The Lymphatic System

As blood circulates, some fluid leaks from the blood into the surrounding tissues. This isn't an altogether bad thing. A steady flow of fluid helps to maintain an efficient movement of nutrients and salts from the blood into the tissues. However, more than 3 liters of fluid leak from the circulatory system into surrounding tissues every day! If this leakage continued unchecked, the body would begin to swell with fluid—not a very pleasant prospect.

Fortunately, the interrelationship between two body systems does not allow this to happen.   A network of vessels, nodes, and organs called the lymphatic system collects the fluid that is lost by the blood and returns it back to the circulatory system. The fluid is known as lymph (LIMF). Lymph collects in lymphatic capillaries and slowly flows into larger and larger lymph vessels. Like large veins, lymph vessels contain valves that prevent lymph from flowing backward. Ducts collect the lymph and return it to the circulatory system through two openings in the superior vena cava. The openings are under the left and right clavicle bones just below the shoulders. The figure at right shows the lymphatic system.

Lymphatic System

Along the length of the lymph vessels are small bean-shaped enlargements called lymph nodes. Lymph nodes act as filters, trapping bacteria and other microorganisms that cause disease. When large numbers of microorganisms are trapped in the lymph nodes, the nodes become enlarged. If you have ever had “swollen glands,” you actually had swollen lymph nodes.

Lymph vessels do not merely return excess fluid to the circulation. They also play a very important role in nutrient absorption. Lymph vessels lie near the cells that line the intestines, where they absorb fats and fat-soluble vitamins from the digestive tract and carry them to the blood. Lymph moves through the lymphatic system under osmotic pressure from the blood and is pushed along by the contractions of nearby skeletal muscles. It is important that there is a steady flow of lymph. Edema, a swelling of the tissues due to the accumulation of excess fluid, can occur when lymphatic vessels are blocked due to injury or disease.

In addition to the lymph vessels and lymph nodes, the thymus and spleen also have important roles in the lymphatic system. The thymus is located beneath the sternum. Certain lymphocytes called T cells mature in the thymus before they can function in the immune system. T cells are the cells that recognize foreign “invaders” in the body. The spleen helps to cleanse the blood and removes damaged blood cells from the circulatory system. The spleen also harbors phagocytes that engulf and destroy bacteria and other microorganisms.