descending limb of loop of Henle

The portion of the renal tubule that constitutes the proximal part of the loop of Henle, has low permeability to ions and urea, and is highly permeable to water; it consists of an initial short thick segment lined by low simple cuboidal epithelium and a long thin segment lined by simple squamous epithelium; however, this distinction is not as important physiologically as in the ascending limb, so often the two are treated as one structure.