Thin ascending limb of loop of Henle

Overview
As the filtrate passes back up the thin ascending limb of loop of Henle, the concentration of the surrounding medulla decreases. The thin ascending limb is not permeable to water, but it is permeable to ions.

Sodium ions (Na+) are passively transported down this concentration gradient, from the thin ascending limb into the medulla.

Histology
As in the descending limb, the epithelium is low simple cuboidal.