Final answer:
The valves located at the entrances of the pulmonary trunk and the aorta are the semilunar valves (pulmonary semilunar and aortic semilunar) that prevent backflow of blood into the ventricles.
Step-by-step explanation:
The valves found at the entrances of the pulmonary trunk and the aorta are the semilunar valves. Specifically, these are the pulmonary semilunar valve and the aortic semilunar valve. The other valves in the heart are the atrioventricular valves, which include the tricuspid valve and the bicuspid, or mitral, valve. Unlike the atrioventricular valves, the semilunar valves are shaped like half-moons, and they operate to prevent backflow of blood from the arteries into the ventricles during relaxation of the heart.
During ventricular contraction, the atrioventricular valves close to prevent backflow of blood into the atria, while the semilunar valves open to allow blood to eject into the pulmonary trunk and aorta. This ensures the unidirectional flow of blood through the heart, critical for effective cardiovascular function.