The role of the collecting duct in urinary concentration

The various segments of the collecting duct have interesting and important roles in the urinary concentrating mechanism. On one hand, the collecting duct contributes insignificantly to the medullary interstitial concentrating process, but on the other hand, urinary concentration occurs as the result of water abstraction from the collecting duct as it courses from the cortex to the papillary tip. Thus, the collecting duct seems to do little work for medullary concentration. It, nevertheless, gets credit for regulation of the final urine osmolality. The latter occurs as a consequence of finely tuned mechanisms which regulate the collecting duct's water permeability in an ambience where the interstitial osmolality is controlled by highly integrated mechanisms involving numerous different nephron segments. The purpose of this article is to review the specific role of each segment of the collecting duct in the urinary concentrating mechanism, while other contributions in this Symposium will discuss the water permeability of the collecting duct and the generation and maintenance of the medullary interstitial osmolality