In anatomy, lymph vessels are thin walle, valved structures that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the vascular system. In contrast to the vascular system, which carries blood under pressure to the entire body, lymph is not under pressure and is propelled in a passive fashion, assisted by the aforementioned valves. Fluid that leaks from the vascular system is returned to general circulation via lymphatic vessels.