Physiology: Lymph Flow

The lymphatic system is an elaborate network of vessels that act harmoniously to pump fluid and cells, collectively called lymph, from the interstitial space into the blood circulation. The journey of lymph begins with the extravasation of fluid and cells from the blood capillaries into the interstitium. These are then taken up by the initial lymphatic vessels and are termed lymph. The initial lymphatic vessels have unique button-like junctions that form structures called «primary lymphatic valves» which allow unidirectional flow of interstitial fluid into the initial lymphatics. The fluid is then drained into the pre-collecting and the collecting lymphatic vessels. Bileaflet valves, found throughout the collecting lymphatic vessels, prevent the backflow of lymph and segment the vessels into discrete sections known as lymphangions. Lymphangions act as discretized pumps due to their ability to rapidly and phasically contract their walls through the action of lymphatic muscle cells, propelling lymph downstream. Collecting vessels connect to lymph nodes where lymph is filtered. A small fraction of entering lymph flows deeper into the lymph node and enters the blood circulation through lymph node venules. The majority of lymph flows into efferent lymphatics and is eventually merging into the thoracic duct and is deposited into the subclavian vein after passing through a final lymphovenous valve.