Which segment of the nephron is primarily responsible for water reabsorption and countercurrent exchange?

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Multiple Choice

Which segment of the nephron is primarily responsible for water reabsorption and countercurrent exchange?

The important idea is how the kidney uses osmotic gradients to pull water back into the body. In the descending limb of the loop of Henle, the filtrate moves into a region with a very high osmolality in the surrounding medullary interstitium. This limb is highly permeable to water but not to solutes, so as filtrate descends, water exits into the interstitium by osmosis. The filtrate thus becomes more concentrated as it goes deeper. This water-reabsorption process is a key part of the countercurrent mechanism that concentrates urine. The gradient that drives this reabsorption is maintained by the overall arrangement of the loop of Henle and the surrounding blood vessels (the vasa recta), which run in the opposite direction and exchange substances in a way that preserves the gradient. The other segments behave differently: the ascending limb is water-impermeable and actively transports solutes out, helping to set up the gradient but not reabsorb water; the proximal tubule reabsorbs a large amount of water early on, but not in the context of the loop’s countercurrent mechanism; the collecting duct can reabsorb water under control of ADH, but it is not the primary site that establishes the medullary gradient used for the countercurrent exchange. Therefore, the descending limb best fits as the segment primarily responsible for water reabsorption and supporting countercurrent exchange.

The important idea is how the kidney uses osmotic gradients to pull water back into the body. In the descending limb of the loop of Henle, the filtrate moves into a region with a very high osmolality in the surrounding medullary interstitium. This limb is highly permeable to water but not to solutes, so as filtrate descends, water exits into the interstitium by osmosis. The filtrate thus becomes more concentrated as it goes deeper.

This water-reabsorption process is a key part of the countercurrent mechanism that concentrates urine. The gradient that drives this reabsorption is maintained by the overall arrangement of the loop of Henle and the surrounding blood vessels (the vasa recta), which run in the opposite direction and exchange substances in a way that preserves the gradient.

The other segments behave differently: the ascending limb is water-impermeable and actively transports solutes out, helping to set up the gradient but not reabsorb water; the proximal tubule reabsorbs a large amount of water early on, but not in the context of the loop’s countercurrent mechanism; the collecting duct can reabsorb water under control of ADH, but it is not the primary site that establishes the medullary gradient used for the countercurrent exchange. Therefore, the descending limb best fits as the segment primarily responsible for water reabsorption and supporting countercurrent exchange.

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