Lymphatic Vessel Histology Key points: • Collect interstitial fluid, carry it to lymph nodes, and ultimately return it to systemic circulation via the thoracic ducts. - Interstitial fluid comprises blood plasma, white blood cells, and other elements. • Have multiple layers of smooth muscle that contract to propel the lymph. • Larger lymph vessels have valves which that unidirectional flow of lymphatic fluid by preventing backflow. • Valves arise from the tunica intima to promote unidirectional flow. • Anchoring filaments attach the vessel to surrounding interstitium. • Interstitial lymph enters through the openings in the vessel wallClinical Correlations: • Because the valves prevent backflow, lymph fluid travels unidrectionally. If the valves fail, lymph fluid pools and causes lymphedema (as with varicose veins, this is particularly relevant in the extremities). • Cancer cells can break from a tumor and travel within the lymph vessels; These cells can settle and proliferate to form new tumors in new areas (metastasis).