The countercurrent multiplier system creates hyperosmotic urine
The term countercurrent indicates a flow of fluid in adjacent structures in opposite directions. The ability to excrete hyperosmotic urine depends on the countercurrent multiplier system that involves three structures:
• Loop of Henle, which acts as a countercurrent multiplier. The ultrafiltrate moves within the descending limb of the thin segment of the loop toward the renal papilla and moves back toward the corticomedullary junction within the ascending limb of the thin segment. The osmotic gradients of the medulla are established along the axis of the loop of Henle.
• Vasa recta, form loops parallel to the loop of Henle. They act as countercurrent exchangers of water and solutes between the descending part (arteriolae rectae) and ascending part (venulae rectae) of the vasa recta. The vasa recta help to maintain the osmotic gradient of the medulla.
• Collecting duct in the medulla acts as an osmotic equilibrating device. Modified ultrafiltrate in the collecting ducts can be further equilibrated with the hyperosmotic medullary interstitium. The level of equilibration depends on activation of ADH-dependent water channels (AQP-2).
A standing gradient of ion concentration produces hyperosmotic urine by a countercurrent multiplier effect
The loop of Henle creates and maintains a gradient of ion concentration in the medullary interstitium that increases from the corticomedullary junction to the renal papilla. As noted above, the thin descending limb of the loop of Henle is freely permeable to Na+, CI", and water, whereas the ascending limb of the loop of Henle is impermeable to water. Further, the thin ascending limb cells add Na+ and Cl~ to the interstitium.
Because water cannot leave the thin ascending limb, the interstitium becomes hyperosmotic relative to the luminal contents. Although some of the CI" and Na+ of the interstitium diffuses back into the nephron at the thin descending limb, the ions are transported out again in the thin ascending limb and distal straight tubule (thick ascending limb). This produces the countercurrent multiplier effect. Thus, the concentration of NaCl in the interstitium gradually increases down the length of the loop of Henle and, consequently, through the thickness of the medulla from the corticomedullary junction to the papilla.
Vasa recta containing descending arterioles and ascending venules act as countercurrent exchangers
For an understanding of the countercurrent exchange mechanism, it is necessary to resume the description of the renal circulation at the point at which the efferent arteriole leaves the renal corpuscle.
The efferent arterioles of the renal corpuscles of most of the cortex branch to form the capillary network that surrounds the tubular portions of the nephron in the cortex, the peritubular capillary network. The efferent arterioles of the juxtamedullary renal corpuscles form several unbranched arterioles that descend into the medullary pyramid. These arteriolae rectae make a hairpin turn deep in the medullary 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 rectae is required for concentrating urine by the countercurrent exchange mechanism
Because the thin 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 hypoosmotic. 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 also becomes isosmotic, partly by loss of water to the interstitium and partly by addition of ions other than Na+ and CI" 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 above, the vasa recta 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 CI" in the epithelium of the ascending limb of the loop of Henle.
The vasa recta form a countercwrent 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 cortico-medullary 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 CI" out of the cells of the water-impermeable ascending limb of the loop of Henle. The countercurrent exchange system and other movement of molecules in different parts of the nephron are shown in Figure 19.20.
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