[Renal medulla]

The ability to produce hyperosmotic urine allows mammals, including humans, to excrete their soluble mineral and organic waste products in the urine with a limited amount of water. The urinary concentrating capacity depends primarily on a special "loop-shaped" architecture of the nephrons and vessels, observed only in mammals. It also depends on the influence of antidiuretic hormone on the permeability to water of the collecting ducts on their entire length, and on the permeability to urea limited to the terminal portion of these ducts in the inner medulla. The ability to concentrate urine also requires (1) an "engine" able to produce an active (energy demanding) transport that generates a transepithelial concentration difference leading to increase the solute concentration in the surrounding interstitium; and (2) the expression of several membrane transporters or channels localized very specifically to limited portions of some nephron segments, collecting ducts and arterial vasa recta. These transporters and channels greatly accelerate the transport of water and some solutes across the cell membranes. Even if some nephron segments and parts of the collecting system are present in both the renal cortex and medulla (namely, the proximal tubule, the thick ascending limb and the collecting duct), their functions in the two renal zones may not be strictly similar, owing to their different peritubular environment, to the composition of the fluid running in their lumen, and to some differences in their epithelium. This paper describes some characteristics and specific functions of these structures in the renal medulla, as opposed to their corresponding structures in the cortex. These specific functions operating in the medulla may lead to various adverse consequences in some pathological situations.