ANATOMY

Detailed explanation-1: -The ascending limb of loop of Henle is a segment of the nephron in the kidney divided into a thin and thick ascending limb. The thin ascending limb is impermeable to water and ions, except sodium and chloride, which cross by diffusion.

Detailed explanation-2: -The thick ascending limb of Henle’s loop actively reabsorbs sodium chloride (NaCl) but is impermeable to water. Therefore this segment raises the osmolality of the interstitial fluid, thus generating medullary interstitial hypertonicity and a lumen-to-interstitium osmotic gradient.

Detailed explanation-3: -The ascending limb is permeable to sodium chloride, which passes out of the tubule into the medullary tissue surrounding it.

Detailed explanation-4: -The descending limb is permeable to water but completely impermeable to salt, and thus only indirectly contributes to the concentration of the interstitium.