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Calcium metabolism and Homeostasis : Endocrine Physiology USMLE Step 1

Calcium metabolism and homeostasis play an important role in maintaining normal physiological function in the human body. The regulation of calcium levels is primarily controlled by the parathyroid hormone (PTH), calcitonin, and vitamin D.

Parathyroid hormone is secreted by the parathyroid glands and plays a crucial role in increasing the serum calcium levels. PTH increases serum calcium levels by stimulating osteoclast activity, which leads to bone resorption and the release of calcium into the bloodstream. PTH also increases the reabsorption of calcium in the kidney tubules and increases the production of vitamin D, which enhances intestinal absorption of calcium.

Calcitonin is secreted by the C cells in the thyroid gland and plays a role in reducing serum calcium levels. Calcitonin inhibits osteoclast activity and promotes bone formation, which leads to decreased calcium release from the bone. It also decreases calcium reabsorption in the kidney tubules.

Vitamin D plays a critical role in calcium homeostasis by increasing calcium absorption in the intestines. Vitamin D is synthesized in the skin when exposed to sunlight and is also obtained from dietary sources. In the liver, vitamin D is converted to 25-hydroxyvitamin D [25(OH)D], which is the major circulating form of vitamin D. In the kidney, 25(OH)D is converted to the active form of vitamin D, 1,25-dihydroxyvitamin D [1,25(OH)2D], which promotes calcium absorption in the intestines.

Overall, the regulation of calcium homeostasis involves a complex interplay between the parathyroid hormone, calcitonin, and vitamin D. Any disruptions in this system can lead to disorders such as hypercalcemia, hypocalcemia, and osteoporosis. Understanding these mechanisms is crucial for medical students preparing for the USMLE Step 1 exam.

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