In an excised cat kidney, the subcapsular veins were clipped near their connections with the renal vein and cut open distal to the clamps. The kidney was then placed in an oil-filled lucite chamber and perfused from a donor cat via tubes entering the chamber through its close-fitting lid. The blood leaking from the cut subcapsular veins was led to one flowmeter; that leaving via the cannulated renal vein (supplied now only by the arcuate vein system) was led to another flowmeter. Catecholamines and stimulation of the renal nerves reduced the arcuate blood flow more than the subcapsular blood flow, the two average reductions being 28% and 20% with epinephrine and 22% and 14% with stimulation of the renal nerves. Angiotensin reduced both flows equally. Microspheres reduced the subcapsular flow more than the arcuate flow, the two average reductions being 42% and 25% of the control flows. This effect was probably caused by trapping of the microspheres in the distal ends of the interlobular arteries beyond the inlets to the afferent arterioles, since these vessels are too narrow to allow free entrance of the microspheres. The preferential effect of epinephrine and nerve stimulation on the deep circulation is thought to be the result of the special structure of the juxtamedullary glomerular units. • In the cat kidney, the veins draining the outer cortex are well separated from those draining the inner cortex and the medulla, because the blood from the outer cortex leaves through subcapsular veins which do not join the arcuate vein system until the hilar region (1, 2) (Fig. 1). In this paper, we will describe how the rates of blood flow leaving the subcapsular and the arcuate veins can be measured directly and simultaneously and show that the vessels in the two drainage areas react differently to certain external

The renal arterial pressure was changed in steps of 25 mm Hg, or it was successively varied between two levels (100 and 150 mm Hg) to obtain four or five stable flow recordings at each level. Statistical calculations indicated that the blood flows in the two drainage areas of the cat kidney were autoregulated with equal effectiveness (whether judged from inflow or outflow rates). The flows at 100 mm Hg were approximately 95% of the flows at 150 mm Hg. To establish whether the accuracy of the method was sufficient to detect a possible nonautoregulated fraction of the arcuate flow similar in magnitude to that traversing the medulla, the flow data were compared with the flows calculated from a simple mathematical model consisting of three homogeneous compartments (outer cortical, inner cortical, and medullary), each with its own pressure-flow relationship. If the flows through all cortical glomeruli are similarly autoregulated, the experimental results are incompatible with a flow through juxtamedullary glomeruli which is proportional to pressure and constitutes 5-;7%, or more, of the total renal blood flow.

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