Heart

The heart is a pulsating organ that pushes a liquid medium throughout the body. It may be as simple as one chamber or as complex as four chambers, as in the higher mammals. In all animals, however, it is an organ that must function day after day without pause to keep the blood moving.

In general, blood that returns from the body or from the oxygen exchanging structures returns to an atrium, which is simply a holding chamber. The atrium (plural is atria) empties into another chamber called the ventricle, a muscular chamber that contracts rhythmically to propel the blood through the body. Movement of the blood between chambers and in and out of the heart is controlled by valves that allow movement only in one direction.

The lower vertebrates such as the hagfish and other fish have two-chambered hearts. The ventricle pumps blood forward through the gills to obtain oxygen and dispose of carbon dioxide. From there the blood enters the dorsal aorta and is carried through the body. The blood returns to the heart by means of the sinus venosus, which empties into the auricle or atrium. From there it is passed into the ventricle and the cycle begins again.

Terrestrial vertebrates such as amphibians, have three-chambered hearts and a more complex circulatory system. The third chamber is another auricle or atrium. Unoxygenated blood from the body of the animal returns to the right auricle and the oxygenated blood from the lungs goes into the left auricle. Both auricles contract simultaneously and empty their contents into the single ventricle. The oxygenated and unoxygenated blood does not mix to any degree because of specialized muscle strands in the ventricle. Also, the ventricle contracts immediately after the auricles so the bloods do not have time to mix. When the ventricle contracts, the unoxygenated blood is forced from the heart first and enters the pulmocutaneous vessels leading to the lungs and skin for oxygen exchange. The oxygenated blood enters the truncus arteriosus and flows to the head, arms, body, and hind legs.

Reptiles demonstrate a further step in heart development—a divided ventricle. The wall in the ventricle dividing left from right is incomplete except in crocodiles and alligators. Blood from the body enters the right auricle from which it passes into the right side of the ventricle and is pumped through the lungs. Oxygenated blood from the lungs enters the left auricle and passes into the left side of the ventricle. From there it moves out through the two aortic arches to the body.

Among the crocodilia, however, a peculiar structure of the aortae allows mixing of arterial and venous blood. The left aortic arch rises from the right side of the heart, as does the pulmonary artery. Thus, unoxygenated blood pumped from the right ventricle goes to the lungs through the pulmonary artery and also into the aortic arch and out into the body where it will mix with oxygenated blood. The right aortic arch rises from the left ventricle and carries only oxygenated blood.

Birds also possess four-chambered hearts, in this case with complete separation of the two ventricles. They have only one aortic arch, however, and that is the right arch.

Mammals, including humans, also have a four-chambered heart, but the aortic arch curves up and to the left as it leaves the heart. Here, as with birds, there is no mixing of venous and arterial blood under normal circumstances. Though the heart appears to be a simple organ, it requires a complex series of nerve stimulations, valve openings, and muscle contractions to adequately achieve its purpose.