GENERAL HISTOLOGY

Detailed explanation-1: -Vitamin D is actually a hormone rather than a vitamin; it is required to absorb calcium and phosphate from the gut into the bloodstream. Vitamin D is mostly produced in the skin in response to sunlight and is also absorbed from food eaten (about 10% of vitamin D is absorbed this way) as part of a healthy balanced diet.

Detailed explanation-2: -Sun exposure is the primary source of vitamin D for most people. Upon exposure to ultraviolet-B radiation, previtamin D3 is synthesized in keratinocytes of the epidermis.

Detailed explanation-3: -Vitamin D functions by stimulating intestinal calcium and phosphorus absorption, by stimulating bone calcium mobilization, and by increasing renal reabsorption of calcium in the distal tubule. These functions on bone and possibly kidney, but not intestine, require the parathyroid hormone.