<P> The lymphatic conducting system broadly consists of two types of channels--the initial lymphatics, the prelymphatics or lymph capillaries that specialize in collection of the lymph from the ISF, and the larger lymph vessels that propel the lymph forward . </P> <P> Unlike the cardiovascular system, the lymphatic system is not closed and has no central pump . Lymph movement occurs despite low pressure due to peristalsis (propulsion of the lymph due to alternate contraction and relaxation of smooth muscle), valves, and compression during contraction of adjacent skeletal muscle and arterial pulsation . </P> <P> The lymphatic circulation begins with blind ending (closed at one end) highly permeable superficial lymph capillaries, formed by endothelial cells with button - like junctions between them that allow fluid to pass through them when the interstitial pressure is sufficiently high . These button - like junctions consist of protein filaments like platelet endothelial cell adhesion molecule - 1, or PECAM - 1 . A valve system in place here prevents the absorbed lymph from leaking back into the ISF . There is another system of semilunar (semi = half; lunar = related to the Moon) valves that prevents back - flow of lymph along the lumen of the vessel . Lymph capillaries have many interconnections (anastomoses) between them and form a very fine network . </P> <P> Rhythmic contraction of the vessel walls through movements may also help draw fluid into the smallest lymphatic vessels, capillaries . If tissue fluid builds up the tissue will swell; this is called edema . As the circular path through the body's system continues, the fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body); both ducts drain into the circulatory system at the right and left subclavian veins . The system collaborates with white blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or bacteria . This is known as a secondary circulatory system . </P>

Structure that prevents backflow of fluid in lymphatic vessels