Analysis of countercurrent diffusion exchange in blood vessels of the renal medulla.

MARSH, DONALD J., AND LEE A. SEGEL. Analysis of countercurrent di$usion exchange in blood vessels of the renal medulla. Am. J. Physiol. 221(3): 817-828. 1971 .-A model of the vasa recta was developed to explore the significance of dissimilarities between ascending and descending vasa recta for countercurrent exchange efficiency, and also to examine the interaction between countercurrent exchange and the water transport function of the vasa recta. (The last refers to fluid reabsorbed from tubules which the vasa recta remove from the medulla.) Vasa recta can be characterized by four dimensionless parameters : two embody previous theoretical results which show that interstitial concentration of any solute is directly related to permeability and surface area and inversely to volume flow rate, and also to the rate of solute addition or removal by the tubules. The two new parameters express the numerical relationships among the numbers of ascending and descending vasa recta, their radii, solute permeabilities, and volume flow rates. Estimation of the parameter values leads to the conclusion that real dissimilarities between the two classes of vasa recta serve to improve countercurrent exchanger efficiency. Moreover, interstitial solute concentration either increases or decreases as the intake of fluid from the tubules increases, depending on which other parameters are held constant or allowed to vary.