Editorial review. Recent formulations of the urinary concentrating mechanism: a status report.

The status of the purely passive mode of solute concentration as of 1979 appears to be similar to that of the original countercurrent hypothesis 10 years ago. The passive mode concept has advanced our understanding of the concentrating process by qualitatively incorporating the permeability characteristics of tubule segments and the lack of an active transport process in the thin loop of Henle into a mechanism which has attractive economy and explanatory value. But in the final analysis some assumptions are not borne out by experimental findings (for example, the high urea concentration of fluid in the rat and hamster end-descending limb; the likelihood of net transepithelial addition of sodium chloride to the Psammomys descending limb; the removal of sodium chloride from the hamster ascending limb against an apparent electrochemical gradient under certain circumstances; and the osmotic lag between vasa recta blood and interstitium in the rat). Furthermore, when the known permeability and transport characteristics of the renal tubule are incorporated into a mathematic model of the passive operating mode, numerical simulations fail to establish a progressively hyperosmotic inner medulla. This does not rule out the applicability of the more general model (Eq. 1), particularly if evidence for some form of active transport in the inner medulla, heretofore lacking, is forthcoming.