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count·er·cur·rent ex·chang·er(kown'ter-ker'ent eks-chānj'er),
A system in which heat or chemicals passively diffuse across a membrane separating two countercurrent exchanger streams so that at each end the fluid leaving along one side of the membrane nearly resembles, in temperature or composition, the fluid entering the other; for example, the venae comitantes in the arms serve as a countercurrent exchanger exchanger, the arterial blood serving to rewarm the cooler venous blood.
The exchange of chemicals between two streams of fluid flowing in opposite directions on either side of a permeable membrane. This permits the fluid leaving one side of the membrane to be similar to the composition of the fluid entering the other end of the other stream.