Thin, fibrous flaps called valves lie at the opening of the heart's pulmonary artery and aorta. Valves are also present between each atrium and ventricle of the heart. Valves prevent blood from flowing backward in the heart. In this illustration of the pulmonary valve, as the heart contracts, blood pressure builds and pushes blood up against the pulmonary valve, forcing it to open. As the heart relaxes between one beat and the next, blood pressure falls. Blood flows back from the pulmonary artery, forcing the pulmonary valve to close, and preventing backflow of blood.