DISTRIBUTED SOLUTE AND WATER REABSORPTION IN A CENTRAL CORE MODEL OF THE RENAL MEDULLA

In this model study we investigate the dependence of urine concentrati ng capability on the spatial distribution of solute and water reabsorp tion from Henle's loops. Within the context of model assumptions, urin e concentrating capability is increased by exponential decline in loop population as a function of medullary depth and by solute efflux loca lized near loop bends, in accordance with earlier, but less comprehens ive, studies. Further, we find that water-impermeable prebend enlargem ents of the descending limb may release urine concentrating capacity t hat 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 solut e from the ascending limbs of the inner medulla would not be sufficien t to explain the large concentration gradients generated by some mamma ls. We consider the significance of distributed reabsorption for the o peration of the concentrating mechanisms of the mammalian inner medull a, the mammalian outer medulla, and the avian medullary cone.