How does the water potential in the medulla of the kidney remain lower than the water potential in the loop of Henle?

The loop of Henle is designed so that the water moves out of the thinner walled descending limb by osmosis and then ions such as Na+ and Cl- are actively transported out of the ascending limb. This active transport allows the high salt concentration of the medulla to be maintained and therefore a lower water potential than the descending limb of the loop of Henle for continuous movement of water into the medulla.