The pulmonary vein travels parallel to the pulmonary artery as it carries the blood back up to the heart. When the muscular wall of the right ventricle contacts, the blood inside the heart chamber is put under more pressure, and the tricuspid valve closes. As a result, the only exit is through the pulmonary trunk, which divides to form the right and left pulmonary arteries. At the base of this trunkmycontentbreak is a pulmonary semilunar valve that is made up of three leaflets or cusps. This valve opens when the right ventricle contracts. When the right ventricular muscles relax, blood starts back up the pulmonary trunk, causing the valve to close to prevent the flow from returning into the ventricular chamber.