the loop of henle has been the focus of extensive investigation for the last 50 years since Hargitay and Kuhn (1) first proposed that it acts as a countercurrent multiplier and generates the hypertonic fluid in the renal medulla. Until recently, however, the equally important role of the vasa recta circulation in countercurrent exchange and in the preservation of the corticomedullary osmotic gradient has received much less attention. The review by Pallone and colleagues (2) in this issue of theAmerican Journal of Physiology-Regulatory, Integrative and Comparative Physiology highlights the major advances that have emerged in the last 10 years in videomicroscopy, fluorescence imaging, and molecular biology permitting the dissection and in vitro perfusion of vasa recta capillaries. These advances have transformed our understanding of how the vasa recta circulation functions as a countercurrent exchange. For example, recent studies have indicated that ascending and descending vasa recta exhibit differential permeability to sodium, urea, and protein. This optimizes urine concentration by shunting water from the descending to ascending vasa recta, thereby lowering blood flow and solute washout near the tip of the renal medulla. Other investigators have shown that the vasa recta capillaries are not simply passive U-shaped tubes but that pericytes that surround these capillaries are highly vasoactive and respond to a large number of hormones and autacoids (angiotensin II, nitric oxide, prostaglandins). These findings indicate that tubulovascular coupling in the renal medulla is far more complicated than previously thought. It is now clear that vasoactive agents not only alter urine concentration and tubular function by changing vasa recta blood flow but that changes in tubular function likely produce appropriate adjustments in medullary blood flow through the release of vasoactive substances from the surrounding tubules. Pallone et al.'s review provides the quantitative basis and summarizes the data that have led to a more realistic understanding of how the physical properties of the vasa recta vessels provide for the optimization of the countercurrent urine concentrating mechanism. In summary, the type of detailed information regarding the vasa recta circulation and countercurrent exchange presented in this review is key to those interested in understanding the role of the renal medulla in urine concentration, sodium and water balance, and the long-term control of arterial pressure.
- Copyright © 2003 the American Physiological Society