What hormone regulates the level of calcium in blood plasma?

The hormone that regulates the level of calcium in blood plasma is parathyroid hormone. This hormone is produced by the parathyroid glands and plays a critical role in maintaining calcium homeostasis in the body. When blood calcium levels drop, parathyroid hormone is released, which increases calcium levels by stimulating several processes.

It promotes the release of calcium from bones by activating osteoclasts, which break down bone tissue. Additionally, parathyroid hormone enhances the absorption of calcium from the gastrointestinal tract by promoting the activation of vitamin D. It also acts on the kidneys to decrease the excretion of calcium, allowing for more calcium to be reabsorbed back into the bloodstream. Together, these actions ensure that blood calcium levels remain within a tight range, which is vital for various physiological functions, including muscle contraction, nerve signaling, and blood clotting.

In contrast, insulin and glucagon are involved in glucose metabolism and do not directly impact calcium levels, while thyroid hormone primarily affects metabolism and growth rather than calcium regulation. The specificity of parathyroid hormone's actions on calcium balance makes it the key hormone for this physiological process.