The American journal of physiology
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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 vasa 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. ⋯ 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.