In the renal nephron, different segments have varying permeability to water and solutes, which plays a crucial role in the kidney's ability to concentrate or dilute urine.

- The ascending limb of the loop of Henle is uniquely characterized by its impermeability to water under normal physiological conditions.
- This means that as the filtrate moves up this segment, water cannot follow the solutes by osmosis.
- Instead, the thick ascending limb actively reabsorbs ions such as sodium, potassium, and chloride, which helps to dilute the tubular fluid and contributes to the creation of a hyperosmotic medullary interstitium.

In contrast:
- The proximal convoluted tubule is highly permeable to water, allowing significant reabsorption of water alongside solutes.
- The distal convoluted tubule is relatively impermeable to water but can become permeable in the presence of antidiuretic hormone (ADH).
- The descending limb of the loop of Henle is permeable to water but not to solutes, allowing water to be reabsorbed and concentrating the tubular fluid.

Therefore, the ascending limb of the loop of Henle plays a critical role in creating the dilute tubular fluid necessary for urine concentration, by actively reabsorbing electrolytes while preventing water movement.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th Edition, Chapter 26 - Urine Formation by the Kidneys, Page 317-320