The lymphatic system can be thought of as a secondary circulatory system. The lymph vessels contain a clear, colorless fluid called lymph , which is derived from a network of capillaries which collect this clear fluid as it filters through the capillaries of the blood. Lymph passes from tiny capillaries to lymph vessels and flows through lymph nodes that are located along the course of these vessels. Cells of the lymph nodes phagocytize , or ingest, such impurities as bacteria, old red blood cells, and toxic and cellular waste. Finally, the lymph flows into either the thoracic duct, a large vessel that runs parallel to the spinal column, or into the right lymphatic duct, both of which transport the lymph back into veins of the shoulder areas where is reenters the general circulation.
All lymph vessels contain one-way valves, like the veins, to prevent backflow.
Unlike the blood, which is forced through the arteries by the pumping action of the heart, the flow of lymph is not aided by a pump. The lymph moves by way of the one-way valves, aided by the compression of lymph vessels when the muscles of the body contract. Therefore, exercise and activity is very important for the proper circulation of the lymphatic fluid. The spleen, thymus, and bone marrow manufacture lymphocytes , which are the major cell type of the system. The spleen is also involved in the destruction of old cells and other substances by phagocytosis and plays a role in immune responses.