Question

reabsorbs water and salt; creates a concentration gradient

Answer 1

The process you're describing is primarily associated with the **loop of Henle**, which is a part of the nephron in the kidneys. The loop of Henle plays a critical role in the reabsorption of water and salts (sodium and chloride ions) and is essential for creating a concentration gradient in the medullary interstitium.

Here’s a brief overview of how it works:

1. **Descending Limb**: The descending limb of the loop of Henle is permeable to water but not to salts. As the fluid descends, water is reabsorbed into the surrounding interstitial fluid, concentrating the urine.

2. **Ascending Limb**: The ascending limb, on the other hand, is impermeable to water but actively transports sodium and chloride ions out of the nephron into the surrounding interstitial fluid. This action dilutes the urine while contributing to the establishment of a high osmolarity in the medullary interstitium.

3. **Concentration Gradient**: The movement of water and salts helps create a concentration gradient that helps the kidney concentrate urine. This gradient is crucial for the function of the collecting ducts, where water reabsorption can be adjusted based on the body's hydration needs through the action of antidiuretic hormone (ADH).

Overall, through these mechanisms, the loop of Henle is essential for urine concentration and maintaining water and salt balance in the body.