Before the forming urine flows into the distal tubule of the nephron, it is in the , where water is absorbed as the fluid descends into the deep medulla of the kidney, and salts are absorbed as the fluid flows back up to the cortex of the kidney.

"Before the forming urine flows into the distal tubule of the nephron, it is in the loop of Henle, where water is absorbed as the fluid descends into the deep medulla of the kidney, and salts are absorbed as the fluid flows back up to the cortex of the kidney".

Explanation:

Henle´s loop is a specific section of renal tubules that forms the nephrons.

In this U-shaped region, the two parts of the contorted tubule, the distal and the proximal part, are very close to each other. In the kidney, both parts of the tubules are located in the renal cortex, while the loop of Henle is located within the renal medulla.

The proximal region of the Henle´s loop is a permeable region, unlike the proximal and distal regions of the renal tubule.  

This is why it is possible the interchange of water and ions in the Henle´s loop. In the loop´s walls, cells present aquaporines in their membranes that allow passive water absorption, while they do not allow the transport of ions and urea. In this way 25% of the water that passes through the kidney might be filtered.  

The ascendant side of the loop, which is narrower, the tubule wall is impermeable to water but allows the ions´reabsorption. Here, cells present sodium, potassium, and chloride channels.

Potassium sodium pump transporters are found in the basolateral zone. While in the apical area are active transporters of the 3 ions.

Due to these differences in the water and ions´absorption between the two parts of the Henle´s loop, a countercurrent multiplier effect is produced, stratifying the osmotic compounds of the medullary interstitium, being the part closest to the cortex less concentrated than the part closest to the renal papilla, which is about 4 times more concentrated in ions.

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