Counter-current heat exchange

Counter-current exchange is an anatomical variation of the circulatory system, designed to reduce heat loss and maintain gas exchange.

Animals living in an arctic-type region, with very cold winters, have this variation to keep them warmer in the winter. Examples are polar bears and snowshoe hares. Warm blood flows in arterial vessels next to cold blood flowing in surface veins. Each one travels in opposite direction helping the colder blood stay warm instead of getting colder.

Counter current exchange is also found inside fish gill filaments to aid in oxygen uptake and maintain a gradient.