The ascending loop of Henle is a portion of the nephron in the kidney. It connects to the descending loop of Henle and runs through the medulla of the nephron in the kidney. Together, the descending and ascending loops of Henle reabsorb water and ions for the kidney.
The ascending loop of Henle and the descending loop of Henle have different types of cells which have different permeabilities. The descending portion is permeable to water and allows water to be absorbed by the nephron to be excreted as urine. It can also allow water to be reabsorbed by the body.
The ascending loop of Henle is permeable to ions, such as sodium and potassium, and it is not permeable to water. Ions can leave or enter the urine through the ascending loop of Henle, depending on what the body needs.