The loop of Henle is a U-shaped second segment of the renal tubule which is located between the proximal and distal convoluted tubules.
Reabsorption is minimum in its ascending limb. The ascending limb is impermeable to water but allows transport of electrolytes actively or passively. Therefore, as the concentrated filtrate pass upward, it gets diluted due to the passage of electrolytes to the medullary fluid. The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down.

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