Solute Concentration in the Kidney- I. A Model of the Renal Medulla and Its limit Cases*

We develop a general model of the renal medulla that is similar m geometry to the models reported by others. Two solutes are assumed to be present, one to play the role of salt and the other that of urea and similarly handled compounds. The equations fr’om irreversible thermodynamics are used to describe simultaneous solvent and solute flow across tubular membranes and a salt pump, saturable in the cis concentration and inhibited by trans concentrations is included in the AHL. We examine the input limit cases of the model, i.e., those for zero and infinite inflows in vasa rectae, Henle loop and collecting ducts. High inflow to vasa rectae washes out the gradients. For inflow to DHL there is an optimal value for building up the concentration gradients: for zero inflow, no concentration gradients are developed and as inflow increases above the optimal value, the concentration gradients decrease and all concentrations in DHL, AHL and interstitium tend towards those in the inflow to DHL. A central core model, the limit case for zero inflow in vasa rectae. is then described.