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Module 7: Figure Ca2+ homoeostasis

Regulation of Ca2+ homoeostasis by multiple hormonal and organ effector systems.

The concentration of Ca2+ within the plasma is tightly regulated within a normal range between 1.1 and 1.3 mM. On a daily basis, there is a net input of approximately 200 mg from the intestine, and this is balanced by a similar loss in the urine. The rates of bone resorption and bone formation are normally balanced at about 400 mg/day. Hormonal regulation is based on the release of hormones that respond to either low (hypocalcaemia) or high (hypercalcaemia) plasma Ca2+ concentrations. Low Ca2+ together with the resulting release of parathyroid hormone (PTH) can also activate vitamin D metabolism to form the hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], which also functions in the regulation of Ca2+ homoeostasis.