Answer: The heart valves prevent blood from flowing backwards.
Step-by-step explanation:
The openings between atria and ventricles are guarded by atrioventricular valves also called as AV valves. AV valve between right atrium and right ventricle has three flaps or cusps and is therefore called the tricuspid valve. The AV valve between the left atrium and left ventricle has two flaps or cusps and is therefore called bicuspid valve or mitral valve. During ventricular contraction these valves close to prevent the back flow of blood into the atria.
Three semilunar valves are found at the points where the pulmonary artery and aorta leave the heart. These valves prevent blood from flowing back into the ventricles.