BLOOD VESSELS OF THE KIDNEY
The pathway of blood flow through the kidney is an essential part of the process of urine formation. Blood from the abdominal aorta enters the renal artery, which branches extensively within the kidney into smaller arteries (see Fig. 18–2). The smallest arteries give rise to afferent arterioles in the renal cortex (see Fig. 18–3). From the afferent arterioles, blood flows into the glomeruli (capillaries), to efferent arterioles, to peritubular capillaries, to veins within the kidney, to the renal vein, and finally to the inferior vena cava Notice that in this pathway there are two sets of cap-illaries, and recall that it is in capillaries that exchanges take place between the blood and surrounding tissues. Therefore, in the kidneys there are two sites of ex-change. The exchanges that take place between the nephrons and the capillaries of the kidneys will form urine from blood plasma.
Figure 18–2 shows two views of a vascular cast of a kidney; the shape of the blood vessels has been pre-served in red plastic. You can see how dense the vas-culature of a kidney is, and most of these vessels are capillaries.