In the arteriole it is returned to pyramid. Similarly, water leaves the arteriole and flows into the venule. The sodium is thus carried back to the base of the medulla in a continual flow and is prevented from escaping, while the osmotic gradient is preserved. The loop of Henle is slightly more complex. The descending limb is permeable to water, but not to sodium.
Water is drawn out by osmotic pressure, and the urine is concentrated.
The ascending limb is permeable to sodium, but not to water. The urine returning to the cortex is much reduced in volume and water content compared to the filtrate in the first part of the nephron. Urine may be concentrated still further as it descends once more into the medulla through the collecting duct. The collecting duct is impermeable to sodium, but allows water to escape. At its most permeable, the collecting duct concentrates urine to nearly the same level as the tissues of the medulla.