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
References and Links
- CCC – Calcium metabolism
- CCC – Calcium basics
- CCC – Hypercalcaemia Overview
- CCC – Hypercalcaemia DDx
- CCC – Hypocalcaemia
- CCC – Hypocalcaemia DDx
- CCC – Calcium replacement (advantages/disadvantages)
- CCC – Calcium as an inopressor
Critical Care
Compendium
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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!