The heart is a hollow, muscular organ in vertebrates that pumps blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods.
The heart is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ.
The function of the right side of the heart is to collect deoxygenated blood, in the right atrium, from the body and pump it, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas exchange).
This happens through a passive process called diffusion.
The left side collects oxygenated blood from the lungs into the left atrium.
From the left atrium the blood moves to the left ventricle which pumps it out to the body.
On both sides, the lower ventricles are thicker and stronger than the upper atria.
The muscle wall surrounding the left ventricle is thicker than the wall surrounding the right ventricle due to the higher force needed to pump the blood through the systemic circulation.