Distributed solute and water reabsorption in a central core model of the renal medulla

In this model study we investigate the dependence of urine concentrating capability on the spatial distribution of solute and water reabsorption from Henle's loops. Within the context of model assumptions, urine concentrating capability is increased by exponential decline in loop population as a function of medullary depth and by solute efflux localized near loop bends, in accordance with earlier, but less comprehensive, studies. Further, we find that water-impermeable prebend enlargements of the descending limb may release urine concentrating capacity that would otherwise be needed to concentrate the fluid flowing in the prebend enlargements. Calculations reported here suggest that without some distributed features, even vigorous net active transport of solute from the ascending limbs of the inner medulla would not be sufficient to explain the large concentration gradients generated by some mammals. We consider the significance of distributed reabsorption for the operation of the concentrating mechanisms of the mammalian inner medulla, the mammalian outer medulla, and the avian medullary cone.