ABSTRACT
The fish heart consists of four chambers: the sinus venosus, atrium, ventricle, and bulbus arteriosus. The sinus venosus is a thin-walled chamber whose composition varies between species. An important characteristic of the sinus is that it contains the heart pacemaker. The teleost atrium is a single chamber that shows considerable variability in size and shape between species. It is formed of an external rim of the myocardium with thin trabeculae. The ventricle 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. Elastic arteries (ventral aorta or gill arteries) are found near the heart, and their media is rich in elastin. Veins are structurally similar to those in mammals but have thinner walls and less abundant smooth muscle. Special types of blood vessels are also recorded. Blood capillaries are histologically like those found in mammals, but they are much more permeable. The main blood cells in fishes are erythrocytes, neutrophils, eosinophils, basophils, heterophils, monocytes, lymphocytes, and thrombocytes.
