Which structure prevents backflow of lymph toward the tissues?

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Valves in lymph vessels are the mechanism that prevents backflow. They act as one-way flaps that open to let lymph move forward toward larger vessels and eventually into the bloodstream, but they close if lymph tries to flow backward toward the tissues. This unidirectional setup is essential because lymph is collected from interstitial fluid in tissues and needs to return to the circulatory system efficiently. Endothelial cells line the inside of lymph vessels but don’t stop backward flow by themselves; they form the inner lining. The interstitium is the tissue space where lymph originates and accumulates, not a structural valve. Lymphatic tissue, like lymph nodes, filters lymph but isn’t what prevents backward movement. So the valves are the correct structure for preventing backflow.