I believe it is the ascending loop of henle. In the ascending portion, the loop becomes impermeable to water and instead the cells of the loop, actively reabsorb the solute; thus water is not reabsorbed but ions are readily reabsorbed. As the ions are reabsorbed the concentration becomes more and more hypotonic until it reaches 100-150 mOsm/L. Therefore, the ascending loop is called the diluting segment of the nephron due to its ability to dilute the fluid in the loop from 1200 to 100 mOsm/L.
