The loop of Henle, which has a thick descending portion (pars recta), a thin descending portion, a thin ascending portion, and a thick ascending portion.
The ascending and descending loops lie next to each other, and there is an increasing osmotic gradient from the cortex to the tip. Urea, sodium and chloride ions are actively transported into the interstitial space of the medulla, by the ascending limb, and these are taken up by the descending limb, which has a lower osmotic concentration.
This means that moving from the cortex into the medulla, the salt concentration in the interstial space increases becoming more and more hypertonic, relative to the fluid passing through the collecting ducts, helping to extract water from the filtrate, and concentrate the urine.
The lumen is made up of simple squamous epithelium. This part of the nephron is hard to tell apart from adjacent capillaries, except that there are no red blood cells in the lumen.