The lymphatic system is a separate entity within the circulatory system and acts as an additional drainage system. The fluid circulating in the lymphatic system is known as lymph; it consists of plasma and excess interstitial fluid which drains from tissue into the lymph vessels, which will ultimately return the fluid into the systemic circulation. It contains white but no red blood cells.  Lymph vessels are very thin-walled and also have valves to prevent a backflow of lymph. The small lymph vessels drain into larger vessels which eventually join to form the main thoracic lymph duct, the Ductus thoracicus. This empties into the venous system where the lymph is returned to the blood.

In addition to a drainage system, the lymphatic system also plays an important role in an organism’s immune system. The lymphatic system is interspersed with a multitude of lymph nodes, usually at the confluence of several lymph vessels. These nodes contain a multitude of white blood cells or lymphocytes (and other white blood cells) which screen the incoming fluid and can filter out and neutralise pathogens, foreign bodies and pathologically changed cells (e.g. tumour cells).