Major arteries and veins run longitudinally to and from the red swimming muscles, which are found close to the spinal column, just underneath the skin. Small arteries branch off and penetrate the red muscle, delivering oxygenated blood, whereas veins take deoxygenated blood back to the heart. The red muscles also have a high myoglobin content and capillary density, where many of the capillaries branch off. This helps increase surface area and red-cell residence time. The veins and arteries are organized in a way that allows countercurrent heat exchange. They are juxtaposed and branched extensively to form rete mirabile. 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). High cardiac outputs in southern bluefin tuna are necessary to achieve their maximum metabolic rates. The bulbus arteriosus can take up an entire stroke volume, maintaining a smooth blood flow over the gills through diastole. This might, in turn, increase the rate of gas exchange. Their heart rate is also affected by temperature; at normal temperatures can it reach up to 200 beats/min.