How does the body maintain a steady supply of calcium for its tissues to support functions such as muscle contractions, blood pressure regulation, and the conduction of neural impulses?
Vitamin D (as calcitriol) simultaneously increases calcium absorption in the small intestine, decreases calcium excretion in the urine, and facilitates the release of calcium from bones:
• Small intestine: Calcitriol is required for calcium absorption in the small intestine. Acting as a cell signal, it up-regulates several genes that code for proteins that transport dietary calcium into enterocytes. (Without vitamin D, these proteins are not made, and calcium absorption is severely limited.)
• Kidney: Calcitriol and PTH cause the kidneys to reduce their excretion of calcium into the urine. As a result, more calcium remains in the blood.
• Bone: Calcitriol and PTH together stimulate bone breakdown by osteoclasts, in turn resulting in the release of calcium into the blood.
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