Countercurrent multiplication

refers to the process by which a small osmolality difference, at each level of the outer medulla, between fluid flows in ascending and descending limbs of the loops of Henle, is multiplied by the countercurrent flow configuration to establish a large axial osmolality difference from 300 mOsm/L by the end of PCT to 1200 mOsm/L at the tip of the loop of Henle.