The heart is a muscular pump which serves to circulate the blood throughout the body. The structure of the heart in vertebrates (organisms with backbones) is remarkable similar. All vertebrates have at least one atrium (a chamber which receives blood returning from circulation throughout the body) and at least one ventricle (a chamber which pumps blood into the arteries which circulate it throughout the body). As organisms become more complex, so do their hearts. Therefore, fish have only one atrium and one ventricle; amphibians have two atria and one ventricle; reptiles have two atria and two ventricles, although with an incomplete wall separating the two ventricles); birds and mammals have two atria and two ventricles, with complete separation between all four chambers.

In the human heart, the major job of the right side of the heart is to receive oxygen-poor (deoxygenated) blood, and then pump it back to...