The medullary osmotic gradient depends on the permeability of what?

Question

Is that the ascending or descending loop?

Answer 1

The ascending limb of the Henle's loop.

Unlike the descending limb, the ascending limb of Henle's loop is impermeable to water, a critical feature of the countercurrent exchange mechanism employed by the loop. The ascending limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives countercurrent exchange. In passing through the ascending limb, the filtrate grows hypotonic since it has lost much of its sodium content.

Answer 2

The Loop of Henle.