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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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The process of urine concentration in mammals occurs due to the interaction of the loop of Henle and its nearby collecting tubule. When someone is dehydrated, the kidney will aim to a maximal water reabsorption. To concentrate urine, a maximal amount of ions will be reabsorbed. This is to maximalize the concentration gradient for water. However, the main driving force for water reabsorption is the urea concentration gradient (created by the loop of Henle). Then aquaporins will allow water to flow from the collecting tubule to the nearby medulla because of the presence of the neighboring loop of Henle. It is this proximity between the loop of Henle and the collecting tubule that allows such a high efficiency for water reabsorption.

Concentrating urine is not about leaving more ions in the water, it is about reabsorbing a maximal amount of water.

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