' calcium (~50%)
- calcium bound to albumin (~40%)
- unionised calcium salts, e.g. bicarbonate, citrate and phosphate (~10%)
Hydrogen ions compete with calcium for the binding sites on albumin. Any decrease in pH (i.e. acidosis/rise in hydrogen ion concentration) will cause calcium to become displaced. This will increase the unbound fraction and in extreme cases cause hypercalcaemia.
The main organs involved in calcium homeostasis are:
- parathyroid gland
- GI tract
The main hormones involved in calcium homeostasis are:
It is the 'free' portion of calcium that has its effects physiologically although this is very difficult to measure in the laboratory. Total calcium is reported and thus an adjustment must be applied to this figure. This is calculated by adding (or subtracting) 0.02 mmol of calcium for each g/l that albumin is below (or above, respectively) 40g/l.
- About 5% transient incidence after parathyroidectomy
- In acute hypocalcaemia (causing tetany) a soluble preparation such as calcium gluconate 10% is given slowly: typically for an adult 10ml but see local protocols as there is likely to be a need for continuous infusion of up to 9mmol/day until other measures kick in.
- Osteoporosis with calcium deficiency
- Other calcium deficiency states
- Hyperkalaemia as a temporary cardiac stabilising measure iv at similar doses to acute hypocalcaemia
- ↑ Tang BM, Eslick GD, Nowson C, Smith C, Bensoussan A. Use of calcium or calcium in combination with vitamin D supplementation to prevent fractures and bone loss in people aged 50 years and older: a meta-analysis. Lancet. 2007 Aug 25; 370(9588):657-66.(Link to article – subscription may be required.)
- ↑ Lappe JM, Heaney RP. Results may not be generalisable. BMJ (Clinical research ed.). 2008 Feb 23; 336(7641):403.(Link to article – subscription may be required.)
- ↑ Bolland MJ, Barber PA, Doughty RN, Mason B, Horne A, Ames R, Gamble GD, Grey A, Reid IR. Vascular events in healthy older women receiving calcium supplementation: randomised controlled trial. BMJ, doi:10.1136/bmj.39440.525752.BE (published 15 January 2008 accessed 16 January 2008)