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I know that the ascending loop of Henle is impermeable to water and ions, and, by end of the ascending limb, the osmolarity of the tubular fluid is very low due to the active transport of ions out (e.g, sodium, potassium, chloride).

However, what if the person had to produce hypertonic urine due to dehydration or some other circumstance? Would the increase in osmolarity be done through a decrease in removal of ions in the descending limb, or has the distal tubule and collecting ducts got something to do with it?

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aquaporins in the distal tubule absorb water. they are controlled by vasopressin (anti-diuretic hormone) –  WYSIWYG Apr 19 '13 at 4:21

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