Which of these structures contains valves to prevent blood from flowing in the wrong direction?

The correct answer is that veins contain valves to prevent blood from flowing in the wrong direction. Veins are responsible for returning deoxygenated blood back to the heart, and they often operate against gravity, especially in the limbs. The presence of valves in veins is essential; these structures open to allow blood to flow towards the heart and close to prevent any backflow. This mechanism is crucial for maintaining proper circulation and efficient blood return, particularly in the extremities where the force of gravity can impede upward blood flow.

Arteries, on the other hand, carry blood away from the heart under high pressure and do not require valves to prevent backflow, as the high pressure inherently keeps the blood moving in the correct direction. Similarly, capillaries are the sites of exchange between blood and tissues and do not have valves, since their function is to facilitate diffusion rather than to direct blood flow. Lymph ducts, while also having valves, are not the focus of this question because the context specifically concerns blood circulation rather than the lymphatic system. Thus, the presence of valves in veins is a key feature that helps ensure unidirectional blood flow back to the heart.