Calcium

Calcium is the most abundant mineral in the body

1.5% of body weight; 99% is in bone matrix; 1100gramm (27mmol)
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 dependant

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 hypoalbuminemia 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

Disorders of serum Calcium: