The Calcium Homeostatic Mechanism

Diagramatic repiesentation of calcium homeostatic mechanisms involving the vitamin D endocrine system located in the kidney. Note that the calcium sensing organs are the para-thryoid glands (hypocalcemia) and the C-cells of the thyroid (hypercalcemia). Note that the vitamin D hormone acts by itself on the intestine and together with parathyroid hormone in bone and kidney 

It is now clear that the vitamin D endocrine system is a major factor in the control of plasma calcium and the overall calcium economy of terrestial vertebrates The parathyroid glands monitor calcium concentration of the plasma (Fig. 5) and in response to low blood calcium secrete the parathyroid hormone Parathyroid hormone is taken up by the kidney and bone In the kidney, parathyroid hormone stimulates production of 1,25-(OH)2D3 The 1,25 (OH)2D3 then 

Since 1,25-(OH)2D3 production responds slowly to parathyroid hormone a more quickly-reacting system is needed to prevent low calcium tetany. Parathyroid hormone secretion and action is very rapid as is the lifetime of the hormone itself Thus parathyroid hormone can, with endogenous 1,25-(0H)2D3, stimulate the mobilization of calcium from bone and renal reabsorption of calcium but not intestinal absorption Nevertheless, plasma calcium rises but 

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