Reabsorption is minimum in its ascending limb which plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid.

The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes so water can flow out. This concentrates the filtrate as it moves down.

The ascending limb is impermeable to water but allows transport of electrolytes. Therefore, as the concentrated filtrate passes through the ascending limb, it gets diluted due to the passage of electrolytes to the medullary fluid.