What do veins contain to prevent backflow of blood?

Veins contain valves that play a crucial role in preventing the backflow of blood. These valves are formed from folds of the inner lining of the vein and are structured to open in response to the flow of blood towards the heart and close to prevent it from flowing backwards. This is particularly important in the limbs, where blood must travel against the force of gravity to return to the heart. The presence of valves in veins ensures efficient circulation, especially during activities such as walking, where muscle contractions help push blood through the veins.

Muscle layers are present in the walls of veins to help propel blood, but they are not the primary structure responsible for preventing backflow. Connective tissue provides support but does not actively prevent the reverse flow of blood like valves do. Arteries, on the other hand, do not contain valves because blood is under higher pressure and flows away from the heart. Therefore, the function of valves is specific and essential in veins to maintain proper blood flow direction.