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Calcium physiology (Multimedia)
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Total body calcium amounts to around 1-2 kg (98% in skeleton).
The plasma level is around 8.5-10.4 mg%. In plasma it is found in 3 main
- Free ionized (Active) 45%
- Bound (Proteins) 40%
- Combined diffusible (Salts)15%
About 2/3 of the daily quantity of ingested calcium is excreted by the
gut the remaining 1/3 is absorbed into the circulation where it reaches
the extracellular fluid. From the extracellular fluid calcium reaches bone.
Finally it reaches the kidneys, here calcium is partially reabsorbed back
into the circulation. The remainder that is excreted is the same as the
amount absorbed from the intestines.
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Control of serum calcium
The level of calcium in the extracellular fluid is under hormonal control.
Parathyroid hormone secretion is stimulated once calcium level decreases
by a negative feedback mechanism. Parathyroid hormone secretion stimulates
kidney and bone to increase the plasma level of calcium. The kidneys do
so by distal tubular reabsorption of calcium and the activation of vitamin
D. Bone responds to parathormone by increasing osteolysis. Other hormones
involved include vitamin D and calcitonin. Vitamin D is responsible for
increasing the intestinal absorption of calcium while calcitonin antagonizes
the action of parathormone in bone by decreasing osteolysis.