PATHOLOGY

Detailed explanation-1: -The urea reabsorbed increases the medullary concentration of the solute, which is critical for the reabsorption of water from the thin inner medullary part of the descending limb of the loop of Henle. Here, there is no osmotic gradient to cause water movement in the diluting kidney.

Detailed explanation-2: -Vasopressin, upon binding to V2 receptors in the inner medullary collecting ducts, increases urea permeability through activation of UT-A1 molecules, thus enabling urea to diffuse into the inner medullary interstitium.

Detailed explanation-3: -In effect, reabsorbed urea is re-excreted into the loop of Henle, thereby augment-ing urea clearance. The efficiency of this cycle may be increased by the countercurrent exchange of urea in the vasa recta.