Heat is transferred from one liquid or air supply to another without any actual mixing or contact.
Structures employing similar principles have been discovered in several animals, including the legs of wading birds and flukes or flippers of marine mammals. Thus, warm blood in the arteries from the body passes very close to cooled blood returning in the veins to the body from the extremities. Heat is transferred from the warmer blood to the cooler as the blood flows through these vessels, thus conserving body heat. Such an arrangement can prevent hypothermia or excessive loss of body heat to the environment. Even though these animals often live in an environment that is much cooler than their body temperature, they can conserve body heat through such counter-current heat exchangers.