CURRENT CONCEPTS OF THE COUNTERCURRENT MULTIPLICATION SYSTEM

The production of a concentrated urine is achieved by countercurrent m ultiplication in the renal medulla. While the single effect in the out er medulla is known to be active NaCl reabsorption in the thick ascend ing limb, the single effect in the inner medulla is not definitively e stablished. However, the passive model of Kokko and Rector [1] and Ste phenson [2] remains the most widely accepted mechanism for the single effect in the inner medulla. Continued experimental studies of transpo rt in perfused inner medullary nephron segments and mathematical simul ations that incorporate these new experimental values and anatomic com plexity will be needed to fully elucidate the process of urinary conce ntration. In addition, the availability of molecular reagents will per mit investigation into the molecular mechanisms that regulate transpor t proteins which play crucial roles in the urinary concentrating mecha nism.