In the inner medulla, urea recirculates, creating a greater osmotic gradient. It follows the concentration gradient formed by ADH-sensitive water reabsorption into the interstitial space from the collecting duct. The remaining water is drawn from the descending structures by increased interstitial osmolarity.
To maintain the vertical osmotic gradient, the blood supply to the renal medulla serves as a countercurrent exchanger. - The medullary vertical osmotic gradient is maintained by the vasa recta.
It enhances both water and urea permeability in the inner medullary collecting ducts, allowing urea to diffuse passively down its concentration gradient into the interstitial fluid. This contributes to the osmotic gradient and aids water absorption.
