How is blood calcium level primarily regulated?

Blood calcium levels are primarily regulated by parathyroid hormone (PTH) and vitamin D. Parathyroid hormone is secreted by the parathyroid glands in response to low blood calcium levels. It then acts on bones, kidneys, and the intestines to increase calcium levels in the blood. In bones, PTH stimulates the release of calcium by promoting the activity of osteoclasts, which break down bone tissue. In the kidneys, PTH reduces the excretion of calcium, leading to increased reabsorption. Additionally, PTH influences vitamin D metabolism, promoting the conversion of vitamin D into its active form (calcitriol), which enhances calcium absorption from the intestines.

Vitamin D itself is crucial for calcium homeostasis as it facilitates the intestinal absorption of calcium and helps maintain calcium and phosphate balance. The combined action of parathyroid hormone and vitamin D ensures that blood calcium levels remain within a narrow, healthy range, which is vital for various physiological functions, including bone health, muscle function, and nerve signaling.