What feature do lymphatic vessels share with veins that allows lymph to flow in only one direction?

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Valves in lymphatic vessels are the feature that enforces one-way flow. These are flap-like closures that open to let lymph move forward when pressure from surrounding tissues or muscle contractions pushes it along, and then snap shut to prevent it from moving backward. This setup keeps lymph moving toward the venous system even though the overall pressure is low and there’s no central pump like the heart.

The other ideas don’t fit as well: semilunar valves are in arteries, not lymphatics, and they regulate blood leaving the heart. Lymph movement isn’t driven by high pressure; there isn’t a heart-like pump in the lymphatic system, and propulsion mainly comes from surrounding muscle activity and vessel contractions. Gravity can assist, but without valves, lymph could reflux; valves provide the unidirectional guarantee.