<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> <P> The lymph capillaries drain the lymph to larger contractile lymphatics, which have valves as well as smooth muscle walls . These are called the collecting lymphatics . As the collecting lymph vessel accumulates lymph from more and more lymph capillaries in its course, it becomes larger and is called the afferent lymph vessel as it enters a lymph node . Here the lymph percolates through the lymph node tissue and is removed by the efferent lymph vessel . An efferent lymph vessel may directly drain into one of the (right or thoracic) lymph ducts, or may empty into another lymph node as its afferent lymph vessel . Both the lymph ducts return the lymph to the blood stream by emptying into the subclavian veins </P>

Where do lymph vessels join the circulatory system