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Human stomach. Computer artwork of a human stomach showing its blood supply and lymph nodes. Oxygenated blood flows in the arteries (red) and de-oxygenated blood in the veins (blue). Lymph nodes are found throughout the body and are linked by lymphatic vessels. They act as filters or traps for foreign particles and are an important for the proper functioning of the immune system. (KEYSTONE/SCIENCE PHOTO LIBRARY/BO VEISLAND/SCIENCE PHOTO LIBRARY)