The heart is a muscle. It has four chambers that pump blood. The upper two chambers are called the "atria" and the lower two chambers are called the "ventricles". The heart is really two pumps that work together to move blood through the body.
The right-sided pump fills with blood returning from the body and pumps it to the lungs to receive oxygen. This right-sided pump is thinner than the left-sided pump because it only has to pump blood to the lungs.
The left side of the heart receives blood from the lungs and pumps it out to all parts of the body. This left-sided pump is a thick, strong muscle. The heart has an electrical system that sends impulses to tell the heart muscle when to pump.
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