As the de-saturated blood leaves the right ventricle, it passes through the pulmonary valve, which has been closed as the right ventricle was filling. The pulmonary valve opens, allowing the blood to leave the right ventricle and flow to the lungs via the pulmonary artery. Once the right ventricle has emptied, the pulmonary valve closes, thereby keeping the blood from re-entering the right ventricle.
