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There are a couple of primary differences between the atria and ventricles. The heart has four chambers. Atria make up the top chambers of the heart whereas ventricles make up the bottom chambers. This means that there are atrium and ventricle on both, right and left sides of the heart. The right part of the heart receives unoxygenized (impure) blood and sends it from ventricles to the atria. Atria are small cavities whereas the ventricles tend to be larger cavities.
The atria have relatively thin walls, but are sufficiently thick to serve their function, which is containing and carrying blood. The right atrium receives old blood, whose oxygen has been distributed to the tissues of the body in exchange for CO2 and other tissue waste materials. The blood from the upper body is supplied from the superior vena cave, while the inferior vena cave supplies the right atrium with blood from the lower body. The left atrium receives filtered, purified blood from the lungs through the pulmonary veins.
The ability to pump blood is another difference between the atrium and ventricles. The ventricles have thick walls, which help them contract, pumping blood in and out of the heart. The right ventricle receives the deoxygenated blood from the right atrium and pumps it into the lungs for purification. The left ventricle receives the clean blood from the left atrium and pumps it into the aorta, which is the largest artery in the body. The aorta then carries the blood to the rest of the body.
Another difference between the atrium and ventricles is the valves they are attached to. Heart valves serve as a sort of seal, which open when blood is pumped from one area to the other and closes to prevent the blood from leaking backward. The tricuspid valve separates the right atrium from the right ventricle, which seals when the right ventricle receives the blood to prevent splash back into the right atrium. The pulmonary valve opens to allow the right ventricle to pump blood into the pulmonary artery. The mitral or bicuspid valve separates the left atrium from the left ventricle. Lastly, the aortic valve serves as a barrier between the left ventricle and the aorta.