A mathematical model of transport in the renal medullary microcirculation was used to investigate the role of the UTB urea transporter expressed in descending vasa recta (DVR) endothelia and red blood cell (RBC) membranes. Our simulations suggest that UTB raises RBC, plasma and interstitial urea concentrations by facilitating radial diffusion of the solute, and therefore serves to increase the contribution of urea to the cortico-medullary osmolality gradient, assuming no secondary effects on tubular transport. However, by lowering transmural urea concentration gradients, UTB reduces water efflux from DVR through aquaporin-1 (AQP-1) water channels, thereby decreasing plasma sodium concentration. The net result of these competing effects on the osmolality gradient depends upon the fraction of filtered urea that is reabsorbed by vasa recta. We also found that the contribution of UTB to water transport across DVR and RBCs is negligible, even in the absence of AQP-1. Our model predicts that UTB plays a significant role, however, in reducing the shrinking and swelling of RBCs as blood flows along the medulla.