Sodium chloride, water and urea handling in the rat renal medulla: a computer simulation.

A dynamic rat renal medullary model was employed to study how independent control of excretion of various substances is achieved. Salt and water reabsorptive coupling was heterogeneous in the distal nephron. Diversion of filtrate to juxtamedullary nephrons retained salt with little effect on water or urea, a response enhanced by concomitant reduction in inner medullary blood flow. Urine concentration required high permeability and low flow in medullary blood vessels and active inner medullary salt transport. Collecting duct urea movement was closely coupled to that of water and urea recycling was enhanced by incorporation of nephrovascular bundles.