I n the mammalian kidney the renal artery divides into the interlobar arteries radiating from the hilus and passin.g along the pyramids before penetrating the renal parenchyma at the level of the cortico-medullary border. Here they give rise to several branches, the arcuate arteries, which pass as arches along the corticomedullary border (Fig. 1). From these arteries arise the interlobular arteries, which extend into the cortical parenchyma. O n their way through the cortex they give off, at more or less acute angles, the afferent glomerular arterioles. I n man and in the dog and cat these arteries finally pierce the capsule, where they end in a number of branches anastomosing with extrarenal arteries. The afferent glomerular arteriole breaks up into the glomerular capillary system, the vessels of which reunite to form the efferent glomerular arteriole. The efferent arteriole of the cortical glomeruli then breaks up into the peritubular capillaries supplying the cortical tubuli an.d collecting ducts. These capillaries then empty into the interlobular veins accompanying the interlobular arteries. In some animals, including the dog and man, there is, in addition, a superficial venous drainage. The interlobular veins then drain into the arcuate veins joining to form the interlobar veins, which in turnjoin to form the renal vein.
The medullary parenchyma derives its blood supply from the juxtamedullary glomeruli located in the inner part of the cortical parenchyma. The number of these glomeruli averages, according to Edwards (1956), 18 24 of the total number of glomeruli,