Reabsorption Of Calcium & Magnesium from the Kidneys : Renal physiology USMLE Step 1

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Reabsorption Of Calcium & Magnesium from the Kidneys : Renal physiology USMLE Step 1

Calcium and magnesium are two important minerals that are essential for various physiological processes in the human body. Both of these minerals are filtered by the kidneys and reabsorbed back into the blood, where they can be used by the body. Here is a detailed explanation of the reabsorption of calcium and magnesium from the kidneys:

Reabsorption of Calcium:
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Filtration: Calcium is filtered from the blood in the glomerulus of the nephron, which is the basic functional unit of the kidney.

Proximal Tubule: About 65% of the filtered calcium is reabsorbed in the proximal tubule of the nephron. This process is passive and occurs through paracellular transport, which means that calcium moves between the cells of the tubule rather than through the cells themselves.

Loop of Henle: The descending limb of the loop of Henle is impermeable to calcium, while the ascending limb actively transports calcium out of the tubule and into the interstitial fluid surrounding the tubule.

Distal Tubule: In the distal tubule, the reabsorption of calcium is regulated by the hormone parathyroid hormone (PTH), which is released by the parathyroid gland in response to low levels of calcium in the blood. PTH stimulates the reabsorption of calcium by increasing the expression of calcium channels and pumps in the cells of the distal tubule.

Collecting Duct: The collecting duct is the final site of calcium reabsorption in the nephron. The reabsorption of calcium in this segment is also regulated by PTH, as well as by the hormone calcitriol, which is the active form of vitamin D. Calcitriol stimulates the production of calcium-binding proteins in the cells of the collecting duct, which facilitate the reabsorption of calcium.

Reabsorption of Magnesium:
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Filtration: Magnesium is filtered from the blood in the glomerulus of the nephron.

Proximal Tubule: About 20-25% of the filtered magnesium is reabsorbed in the proximal tubule of the nephron. This process is passive and occurs through paracellular transport, similar to the reabsorption of calcium in this segment.

Loop of Henle: The ascending limb of the loop of Henle actively transports magnesium out of the tubule and into the interstitial fluid surrounding the tubule. This segment is particularly important for the reabsorption of magnesium, as it accounts for about 50-60% of the total reabsorption.

Distal Tubule: The reabsorption of magnesium in the distal tubule is regulated by the hormone PTH, which stimulates the expression of magnesium channels and pumps in the cells of the tubule.

Collecting Duct: The collecting duct is the final site of magnesium reabsorption in the nephron. The reabsorption of magnesium in this segment is also regulated by PTH, as well as by the hormone calcitonin, which is released by the thyroid gland in response to high levels of magnesium in the blood.

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