Tunas have type IV hearts, which have more than 30% compact myocardium with coronary arteries in compact and spongy myocardium. Their ventricles are large, thick-walled, and pyramidal in shape, allowing for generation of high ventricular pressures. The muscle fibers are arranged around the ventricle in a way that allows rapid ejection of stroke volume, because ventricles can contract both vertically and transversely at the same time. Myocardium itself is well vascularized, with highly branched arterioles and venules, as well as a high degree of capillarization.