The left atrium receives oxygenated blood through a smooth-walled portion of its construction from the pulmonary veins. It releases the blood into the left vetricle, which connects via the left AV or mitral valve. The right atrium receives blood from other parts of the body through the sinus venarium, which connects to the vena cava. It releases blood into the right ventricle through the right AV or tricuspid valve.