What happens in the ascending loop of Henle?

• A. It reabsorbs water and makes filtrate more concentrated.
• B. It is permeable to water and concentrates filtrate.
• C. Na+ is actively pumped out and it is not permeable to water, decreasing filtrate concentration.
• D. It is the main site of glucose reabsorption.

Answer: (D)

The ascending loop of Henle functions to dilute the tubular fluid. Its walls are impermeable to water, while ions are actively pumped out of the filtrate into the surrounding tissue—primarily Na+, K+, and Cl− via transporters in the thick ascending limb. This active ion reabsorption lowers the osmolality of the filtrate as it ascends, making the filtrate dilute, even as the interstitial fluid around the loop becomes more concentrated. This countercurrent multiplication helps establish the medullary gradient used later to concentrate urine.

Water reabsorption and concentrating filtrate occur mainly in the descending limb and the collecting ducts, not in the ascending limb. Glucose reabsorption happens predominantly in the proximal tubule, not in the loop of Henle.