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 [ http://www.informatics.jax.org/accession/anna ]