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How do the kidneys regulate water balance in the body?
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renal system
Diagram showing the location of the kidneys in the abdominal cavity and their attachment to major arteries and veins.
The kidneys regulate water balance by adjusting the volume of water reabsorbed into the bloodstream during the formation of urine. This process relies on the nephron, the kidney's functional unit, which filters blood and selectively reabsorbs water and essential solutes.123
Mechanisms of Water Regulation
- Filtration and Reabsorption: Every day, the kidneys filter a massive volume of fluid from the blood. The vast majority of this water is reabsorbed back into the blood as it passes through the renal tubules.12
- Countercurrent Multiplier System: The loop of Henle creates a high concentration of salt in the medullary fluid. This osmotic gradient is essential for the kidney's ability to concentrate urine and conserve water.4
- Hormonal Control: The hormone antidiuretic hormone (ADH) is the primary regulator of water excretion. When the body is dehydrated, ADH increases the permeability of the collecting ducts, allowing more water to be reabsorbed into the blood. Conversely, when water intake is high, ADH levels drop, making the ducts less permeable and resulting in the excretion of dilute urine.45
This feedback system, sensitive to plasma tonicity and blood volume, ensures that the body maintains internal equilibrium despite varying environmental conditions or fluid intake.14