The distal part of the loop of Henle that is impermeable to water and actively pumps sodium out of the filtrate to generate the hypertonic interstitium that drives countercurrent exchange; it consists of an initial very thin segment lined by simple squamous epithelium followed by a distal thick segment lined by simple cuboidal epithelium[MP]. [ http://en.wikipedia.org/wiki/Ascending_limb_of_loop_of_Henle ]