Blood Supply and Circulation

1. Each kidney receives a renal artery-a branch from the abdominal aorta.

2. Anterior and posterior branches arise from the renal artery before it reaches the renal hilum

3. Interlobar arteries arise from the anterior and posterior branches in the renal hilum and penetrate the medulla between the medullary pyramids.

4. Arcuate arteries arise from the interlobular arteries and course along the arched border between the cortex and medulla.

5. Interlobular arteries arise at right angles from the arcuates; they penetrate the cortex between the medullary rays and lie at the borders between neighboring renal lobules.

6. Many afferent arterioles arise from each interlobular artery. Each afferent arteriole supplies a glomerulus.

7. An efferent arteriole carries blood away from the glomerulus.

a. Efferent arterioles of cortical nephrons branch to form a profusion of peritubular capillaries that carry absorbed products away from the proximal and distal tubules and converge to form the stellate veins of the peripheral cortex. These drain into the inter lobular veins.

b. Efferent arterioles ofjuxtamedullary nephrons give rise to numerous straight capillary loops--vasa recta-that descend into the medulla.

8. Vasa recta arise mainly from the efferent arterioles of the juxtamedullary nephrons; some may arise from the arcuate artery. The descending parts of the vasa recta carry isotonic blood into the medulla.

The blood loses water and picks up sodium as it passes deeper into the medulla. Unlike the loop of Henle, the ascending parts of the vasa recta are as permeable to salt and water as are its descending parts. As the blood ascends through the gradient, its tonicity equilibrates with that of its surroundings.

The blood carried away from the medulla is thus once again isotonic. The passive exchange of salt and water between the vasa recta and the interstitium is known as the countercurrent exchange mechanism.

It is important in carrying away water lost to the filtrate during its descent into the medulla and thus in maintaining the osmotic gradient set up by the countercurrent multiplier system of Henle's loop.

Blood in the ascending portions of these vessels drains into interlobular veins and exits through the veins that accompany the larger arteries. Urinary system