What functional property of veins contributes to unidirectional flow?

The functional property of veins that specifically contributes to unidirectional flow is the presence of venous valves. These valves are crucial in preventing the backflow of blood as it returns to the heart, especially in the lower extremities where blood must travel against gravity. When these valves are functioning properly, they open to allow blood to flow toward the heart and then close to prevent it from flowing backward. This mechanism is vital in maintaining effective venous return and ensuring that blood circulates efficiently throughout the body.

The other choices do not directly contribute to the unidirectional flow of blood in the same way. For instance, while thick walls and elastic recoil are more relevant to arteries, since they deal with higher pressures and need to accommodate pulsatile blood flow, they do not specifically relate to the prevention of backflow. Surface tension is more applicable in other biological contexts, such as fluid dynamics in capillaries, but does not play a significant role in the unidirectional flow of blood in venous structures. Thus, venous valves are key to maintaining the directionality of blood flow in the venous system.