Lymphangion

A lymphangion is the functional unit of a lymph vessel that lies between two semilunar (half moon-shaped) valves.

[1] [2] Lymph vessels are channels larger than the lymph capillaries that have thicker walls, valves in their lumen and smooth muscles in their walls, thus lymph vessel lymphangion is muscular and capable of contracting on its own.

Additionally the lymph in it is propelled forward only because of force exerted on its walls from the exterior.

Such forces include skeletal muscle contractions and arterial pulsations.

In conditions when the pressure in a lymphatic is sufficiently great, the valves may fail, and there can indeed be backward flow of lymph resulting in edema of the drained region.