Tuna, like mammals and birds, are homeothermic (warm-blooded) animals. This means that they are able to maintain their body temperature a few degrees above or below the environment temperature.
However, tunas use a thermoregulation system which is different from that of land-based homeotherms; they use a system called rete mirabile. This is a set of small arteries and veins arranged in parallel that act as a heat exchanger. The arterial blood, oxygenated and cooled in the gills, takes heat from the venous blood, heated by the metabolism of the fish. This avoids the loss of heat through the gills, leaving it trapped to warm the body.