Active salt transport and countercurrent exchange as the basis of urine concentration: a proposal grounded on the microanatomy of the renal medulla.

The longitudinal arrangement of tubules and vessels in the renal medulla has in the early fifties of the last century given birth to the concept of countercurrent multiplication (CCM). Since then, enormous research has been expended, but a consent on a common concept of the urinary concentrating mechanism has not been reached. Strictly based on the microanatomy of the renal medulla and the transport capacities of the involved tubules and vessels, we propose a mechanism, in which countercurrent exchange (CCE) operates at different levels in the outer and inner medulla to create gradients of salt and urea and to impede their dissipation. Salt release by ascending thick and thin loop limbs (TALs and ATLs) including the bend segments and urea release by terminal collecting ducts (CDs) increase at any medullary level solute concentrations in ascending vasa recta (AVRs) above those in descending loop limbs (DLs) and CDs leading to water extraction from both resulting in final urine concentration. Solute loss by washout is minimized by counter-current exchange between AVRs and descending vasa recta (DVRs), continuously balanced through addition of salt by ATLs and urea by terminal CDs.