What part of the heart stops the blood from flowing backward?

The part of the heart that prevents blood from flowing backward is primarily the valves. Specifically, the atrioventricular (AV) valves, which include the mitral and tricuspid valves, prevent backflow from the ventricles to the atria, while the semilunar valves (aortic and pulmonary valves) prevent backflow from the aorta and pulmonary artery into the ventricles. These valves ensure unidirectional blood flow through the heart and into the circulatory system.