Answer to Question 1
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.
Answer to Question 2
Joan is correct. Regardless of whether it is consumed in the diet or produced in the skin, cholecalciferol must be metabolized further before it can be used. This two-step process occurs in the liver and kidneys. First, cholecalciferol (vitamin D3) is converted to 25-hydroxyvitamin D 25-(OH) D3 in the liver. Then 25-(OH) D3 is circulated in the blood to the kidneys, where it is converted to 1,25-dihydroxyvitamin D 1,25-(OH)2 D3. It is the 1,25-(OH)2 D3 form, also called calcitriol, that is active in the body. Calcitriol is important for calcium absorption, and conversion of 25-(OH) D3 to calcitriol (1,25-(OH)2 D3) increases when calcium concentration in the blood is low. This increase is stimulated by the actions of parathyroid hormone (PTH) produced in the parathyroid glands when more calcium is needed.
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