Effect of vasa recta flow on concentrating ability of models of renal inner medulla

In this study we extend the analysis of the preceding two studies [J. L. Stephenson, J. F. Jen, H. Wang, and R. P. Tewarson. Am. J. Physiol. 268 (Renal Fluid Electrolyte Physiol. 37): F698-F709, 1995; and J. F. Jen, H. Wang, R. P. Tewarson, and J. L. Stephenson. Am. J. Physiol. 268 (Renal Fluid Electrolyte Physiol. 37): F000-F000, 1995] to a model that includes vasa recta. Distribution of nephron and vasa recta lengths is represented by shunting from descending to ascending flow. It is found that the effect of radial separation of structures on concentrating ability is closely linked to vasa recta flow. With minimal or no vasa recta flow the extent of radial mixing has little effect on concentrating ability. As vasa recta flow increases, concentrating ability is decreased by radial mixing. Convective uphill transport of NaCl is again observed, but concentrating ability appears to depend primarily on urea delivery to the inner medulla from the collecting duct rather than on the mechanism of salt transport out of thin ascending limb. Central core models give an upper bound on concentrating ability but do not attain the maximum urine osmolality of the rat with experimental values of tubular permeabilities.