The heart of fish consists of four chambers, namely
1) sinus venosus,
2) an atrium,
3) a ventricle,
4) conus arteriousus.
A thin walled sinus venosus receives blood returning from the tissues and pumps it into the atrium. The atrium contracts and then pumps blood into the ventricle. Ventricle has thick muscular wall. Next, the ventricle pumps blood into an elastic conus arteriosus, which does not contract, and blood flows into aorta.
|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.