Heart
In humans, the four chambers are two atria and two ventricles. Atria is talking about two chambers; atrium is talking about one chamber. There is a right atrium and right ventricle. These get blood that comes to the heart. They pump this blood to the lungs. In the lungs blood picks up oxygen and drops carbon dioxide. Blood from the lungs goes to the left atrium and ventricle. The left atrium and ventricle send the blood out to the body. The left ventricle works six times harder than the right ventricle because it carries oxygenated blood.