<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> <P> The functional unit of a lymph vessel is known as a lymphangion, which is the segment between two valves . Since it is contractile, depending upon the ratio of its length to its radius, it can act either like a contractile chamber propelling the fluid ahead, or as a resistance vessel tending to stop the lymph in its place . </P> <P> Lymph vessels act as reservoirs for plasma and other substances including cells that have leaked from the vascular system and transport lymph fluid back from the tissues to the circulatory system . Without functioning lymph vessels, lymph cannot be effectively drained and edema typically results . </P> <P> The afferent lymph vessels enter at all parts of the periphery of the lymph node, and after branching and forming a dense plexus in the substance of the capsule, open into the lymph sinuses of the cortical part . It carries unfiltered lymph into the node . In doing this they lose all their coats except their endothelial lining, which is continuous with a layer of similar cells lining the lymph paths . </P>

What is the main function of the lymph vessels