What does Parathyroid hormone (PTH) regulate?

Parathyroid hormone (PTH) plays a crucial role in the regulation of calcium levels in the body. It primarily exerts its effects on the bones, kidneys, and gastrointestinal (GI) tract. When blood calcium levels are low, PTH is released from the parathyroid glands. In bones, PTH stimulates the release of calcium by promoting the activity of osteoclasts, which break down bone tissue. This process leads to an increase in the calcium released into the bloodstream.

In the kidneys, PTH enhances the reabsorption of calcium, thus reducing calcium excretion in the urine. It also promotes the conversion of vitamin D into its active form, which facilitates the absorption of calcium and phosphate from the diet in the GI tract. Together, these actions work to restore and maintain optimal calcium levels in the bloodstream.

The regulation of calcium is vital for numerous physiological functions, including muscle contraction, blood clotting, and nerve function, making PTH integral to maintaining homeostasis within the body. This emphasizes why the correct answer focuses on calcium balance in bones, kidneys, and the GI tract.