Which hormone primarily regulates calcium levels in the blood?

Parathyroid hormone (PTH) is primarily responsible for regulating calcium levels in the blood. It is secreted by the parathyroid glands and plays a crucial role in maintaining calcium homeostasis. When blood calcium levels drop, PTH is released into the bloodstream, which increases calcium levels by promoting several key processes.

PTH enhances the reabsorption of calcium from the kidneys, reducing its excretion in urine. It also stimulates the release of calcium from bones by promoting the activity of osteoclasts, which break down bone tissue and release stored calcium into the bloodstream. Additionally, PTH increases the intestinal absorption of calcium by promoting the activation of vitamin D, which facilitates the absorption of calcium from dietary sources.

Overall, PTH works to elevate blood calcium levels when they are low, ensuring that critical physiological processes that depend on calcium, such as muscle contraction and nerve function, can occur effectively.