Water permeability of the epithelium of the collecting ducts is An increase in plasma osmolality or a decrease in blood pyramid and ascend as the venulae rectae. Together, the descending arterioles and the ascending venules are called the vasa recta. The arteriolae rectae form capillary plexuses lined by fenestrated endothelium that supply the tubular structures at the various levels of the medullary pyramid.
Interaction between collecting ducts, loops of Henle, and vasa recta is required for concentrating urine by the countercurrent exchange mechanism.
Because the thick ascending limb of the loop of Henle has a high level of transport activity and because it is impermeable to water, the modified ultrafiltrate that ultimately reaches the distal convoluted tubule is hyposmotic. When ADH is present, the distal convoluted tubules, the collecting tubules, and the collecting ducts are highly permeable to water. Therefore, within the cortex, in which the interstitium is isosmotic with blood, the modified ultrafiltrate within the distal convoluted tubule equilibrates and becomes isosmotic, partly by loss of water to the interstitium and partly by addition of ions other than Na and Cl to the ultrafiltrate. In the medulla, increasing amounts of water leave the ultrafiltrate as the collecting ducts pass through the increasingly hyperosmotic interstitium on their course to the papillae.
As noted previously, the vasa rectae also form loops in the medulla that parallel the loop of Henle. This arrangement ensures that the vessels provide circulation to the medulla without disturbing the osmotic gradient established by transport of Cl in the epithelium of the ascending limb of the loop of Henle.
The vasa recta form a countercurrent exchange system in the following manner: Both the arterial and venous sides of the loop are thin-walled vessels that form plexuses of fenestrated capillaries at all levels in the medulla. As the arterial vessels descend through the medulla, the blood loses water to the interstitium and gains salt from the interstitium so that at the tip of the loop, deep in the medulla, the blood is essentially in equilibrium with the hyperosmotic interstitial fluid.
As the venous vessels ascend toward the corticomedullary junction, the process is reversed (i.e., the hyperosmotic blood loses salt to the interstitium and gains water from the interstitium). This passive countercurrent exchange of water and salt between the blood and the interstitium occurs without expenditure of energy by the endothelial cells. The energy that drives this system is the same energy that drives the multiplier system, namely, the movement of Na and Cl out of the cells of the water-impermeable ascending limb of the loop of Henle. The counter-current exchange system and other movements of molecules in different parts of the nephron are shown in Figure 20.23.
Some aspects of the blood supply of the kidney have been described in relation to specific functions (i.e., glomerular filtration, control of blood pressure, and countercurrent exchange). It remains, however, to provide an overall description of the blood supply of the kidney.
Each kidney receives a large branch from the abdominal aorta, called the renal artery. The renal artery branches within the renal sinus and sends interlobar arteries into the substance of the kidney (Fig. 20.24). These arteries travel between the pyramids as far as the cortex and then turn to follow an arched course along the base of the pyramid between the medulla and the cortex. Thus, these interlobar arteries are designated arcuate arteries.