In order for the countercurrent multiplier system to be effective, most of the salt that is extruded from the ascending limbs must remain in the interstitial fluid of the medulla, while most of the water that leaves the descending limbs must be removed by the blood. This is accomplished by the vasa recta—long, thin-walled vessels that parallel the loops of Henle of the jux-tamedullary nephrons (see fig. 17.19). The descending vasa recta have characteristics of both capillaries and arterioles because their continuous endothelium is surrounded by smooth muscle remnants. These vessels have urea transporters (for fa-cilitative diffusion) and aquaporin proteins, which function as water channels through the membrane (chapter 6). The ascending vasa recta are capillaries with a fenestrated endothelium. As described in chapter 13, the wide gaps between endothelial cells in such capillaries permit rapid rates of diffusion.
The vasa recta maintain the hypertonicity of the renal medulla by means of a mechanism known as countercurrent exchange. Salt and other dissolved solutes (primarily urea, described in the next section) that are present at high concentrations in the interstitial fluid diffuse into the descending vasa recta. However, these same solutes then passively diffuse out of the ascending vasa recta and back into the interstitial fluid to complete the countercurrent exchange. They do this because, at each level of the medulla, the concentration of solutes is higher in the ascending vessels than in the interstitial fluid, and higher in the interstitial fluid than in the descending vessels. Solutes are thus recirculated and trapped within the medulla.
The walls of the vasa recta are freely permeable to water and to dissolved NaCl and urea. Plasma proteins, however, do not easily pass through the capillary walls of the vasa recta. The colloid osmotic pressure (oncotic pressure) within the vasa recta, therefore, is higher than in the surrounding interstitial fluid. This is similar to the situation in other capillary beds (chapter 14) and results in the osmotic movement of water into both the descending and ascending limbs of the vasa recta. The vasa recta thus trap salt and urea within the interstitial fluid but transport water out of the renal medulla (fig. 17.17).
■ Figure 17.17 Countercurrent exchange in the vasa recta. The diffusion of salt and water first into and then out of these blood vessels helps to maintain the "saltiness" (hypertonicity) of the interstitial fluid in the renal medulla. (Numbers indicate osmolality.)
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