The osmolality of the urine excreted by the body can be as high as the osmolality in the medulla's interstitial fluid.

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The high osmolality in the medulla's interstitial fluid creates a concentration gradient that specifically allows for the reabsorption of water. This concentration gradient is created by the active transport of sodium and other solutes out of the ascending limb of the loop of Henle and the passive transport of water out of the descending limb, which concentrates the filtrate. As the filtrate travels through the collecting duct, it passes through areas of varying osmolality created by the countercurrent mechanism of the loop of Henle, allowing for water to be reabsorbed or permeability to be decreased depending on the body's need for water conservation or excretion. Ultimately, the urine excreted can reach the same high osmolality as the interstitial fluid in the medulla.

The Osmolality of the Urine Excreted by the Body Can