Theories which attempt to explain urinary concentrating ability generally neglect the role of medullary microcirculation. The anatomical heterogeneity of the outer medulla and inner medulla as well as the marked difference between descending and ascending vasa recta ultrastructure provide evidence that these capillaries are highly evolved to participate in this process. Our overall objective is to measure transport characteristics of the vasa recta and employ that information to predict countercurrent exchanger efficiency of small solutes, macromolecules and water. The specific aims of this project are the following. 1) To determine the effect of albumin concentration on hydraulic conduc- tivity. The change in transcapillary volume flux will be measured as albumin concentration of the bath and perfusate is simultaneously altered. 2) To determine hydraulic conductivity. An increase in transcapillary volume flux will be induced with a high molecular weight dextran and measured. 3) To determine the reflection coefficient to albumin. The increase in transcapillary volume flux induced by albumin will be measured. 4) To determine the diffusive permeability to albumin. The lumen-to-bath flux of radiolabeled albumin will be measured. 5) To measure the transcapillary flux of neutral dextran probes. Pore theory will be applied to calculate the pore radius of the capillary wall. Finally, we will extend previous mathematical models of the micro- circulation to address both the importance of the anatomical heterogeneity of the inner and outer medullary microcirculatory beds and the transcapillary transport of albumin.