Why is the selective permeability of the loop of Henle important for water reabsorption?

Question

What will be an ideal response?

Answer 1

For water to move out of a nephron by osmosis, the interstitial fluid surrounding the nephron must have a higher solute concentration than the filtrate inside the nephron. The solute concentration in the interstitial fluid is constant throughout the cortex, but increases with the distance into the medulla. Ions, particularly sodium, being pumped out of the ascending loop contribute to the high solute
concentration in the interstitial fluid.

The loop of Henle carries filtrate down into the medulla, then back into the cortex. The ascending and descending arms of the loop differ in their permeabilities to water and sodium. The descending arm of the loop is permeable to water but not to sodium. The loop's ascending arm is impermeable to water and actively pumps sodium into the interstitial fluid. A high solute concentration in interstitial fluid draws
water out of filtrate as it flows through the descending loop of Henle. As water is reabsorbed out of the filtrate the ion concentration builds up within the filtrate. Then, ions are actively transported out of the filtrate as it flows through the ascending loop.

Therefore, the difference in permeabilities allows this concentration gradient to be set up and maintained, which enhances water reabsorption.