82The fish heart consists of four chambers; the sinus venosus, the atrium, the ventricle, and the highly elastic bulbus arteriosus. The heart of fishes is consisted of three layers, the epicardium, myocardium, and endocardium. Unlike the mammalian heart, the teleost myocardium is capable of regeneration. The sinus venosus is a thin-walled chamber whose composition varies between species. The volume of the sinus venosus is equivalent to that of the atrium. The sinus venosus conveys the blood into the atrium, from which it is separated by the sinus valve. An important characteristic of the sinus is that it contains the heart pacemaker. The teleost atrium is a single chamber which shows considerable variability in size and shape between species. It is formed of an external rim of myocardium and of a complex network of thin trabeculae. The ventricle is a highly muscular chamber that has the thickest layer of cardiomyocytes. It is characterized by an abundant spongy myocardium that leaves in the lumen, some lacunae in which blood circulates. The external ventricular shape has been grouped into three main categories: tubular, sac-like, and pyramidal, this division has several functional implications. The bulbus arteriosus opens in the ventral aorta and contains connective tissue and elastic fibers. It expands during ventricular ejection to store a large part of the cardiac stroke volume. The inner surface of the bulbus is characterized by the presence of ridges. The wall of arteries and veins is distinguished into three layers: tunica intima, tunica media, and tunica adventitia. Elastic arteries (ventral aorta or gill arteries) are found near the heart, and their media is rich in elastin. The muscular arteries possess a basic structure like that found in higher vertebrates. Veins are structurally similar to those in mammals, but have thinner walls and less abundant smooth muscle. Blood capillaries are histologically like those found in mammals, but they are much more permeable. The capillaries are classified into several types depending on the form and position of the endothelial cells and pericytes. The blood of fishes is a specialized circulating tissue composed of cells suspended in plasma. Blood volumes of teleosts are small, being about 5% of body weight. The main blood cells in fishes are red blood cells (erythrocytes), white blood cells (leucocytes), and thrombocytes.