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countercurrent exchangea biological mechanism designed to enable maximum exchange between two fluids. The mechanism's effect is dependent on the two fluids flowing in opposite directions, and having a concentration gradient between them.
high concentration → low concentration
high concentration ← low concentration
Such mechanisms occur, for example, in oxygen exchange between water and the blood vessels in fish gills, and in the ascending and descending tubules of the LOOP OF HENLE in the mammalian kidney.