The upper chamber of each half of the heart. The right atrium receives deoxygenated blood from the entire body (except lungs) through the superior and inferior venae cavae and coronary sinus; the left atrium receives oxygenated blood from the lungs through the pulmonary veins. Blood passes from the atria to the ventricles through the atrioventricular valves. In the embryo, the atrium is a single chamber that lies between the sinus venosus and the ventricle.