Final answer:
Calcium reabsorption in the kidneys is facilitated by parathyroid hormone (PTH), which is released in response to low blood calcium levels, enhancing kidney reabsorption, releasing calcium from bones, and indirectly promoting intestinal absorption through calcitriol production.
Step-by-step explanation:
Calcium reabsorption by the kidneys is promoted by the hormone called parathyroid hormone (PTH). When blood calcium levels are low, PTH is secreted by the parathyroid glands. It works by stimulating the release of calcium from bone by increasing the activity of osteoclasts, reducing the excretion of calcium in the urine by promoting its reabsorption in the kidney tubules, and indirectly increasing calcium absorption in the small intestine by stimulating the synthesis of calcitriol (the active form of vitamin D).
PTH has several effects on calcium homeostasis: it enhances the reabsorption of calcium ions by the kidneys, stimulates osteoclasts to release calcium from bones, and, indirectly, increases intestinal absorption of calcium through the production of calcitriol. This hormone thus plays a critical role in maintaining calcium balance in the body, especially when calcium levels in the blood begin to drop.