Lymphatic system

In the body of mammals including humans, the lymphatic system is a network of thin tubes that branch, like blood vessels, into tissues throughout the body. Lymphatic vessels carry lymph[?], a colorless, watery fluid. The lymphatic system transports infection-fighting cells called lymphocytes and is part of the body's immune system. It also returns interstitial fluid[?] (fluid in the tissues) to the blood system and transports fats from the small intestine to the blood.

Unlike the circulatory system, the lymphatic system is not closed and has no central pump; the lymph moves slowly and under low pressure. Like veins, lymph vessels have one-way valves and depend mainly on the movement of skeletal muscles to squeeze fluid through them. Rhythmic contraction of the vessel walls may also help draw fluid into the lymphatic capillaries. This fluid is then transported to progressively larger lymphatic vessels culminating in the right lymphatic duct (for lymph from the right upper body) and the thoractic duct[?]; these ducts drain into the circulatory system (the right and left subclavian veins) at two locations near the shoulders.

Along this network of vessels are small organs called lymph nodes. Clusters of lymph nodes are found in the underarms, groin, neck, chest, and abdomen. Lymph nodes act as filters, with an internal honeycomb of connective tissue filled with lymphocytes that collect and destroy bacteria and viruses. When the body is fighting an infection, these lymphocytes multiply rapidly and produce a characteristic swelling of the lymph nodes. Other parts of the lymphatic system are the spleen, thymus, tonsils[?], and bone marrow. Lymphatic tissue is also found in other parts of the body, including the stomach, intestines, and skin.

Diagram: The human lymphatic system

Lymph originates as blood plasma lost from the capillary beds of the circulatory system, which leaks out into the surrounding tissues. Although capillaries lose only about 1% of the volume of the fluid that passes through them, so much blood circulates that the cumulative fluid loss in the average human body is about 3L per day. The lymphatic system collects this fluid by diffusion into lymph capillaries, and returns it to the circulatory system. Once within the lymphatic system the fluid is called lymph, and has almost the same composition as the original interstitial fluid.

Whenever the lymphatic system cannot drain interstitial fluid from tissues faster than they accumulate, the result is swelling known as edema.

Another role of the lymphatic system is to absorb and transport fats from the small intestine, where lymphatic capillaries penetrate the villi[?] of the intestinal lining.