The human heart has four chambers. Two lower chambers are called ventricles. The left ventricle is connected to the left atrium, on the left side of the heart. It receives blood from the left atrium and pumps it to the aorta. The main function of this ventricle is to pump the oxygenated blood to tissues all over the body. Mitral valve (bicuspid valve) is the medium by which blood passes into the left ventricle, from the left atrium.
The right ventricle is located below the right atrium on the right side of the heart. The main function of this ventricle is to pump deoxygenated blood to the lungs. The deoxygenated blood that flows into the right atrium passes through the tricuspid valve and enters the right ventricle. The right ventricle is less muscular as it has to pump the blood to a shorter distance (heart to lung).
The left atrium is the upper-left chamber of the heart. It acts as a vessel for blood returning from the lungs and pumps blood to the left ventricle through the mitral valve. The left atrium holds the oxygen-rich blood that enters from the lungs, through the pulmonary vein.
The right atrium is located on the upper-right side of the heart. It receives deoxygenated blood from the superior and the inferior vena cavae. The deoxygenated blood is pumped to the lungs by the right side of the heart.