Hypo-osmotic Re-absorption due to Active Salt Transport in Perfused Collecting Ducts of the Rat Renal Medulla

Abstract

URINE reaches its maximal osmolality in the collecting ducts of the renal medulla1, but the mechanisms causing the transformation from isotonic tubular fluid are in dispute. Most modern attempts to explain the concentrating mechanism are based on the hypothesis of Wirz, Hargitay and Kuhn2, that the loops of Henle function as countercurrent multipliers to raise the medullary interstitial osmolality so that water can be extracted from the collecting duct urine by simple osmosis. Experimental evidence3,4 supports the validity of this hypothesis only in the outer medulla; the major increase of osmolality occurs in the inner medulla where the loops of Henle are not capable of active salt transport5 and so cannot be countercurrent multipliers. In this communication I shall propose an alternative hypothesis for the inner medulla and present the results of experimental tests of its more unusual features.