Further information on the cortical countercurrent system in rat kidney.

Receintly we reported the existence of a cortical countercurrent system on the surface of the rat kidney(l). The countercurrent system is arranged in suclh a way that proximal tubular fluid andl efferent peritubular blood flow in opposite directions. The following findings may be cited in support of this thesis. 1. The superficial nephrons of the rat kidney are characterized by an efferent arteriole whiclh generally runs at a riglht angle to the surface of the kidney and branches only after lhaving reached the renal surface. Usually this vessel separates into three to four branclhes. Pictures of casts of suclh loilg efferent arterioles were published as early as 1955 by Wirz(2). WVe term this branclhing point of the efferent arteriole the welling poiint because it is possible to observe by means of higlh-frequency microcinematograplhy the appearance of the flow of erytlhrocytes from deeper regions of the kidney. Beginning witlh the three to four branches of the wellinig point the more periplheral capillary bed branches repeatedly and finally is collectedl into the renal venous tree in close proximity of the efferent arteriole. The morphological situation is described by means of a typical cast slhown in Fig. 1. 2. We could dlemonstrate by intravital microphotograplhy and the use of lissanmine green that proximal tubular urine (from glomerulus to pars recta) frequently flows in the opposite dlirection from the capillary blood flow which originates from the welling point. Stated differently, proximal tubular loops situated close to the glomerulus are generally found at a greater distance from the welling poiInt. Inversely, tubular loops at a greater distance from the glomerulus are generally close to the welling point. We are reporting two new techniques which lhave been found useful for the examination of this countercurrent system: