The lymphatic system is a network of vessels that picks up excess fluid from the tissues and returns the fluid to the bloodstream near the heart.
Most of the fluid that leaks out from blood vessels moves back into the bloodstream. The lymph vessels pick up larger particles, like proteins, that are to big to re-enter the blood stream.
Lymph moves along through changes in fluid pressures, body movement, breathing and a one way valve system.
Lymph vessels can expand enormously but usually carry only about one-tenth their maximum load. When the lymph load is too great, swelling occurs.