Tuna are negatively buoyant and must swim continuously to avoid sinking. In addition, they require constant motion to ventilate their gills. Swimming (at a speed of no less than 65 centimeters per second) forces water to flow over the gills through a process called impaction gill ventilation.

Tuna have many lamellae (gill membranes) and very thin lamellar walls that enable them to extract more oxygen from the water than any other fish. Tuna have large hearts and blood volumes. They also have a high percentage of red muscle, which allows them to swim continuously, burying themselves along the center of the spine to conserve heat.

For fast swimmers like the tuna family, cavitation is a barrier to optimizing energy consumption because it limits maximum swimming speed.

Body heat:

Tuna conserve heat generated by their swimming muscles through an arrangement of blood vessels known as the "marvelous web". These vessels act as countercurrent heat exchangers. In any fish, as blood circulates through the gills to receive oxygen, it also cools to the temperature of the surrounding water.

In tuna, this blood is diverted to blood vessels near the outside of the body rather than directly through the core of the fish. Before flowing inward, the cold, oxygenated blood flows backward through a network of small vessels to the warmer blood leaving the swimming muscles, where heat is transferred to the incoming blood.

In this way, most of the heat produced by the swimming muscles is conserved, and in waters between 7 and 30 degrees Celsius, the bluefin tuna's muscles maintain a temperature of between 28 and 33 degrees Celsius. Other tuna maintain a body temperature 3-7 degrees Celsius higher than the surrounding waters.