Kidneys primarily regulate calcium levels by reabsorbing most of the filtered calcium back into the bloodstream; however, they also excrete some calcium in urine to maintain calcium balance.
The kidneys play a crucial role in maintaining calcium homeostasis by filtering calcium from the blood and then either reabsorbing it back into the bloodstream or excreting it in the urine. The balance between reabsorption and excretion determines the final calcium concentration in the body.
Here's a breakdown of how kidneys handle calcium:
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Filtration: The kidneys filter calcium from the blood into the renal tubules.
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Reabsorption: Most of the filtered calcium is reabsorbed back into the bloodstream. According to provided reference, more than 95% of filtered calcium is reabsorbed along the renal tubules. This process occurs in different parts of the renal tubules:
- Proximal Tubules: 60% of filtered calcium is reabsorbed here through passive mechanisms.
- Thick Ascending Limb: 15% of calcium is reabsorbed by paracellular diffusion through paracellin-1 (claudin-16).
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Excretion: Any calcium that is not reabsorbed is excreted in the urine.
In summary, kidneys don't primarily remove calcium, but rather regulate calcium levels by carefully controlling how much is reabsorbed back into the body versus how much is excreted in urine. The majority is reabsorbed, ensuring the body has enough calcium for its various functions.
