The heart is a powerful muscle slightly larger than a clenched fist that pumps oxygen-rich blood throughout all the parts of the body. It is composed of four chambers: two atria (upper chambers) and two ventricles (lower chambers). A human heart beats an average of 100,000 times per day, during which time it pumps more than 4,300 gallons of blood.

Blood begins its journey through the heart when it is returned to the right atrium from the body in need of oxygen. From the right atrium, the blood flows into the right ventricle, which serves as a pump that delivers the blood to the lungs. Within the lungs, the blood releases waste gases and acquires oxygen. This newly oxygen-rich blood returns from the lungs to the left atrium and flows into the left ventricle. Finally, the left ventricle pumps the oxygen-rich blood out of the heart to all parts of the body.

The human body has about 5.6 liters (6 quarts) of blood, all of which circulates through the body three times every minute.

Model based on:
Medical Modeling: The Human Heart. asileFX.
NYU Heart (Hippocrates Project).
MRI data.