The right atrium receives oxygen poor blood from both the superior and inferior vena cava. The coronary sinus also delivers oxygen poor blood from the heart muscle itself to the right atrium. From the right atrium, the blood travels through the tricuspid valve into the right ventricle. The tricuspid valve is composed of three cusps which are connected via chorda tendineae to the papillary muscles. The combination of chorda tendineae and papillary muscles prevent the valve cusps from backflowing into the right atrium during contraction. From the right ventricle, blood moves through the semilunar valve into the pulmonary trunk on its way to the lungs to pick up oxygen, and it then returns to the left atrium. From the left atrium, blood moves through the mitral valve into the left ventricle. From the left ventricle, blood moves from through the aortic semilunar valve to the ascending aorta (Fig. 2.4).