3 When the right ventricle contracts, the blood pressure forces the flaps of the tricuspid valve to close. This prevents backflow of blood into the atrium. The chordae tendineae prevent the flaps from begin reverted into the atrium when the right ventricle contracts. Blood leaves the right ventricle through the pulmonary arch. The pulmonary arch leaves the heart and divides into pulmonary arteries, one to each lung. Semi-lunar valves in the pulmonary arch prevent backflow of blood into the ventricle.