In no more than 2 short sentences describe the tubular mechanisms which result in urine hypertonicity?

Question

Im going through some old exam papers in preperation for a exam next month. Cant find the answer for this question. Something to do with medullary interstitium???

Answer 1

As the filtrate goes down the desending limb of the loop of Henle (down from the renal cortex into the medulla), water is pumped out into the surrounding tissue. So as it goes down, the filtrate gets more and more concentrated. When the filtrate goes back up the ascending limb, chloride is pumped out  into the medulla (and sodium follows, so NaCl is pushed out into the surrounding tissue). So the filtrate osmolarity gets back to what it was to start with as it goes back up. The result is, at the bottom of the loop of Henle, the filtrate is at its most concentrated/hypertonic, with a gradient of osmolarity from the bottom to the top of the loop. Over time the surrounding medullary tissue will equilibrate with this, so you get a very hypertonic medulla. The collecting ducts later plunge back down through the medulla, so if the kidney needs to retain more water, all it has to do is make the collecting ducts more permeable and the water will be drawn into the hypertonic medulla - causing the urine to get more concentrated.

Sorry, not 2 sentences!!