An active (energy-requiring) process occurring in the loops of Henle in the kidney, which is responsible for the production of concentrated urine in the collecting ducts of the nephrons. Sodium and chloride ions are actively pumped from the thick segment of the ascending limb of the loop but water is retained, since the ascending limb is impermeable to water. This creates a concentration gradient in the medulla in which the concentration of sodium and chloride is greatest in the region of the bend of the loop. Fluid passing from the loop of Henle to the distal tubule is less concentrated than that entering the loop, but because of the high osmotic pressure in the medulla water diffuses out of the collecting ducts, producing a concentrated urine. See illustration.

Countercurrent multiplier system in the kidney