Final answer:
Parathyroid hormone (PTH) regulates blood calcium levels by stimulating bone resorption, increasing intestinal calcium absorption through calcitriol, and enhancing calcium reabsorption in the kidneys.
Step-by-step explanation:
Parathyroid hormone (PTH) plays a crucial role in regulating calcium concentration in the blood. When blood calcium levels are low, the parathyroid glands secrete PTH, which then targets several organs to bring about an increase in calcium levels. In the skeleton, PTH stimulates the activity of osteoclasts, leading to bone resorption and the release of calcium into the bloodstream. Furthermore, it inhibits osteoblasts, which decreases the deposition of calcium back into bone tissue. In the intestines, PTH indirectly increases calcium absorption by promoting the formation of calcitriol, the active form of vitamin D3, which is necessary for calcium uptake from the diet. Lastly, in the kidneys, PTH facilitates the reabsorption of calcium back into the bloodstream, preventing its excretion in urine. Altogether, PTH is vital for maintaining calcium homeostasis, a critical function for numerous physiological processes.