What prevents back flow of blood?

Backflow of blood is prevented primarily by the presence of valves within the veins and the heart. In the heart, the atrioventricular (AV) valves and semilunar valves ensure that blood flows in one direction, preventing regurgitation. Additionally, venous valves in the extremities help maintain unidirectional flow toward the heart, especially against gravity. The contraction of surrounding muscles also aids in pushing blood forward and closing the valves to prevent backflow.