Final answer:
Vitamin D3, initially produced in the skin, is converted to its active form, calcitriol, in the kidneys, playing a crucial role in calcium absorption and bone health.
Step-by-step explanation:
The active, or usable, form of vitamin D is indeed synthesized in the kidneys. This process involves the conversion of vitamin D into 1,25-dihydroxyvitamin D3, also known as calcitriol. Initially, the epidermal layer of the skin synthesizes vitamin D3, called cholecalciferol, when it is exposed to UV radiation.
Subsequently, the liver converts cholecalciferol into calcidiol, which travels to the kidneys. In the kidneys, calcidiol undergoes a hydroxylation reaction, where an -OH group is added, transforming it into calcitriol, the biologically active form of vitamin D. This form is essential in various physiological processes, such as facilitating calcium absorption from the small intestine, aiding calcium reabsorption from the kidneys, and contributing to the rebuilding of bone tissue.