A multiunit model of solute and water removal by inner medullary vasa recta.

A recent model of volume and solute microcirculatory exchange in the renal medulla based on a single descending vasa rectum (DVR) was extended to account for the varying number of vessels along the corticomedullary axis. The assumption that concentration polarization at the walls of ascending vasa recta (AVR) during volume uptake eliminates transmural oncotic pressure gradients was examined. In this limiting case, small hydrostatic pressure gradients can drive AVR volume uptake if the pressure in the interstitium exceeds that in the AVR lumen. The calculated hydraulic pressure difference across AVR yielding agreement between predicted and measured values of AVR-to-DVR blood flow rate ratios was found to be smaller than the reported maximum pressure difference AVR can sustain. Simulations also confirmed previous conclusions suggesting that the presence of urea transporters in DVR counterbalances that of water channels that would otherwise decrease the efficiency of small solute trapping in the renal medulla.