In the loop of Henle, what is the primary function of the descending limb?

• A. Reabsorption of water by osmosis
• B. Reabsorption of sodium and chloride
• C. Secretion of hydrogen ions
• D. Reabsorption of glucose

Answer: (A)

Water reabsorption by osmosis is the main action of the descending limb. It is highly permeable to water due to abundant aquaporin channels, and as filtrate moves deeper into the hyperosmotic medullary interstitium, water exits into the surrounding tissue. This leaves the tubular fluid more concentrated. The limb is relatively impermeable to solutes, so ions don’t follow, which helps establish the medullary osmotic gradient that the rest of the loop uses to concentrate urine. The other options don’t fit because sodium and chloride reabsorption happens mainly in the ascending limb (not water-permeable), hydrogen ion secretion occurs in the distal nephron and collecting duct, and glucose reabsorption occurs in the proximal tubule.