Atria are the heart's upper chambers, and ventricles are the heart's lower chambers. They both play a role in pumping blood but differ in function. The brain also has atria and ventricles.
According to the National Institute of Health, atria in the heart receive impure blood from veins, which is then sent to the ventricles. The left atrium accepts blood from the arteries, then it contracts to force the blood into the aorta. The plural form of atrium is atria.
Ventricles contain valves that pump blood in and out of the heart. The right ventricle pumps the impure blood into the lungs so that it can be purified, and the left ventricle receives oxygenated blood pumped from the left atria. Ventricle walls are thicker than atria walls.