Which describes water reabsorption in the descending limb of the loop of Henle?

• A. Aquaporin channels facilitating water reabsorption.
• B. Sodium-potassium pumps drive water movement.
• C. Water reabsorption is minimal.
• D. Water is secreted into the filtrate.

Answer: (A)

Water movement in the descending limb is driven by osmosis through aquaporin channels, making this segment highly permeable to water. As filtrate travels deeper into the medulla, the surrounding interstitial fluid is very hyperosmotic, so water exits the tubule into the interstitium via those aquaporin channels, concentrating the filtrate. This process is passive and not driven by pumps, and the limb is not a site of water secretion or minimal water movement. The pumps that actively move ions occur elsewhere (and create the gradient), not to move water here.