Countercurrent heat exchange in the lateral muscles of bluefin tuna.

The lateral swimming muscles of endothermic fish, such as tunas and white sharks, are also well supplied with blood vessels that function as countercurrent heat exchangers. These heat exchangers heat cool arterial blood as it carries oxygen to the lateral swimming muscles, and by the time this blood delivers its supply of oxygen and nutrients it has been heated to the same temperature as the active muscles. On the return trip the heat in this warm blood is used to heat the newly arriving blood and so, when blood exits the swimming muscles, it is again approximately the same temperature as the surrounding water. The countercurrent heat exchangers of tuna are efficient enough at conserving heat that these fish can elevate the temperature of their swimming muscles up to 14°C above the temperature of the surrounding water. The anatomy of the countercurrent heat exchanger in bluefin tuna muscles