A Hypertonic Medullary Interstitium Is Needed to Form Concentrated Urine

Terrestrial mammals usually produce urine that is concen­trated well above the plasma osmolality. Excretion of con­centrated wastes conserves water and thus reduces the volume of water that must be consumed each day to prevent dehydra­tion. Two of the three factors just mentioned are responsible for the formation of concentrated urine: (I) generation of a hypertonic medullary interstitium and (2) enhanced water permeability in the collecting duct in the presence of ADH.

The hypertonicity of the medullary interstitium is pro­duced and maintained primarily by (I) the reabsorption of osmotically active substances by tubules in the medulla and (2) the removal of water from the medullary interstitium by the vasa recta.