Analysis of microvascular water and solute exchanges in the renal medulla

A theoretical model has been developed to simulate solute and water transport in the medullary microcirculation of the normal hydropenic rat. The model is formulated in terms of a countercurrent vascular unit consisting of one descending (DVR) and several ascending vaso recta (AVR) extending from the corticomedullary junction to the tip of the papilla. Steady-state mass balances relate gradients in NaCl, urea, and plasma protein concentrations and variations in the flow rates of plasma and red blood cells to permeability properties of the vasa recta and erythrocytes. In contrast to previous models, transmural volume fluxes are assumed to be present in both DVR and AVR. Available micropuncture measurements suggesting net volume removal from DVR within the inner medulla are found to be consistent with NaCl reflection coefficients in DVR between 0.10 and 0.80. The hydraulic permeabillity in the DVR is estimated to be >0.18 x 10^-6 cm.s^-1.mmHg^-1. Based on currently available data, reliable bounds cannot yet be placed on the hydraulic permeability of the AVR. The vascular unit is predicted to accomplish substantial net removal of NaCl and water from the inner medullary interstitium but relatively little removal of urea. Red cells leaving the inner medulla in the AVR are found to be slightly dehydrated. It is calculated that at a given blood flow rate, the lower the initial medullary hematocrit, the more effective the vascular unit is at removing water. Several unresolved issues are discussed, including the role of the capillary plexus that joins DVR with AVR. To the extent that the volume uptake observed in the exposed papilla in structures beyond the DVR occurs in the capillary plexus and not in the AVR, estimated values of AVR hydraulic permeability are reduced, as is predicted overall volume uptake by the vascular unit in the inner medulla.