Final answer:
The parathyroid hormone (PTH) is released to raise blood calcium levels by enhancing kidney reabsorption, stimulating osteoclasts, and promoting calcitriol production. Calcitonin works oppositely, decreasing blood calcium when levels are high. Both hormones are vital for calcium homeostasis.
Step-by-step explanation:
Parathyroid Hormone and Calcium Regulation:
The parathyroid hormone (PTH) is crucial in regulating calcium levels in the blood. Produced by the parathyroid glands, PTH is released when blood Ca2+ levels are low. It has three primary functions: (1) it enhances the reabsorption of Ca2+ in the kidneys, (2) stimulates osteoclast activity which releases calcium into the blood by breaking down bone and (3) promotes the synthesis of calcitriol, which increases intestinal absorption of calcium. The actions of PTH raise blood Ca2+ levels to maintain a necessary balance for normal physiological functions.
Comparatively calcitonin which is produced by the thyroid gland, has the opposite effect. When blood calcium levels are too high calcitonin decreases these levels by inhibiting osteoclasts stimulating bone formation, and excreting calcium via the kidneys. This hormone plays a crucial role in children, during pregnancy and in periods of prolonged starvation, although its role in healthy nonpregnant unstarved adults is less clear. The balance between PTH and calcitonin is essential for maintaining the homeostasis of blood calcium levels, ensuring proper function of the muscular and nervous systems, as well as bone health.