Evolution of Vertebrate Heart – Fish
In fishes blood flows through a single circuit passing through a capillary network in the gills, where blood becomes oxygenated and then through capillaries located in other organs of the body. The oxygenated blood is supplied from dorsal aorta through coronary arteries to the heart and is carried back by coronary veins from the heart. The heart of the fishes never receives oxygenated blood. It is only the deoxygenated blood which passes through different chambers of the heart. The valves present in the heart controls the flow of blood in single direction. So the heart of fish functions as a single circuit heart.