structural:scholar_edition, stylistic:style_transfer
Intestinal absorption

The active absorption of calcium from the gut is regulated by the calcitriol (or 1,25 dihydroxycholecalciferol, or 1,25 dihydroxyvitamin D) concentration in the blood. Calcitriol is a cholesterol derivative. Under the influence of ultraviolet light on the skin, cholesterol is converted to previtamin D which spontaneously isomerizes to vitamin D (or cholecalciferol). It is then converted from cholecalciferol to calcifediol in the liver. Under the influence of parathyroid hormone, the kidneys convert calcifediol into the active hormone calcitriol, which acts on the epithelial cells (enterocytes) lining the small intestine to increase the rate of absorption of calcium from the intestinal contents. In short the cycle is following: