An often overlooked part of the circulatory system is the lymphatic system. As blood passes through the capillaries, some of the fluid diffuses into the surrounding tissues. Lymph passes from 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, impurities such as bacteria, old red blood cells, and toxic and cellular waste. Finally, lymph flows into 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 mixes with blood and is returned to the heart.
All lymph vessels contain one-way valves, like the veins, to prevent backflow.
The tissues of the lymphatic system include the spleen. The spleen serves as a reservoir for blood, releasing additional blood into the circulatory system as needed. It is also involved with destruction of old cells and other substances by phagocytosis . The lymphatic system is also responsible for collecting nutrients that the digestive system has extracted from our foods, and is a very important part of the immune system. We will cover the lymphatic system in detail in the lesson on the immune system.