In fishes the heart is two chambered but sinus venosus and conus arteriosus are also found attached to the heart. Blood from the tissues enters a sinus venosus through the veins.
It passes through a sinu-auricular aperture to the auricle, a relatively thin-walled but large chamber, thence it is propelled through auriculo-ventricular valve to the ventricle.
Vigrous ventricular contraction forces the blood into the bulbous (or conus) arteriosus through a third set of valves. In elasmobranchi and dipnoi fishes there are several sets of valves in the conus.
Blood from the conus arteriosus goes into the gills for oxygenation through blood vessels and then into dorsal aorta for distribution.
Blood from the tissues is collected by the veins and comes back into the heart. Blood passes through the heart only once in its complete circuit so this is known as simple circuit of circulation.
Dipnoi or lung fishes show some advancement in the circulatory system. In them the heart has become three chambered as auricle has become partially divided into right and left auricles.
Deoxygenated blood coming to the sinus venosus enters the right auricle, while the oxygenated blood coming from the air bladder enters the left auricle.
In Amphibia and reptiles the heart is three chambered but to some extent double circulation is maintained. Birds and mammals have a complete double circulation.
This situation is achieved by the development of the complete ventricular septum, with the pulmonary artery originating from the right ventricle and a single arota originating from the left.
In birds and mammals the heart is four chambered consisting of two auricles and two ventricles.
From the left ventricle aorta takes its origin which is divided into different arteries which supply different parts of the body.
Blood from different parts of the body is collected by tiny veins which in turn join to form larger veins.
The veins of the posterior part of the body empty into the inferior vena-cava, while of anterior region empty into the superior vena-cava.
Both these larger veins empty into the right auricle. The blood from the right auricle goes into the right ventricle through auriculo-ventricular valve.
From the right ventricle it is pumped into the lungs through the pulmonary artery where it is oxygenated.
After oxygenation blood through pulmonary vein comes into the left auricle from where it is transferred into the left ventricle from where it is pumped into the aorta.
The blood thus circulates continuously. This type of circulation in which the blood passes twice through the heart during one complete circuit is called a double circulation.