The lymphatic system is a collection system which starts in the tissue space as initial lymph collectors that have fenestrated openings to allow fluid and particles to enter. These initial lymph collectors are valveless vessels and go on to form the precollector vessels which have rudimentary valves (which are not considered to be fully functional). These structures go on to form increasingly larger lymphatic vessels which form co-laterals and have lymph-angions (lymph hearts). The lymphatic system, once thought to be passive, is now known to be an active pumping system with active pumping segments with a function similar to that of peristalsis. Lymph hearts have stretch receptors and smooth muscle tissue embedded in their walls. The lymphatic vessels make their way to the lymph nodes and from the lymph nodes the vessels form into trunks which connect to the internal jugular group of veins in the neck, as shown in .
Lymphatic Tissues and Organs
Lymphatic organs play an important part in the immune system, having a considerable overlap with the lymphoid system. Lymphoid tissue is found in many organs, particularly the lymph nodes, as well as in the lymphoid follicles associated with the digestive system such as the tonsils. Lymphoid tissues contain lymphocytes, but they also contain other types of cells for support. The system also includes all the structures dedicated to the circulation and production of lymphocytes (the primary cellular component of lymph), which includes the spleen, thymus, bone marrow, and the lymphoid tissue associated with the digestive system.