The correct answer is: reduce the loss of body heat to the environment. A countercurrent heat exchanger works by transferring heat from one part of the body to another that has cooled down. For example, in dolphins, body parts of a high surface area such as flippers are rich in veins. Blood running through these veins that is cooled down is run close to arteries containing warmed-up blood, and some of the heat energy from the arteries is transferred to the veins, thereby allowing the dolphin to conserve heat energy.