What effect do parathyroid glands (PTH) have on blood calcium levels?

Parathyroid hormone (PTH), produced by the parathyroid glands, plays a crucial role in regulating blood calcium levels. It is responsible for increasing the concentration of calcium in the bloodstream when levels are low. PTH achieves this through several mechanisms:

1. Bone Resorption: PTH stimulates osteoclast activity, leading to the breakdown of bone tissue and the release of calcium into the bloodstream. This is one of the primary ways the body responds when calcium levels drop.

2. Kidney Function: In the kidneys, PTH promotes the reabsorption of calcium and reduces the reabsorption of phosphate. This selective reabsorption maximizes calcium retention while allowing for phosphate excretion, which can further aid in maintaining appropriate calcium levels.

3. Vitamin D Activation: PTH enhances the conversion of vitamin D into its active form, calcitriol, in the kidneys. Active vitamin D facilitates calcium absorption from the intestines, further contributing to increased levels of calcium in the blood.

Through these combined actions, PTH effectively raises blood calcium levels, making the selected answer accurate in describing the hormone's impact on calcium homeostasis. This is crucial for various physiological processes, including muscle contraction, nerve function, and