Transport of plasma proteins across vasa recta in the renal medulla

In this study, we have extended a mathematical model of microvascular excha nge in the renal medulla to elucidate the mechanisms by which plasma protei ns are transported between vasa recta and the interstitium. In contrast wit h other work, a distinction was made between the paracellular pathway and t he transcellular route (i.e., water channels) in descending vasa recta (DVR ). Our model first indicates that concentration polarization on the interst itial side of vasa recta has a negligible effect on medullary function. Our results also suggest that, whereas proteins are cleared from the interstit ium by convection, both diffusion and convection play a role in carrying pr oteins to the interstitium. In those regions where transcapillary oncotic p ressure gradients favor volume influx through the paracellular pathway in D VR, diffusion is the only means by which proteins can penetrate the interst itium. Whether the source of interstitial protein is DV-R or ascending vasa recta depends on medullary depth, vasa recta permeability to proteins, and vasa recta reflection coefficients to small solutes and proteins. Finally, our model predicts significant axial protein gradients in the renal medull ary interstitium.