 This arrangement allows the heat produced by the red muscles to be retained within them, as it can be transferred from the venous blood to the ingoing arterial blood.Tunas have the highest arterial blood pressure among all fishes, due to a high resistance of blood flow in the gills.They also have a high heart rate, cardiac output, and ventilation rate.To achieve high cardiac outputs, tunas increase their heart rate exclusively (other teleosts may increase their stroke volume as well).