What occurs in the descending loop of Henle?

• A. Secretion of waste products.
• B. Reabsorption of water continues through aquaporin channels.
• C. Active reabsorption of Na+.
• D. Water cannot be reabsorbed.

Answer: (C)

The important idea is that the descending limb of the loop of Henle is highly permeable to water but not to solutes. Water exits the filtrate through aquaporin channels into the surrounding hyperosmotic medullary interstitium, so the filtrate becomes more concentrated as it moves downward. There is little to no active transport of ions like Na+ in this segment; active Na+ reabsorption occurs mainly in the ascending limb, which is not water-permeable. Secretion of waste products isn’t a defining feature of the descending limb, and claiming that water cannot be reabsorbed is incorrect because water reabsorption via aquaporins is the key process here.