The hearts can be classified by the number of chambers such that:
- 2 chambers heart contains one ventricul ad one atrium and it can be found in fish
- 3 chambers heart contains one ventricle and 2 atria and it can be found in reptiles and amphibians
- 4 chambers heart contains 2 ventricles and 2 atria and it can be found in mammals, birds and crocodiles
The separate stages of circulation, pulmonary and systemic circulations, only occur in 4 chambers heart, since only the left ventricle of heart can deal with the blood pressure needed for the oxygen-rich blood, returned from lungs, to be pumped around the body. The lungs do not have the capacity to pump oxygen rich blood throughout the body.
The 4 chambered heart separate the blood in need for oxygen from the oxygen rich blood and it send them to different destinations: the oxygen rich blood is pumped throughout the body, while the blood in need for oxygen is pumped to the lungs.
The 3 chambered heart contains only one ventricle in which oxygen poor and oxygen rich intermingle, but not entirely. In amphibians, the pulmonary respiration is improved because they absorbs oxygen through skin also.