Chapter 51 - Salt and Water Balance and Nitrogen Excretion - 51.5 - The Mammalian Kidney Can Produce Concentrated Urine - 51.5 Recap - Learning Outcomes - Page 1109: 2

The concentration of urine begins in the ascending limb of the loop of Henle, which is responsible for reabsorbing salt but is impermeable to water. A fraction of the reabsorbed NaCl enters the descending limb, which traps the salt in the renal medulla and prevents a washed out gradient.

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