Office of Health Education & Promotion

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The Lymphatic System

The circulatory system exchanges substances between blood and tissue fluid. Many substances, however, that can not move through this system, are returned to the blood as lymph. Lymph is a special fluid, transported by lymphatic vessels, back into the circulatory system. Specifically, lymph forms in this way: blood filters out of the capillaries into very small, microscopic spaces between tissue cells because of pressure generated by the heart pumping. There, this substance is called interstitial fluid or tissue fluid. Most of this fluid goes back into the blood by the same route, but the remainder, now called lymph, enters the lymphatic system through a network of lymphatic capillaries, or tubes. These allow excess tissue fluid, and some other substances like protein molecules, to leave the tissue spaces. Lymph moving through lymphatic capillaries go next into larger and larger lymphatic venules and veins, and eventually empty into the right lymphatic duct and the thoracic duct, which empty their lymph into blood in veins in the neck area.

As lymph moves, it is filtered by lymph nodes, which are located in clusters along the pathway of lymphatic vessels. Lymph nodes have two important functions. The first is biological filtration, a process by which cells alter the fluid passing through the lymph node, filtering out bacteria and preventing it from entering the blood. The second is the formation of white blood cells.

The thymus is a small tissue organ and is located in the middle of the chest just beneath the ribcage. It is composed of lymphocytes, or white blood cells, and assists in the development of specialized cells, called T-lymphocytes, that are critical to healthy immune function.

The tonsils are also lymphoid tissue, and they help protect us against bacteria that may invade tissue around the openings between the nasal and oral cavities.

The spleen is the largest lymphoid organ in the body, and is located in the upper left part of the abdomen, parallel to the stomach. The spleen removes many bacteria and other foreign substances from the blood and destroys worn out red blood cells, saving the iron for future use. It is also a reservoir for blood that can be returned to the circulatory system when needed, holding over one pint of blood.

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The Skeletal System
The Muscular System
The Nervous System
The Digestive System
The Respiratory System

The Endocrine System
The Lymphatic System
The Urinary System
The Reproductive System