The lymphatic system is an extensive drainage system that returns water and proteins from various tissues back to the bloodstream. It is comprised of a network of ducts, called lymph vessels or lymphatic, and carries lymph, a clear, watery fluid that resembles the plasma of blood. Lymph is composed of water, protein molecules, salts, glucose, urea, lymphocytes, and other substances. Over time, the lymph slowly works its way around the body, eventually re-entering the heart so that it can be circulated all over again.
Lymph can be found all over the body, filling the spaces between muscles, organs, and other structures in the body, and circulating through a series of valves which allow the lymph to flow only in one direction.
The major parts of the system are the bone marrow, spleen, thymus gland, lymph nodes, and the tonsils. Other organs, including the heart, lungs, intestines, liver, and skin also contain lymphatic tissue. About two thirds of all lymph nodes and lymphatic tissue are within or near the gastrointestinal tract.
