In mammals including humans, the lymphatic vessels (or lymphatics) are a network of thin tubes that branch, like blood vessels, into tissues throughout the body. Lymphatic vessels carry lymph, a colorless, watery fluid originating from interstitial fluid (fluid in the tissues). The lymphatic system transports infection-fighting cells called lymphocytes, is involved in the removal of foreign matter and cell debris by phagocytes and is part of the body's immune system. It also transports fats from the small intestine to the blood.
The spleen, thymus and bone marrow are considered as lymphoid tissue. The spleen acts to filter and modify the blood while the bone marrow and thymus act to produce and mature lymphocytes and other immune cells. The lymphatic system consists of the lymphatics and the lymphoid tissue.
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 thoracic duct (for the rest of the body); these ducts drain into the circulatory system at the right and left subclavian veins (near the shoulders).
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.
Lymph vessels are present in the lining of the gastrointestinal tract. Fats absorbed from food by the small intestine are passed on to the lymph, to be transported to the blood circulation via the thoracic duct. Unlike carbohydrates and proteins, ingested fats therefore pass to the circulation without any processing by the liver.
Whenever the lymphatic system cannot drain interstitial fluid from tissues faster than they accumulate, the resulting swelling is known as lymphedema. Cancerss that develop in the lymphatic system are known as lymphomas.