What role do the kidneys play in calcium metabolism?

The kidneys play a crucial role in calcium metabolism primarily through the conversion of vitamin D3 to its active forms, calcidiol (25-hydroxyvitamin D) and calcitriol (1,25-dihydroxyvitamin D). This process is essential for maintaining calcium homeostasis in the body.

When vitamin D3 (cholecalciferol) is ingested or synthesized in the skin, it first gets converted to calcidiol in the liver. The kidneys then further convert calcidiol to calcitriol, the active form of vitamin D. Calcitriol is significant because it enhances intestinal absorption of calcium and phosphate, increases bone resorption, and reduces renal excretion of calcium, thus playing a key role in the regulation and metabolism of calcium levels in the blood.

The other options related to calcium metabolism do not accurately reflect the specific and active role the kidneys have in this process. For instance, while the body does store calcium in the bones, the kidneys are not responsible for this storage. The secretion of calcium primarily occurs through the urinary system, but it is not the main functional aspect of the kidneys concerning calcium metabolism. Similarly, the kidneys do not transport calcium; instead, their function hinges on the enzymatic conversion