Counter current mechanism: Counter-Current Mechanism The countercurrent mechanism is an

Counter-Current Mechanism The countercurrent mechanism is an essential procedure in the kidney that enables urination to become concentrated by retaining water. It has established an osmotic gradient in the renal medulla, where the water reabsorption from the tubular fluid back into the bloodstream accompanies the excretion of excess solutes. Vasa recta is freely permeable to both solute and water throughout the length. Water diffuses along the osmotic gradient and NaCl diffuses along its concentration gradient. Blood entering the descending limb of vasa recta is ~ 300mOsm/L and Blood leaving the ascending limb of vasa recta is ~ 325mOsm/L. The countercurrent multiplier mechanism in the renal tubule establishes and maintains a concentration gradient that allows the extraction of water from the tubular fluid. This multiplier mechanism is maintained by the countercurrent exchange of solutes in the vasa recta, a process that prevents the washout of solutes from the inner medulla. The intrarenal recirculation of urea from the collecting duct (into the renal medulla and then back into the ascending limb of the loop of Henle) also ... The mechanisms shown here are traditionally called the 'counter-current multiplier' and the 'counter-current exchanger'. The first takes place in the region of the nephron called Henle's loop; the second occurs in a region of the peritubular capillary bed called the 'vasa recta'. Both are involved in establishing an osmotic gradient throughout this region.