Final answer:
Parathyroid hormone (PTH) from the parathyroid glands regulates blood calcium and phosphorus levels by stimulating calcium reabsorption and inhibiting phosphate reabsorption in the kidneys, affecting bone resorption and production of active vitamin D.
Step-by-step explanation:
The parathyroid glands are responsible for producing parathyroid hormone (PTH), which is vital for maintaining the balance between calcium levels in the blood and phosphorus. PTH achieves this by stimulating the reabsorption of calcium in the kidneys, increasing the activity of osteoclasts (which release calcium from bones), and suppressing osteoblast activity (to spare blood calcium).
Additionally, PTH promotes the conversion of calcidiol to calcitriol (active vitamin D) in the kidneys, which further enhances the absorption of calcium from the diet. Conversely, PTH blocks the reabsorption of phosphate, leading to its excretion, which prevents the formation of calcium phosphate and maintains higher circulating calcium levels. When the blood calcium levels rise, the secretion of PTH is inhibited, which is part of a negative feedback loop crucial for calcium homeostasis in the body.