What prevents backflow of blood in the heart?

valves in the veins.
AV (Atrioventricular) Tricuspid between the right atrium and ventricle, Bicuspid (Mitral) between the left atrium and ventricle,

Semilinar valves = pulmonic valve between the right ventricle and the pulmonary trunk, Aortic valve between the left ventricle and aorta
valves