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We are all aware of the circulatory system which carries blood from the heart and lungs to the body via the arteries and capillaries. And of course, the blood returns to the heart via the veins to complete the circuit. However, blood is pumped under pressure, and some of the fluid leaks from the capillaries into the surrounding tissue, carrying nutrients to the cells. Where does that fluid go? Why does it not simply cause the body to swell and the arteries run dry?
The answer: there is a separate system of drainage vessels that carry the fluid (lymph) that bathes the body cells back towards the heart. Along the way, filters known as lymph nodes provide a locale for the the lymphocytes of the immune system to monitor for and fight infection and gobble up the debris of worn-out cells.
The lymphatic vessels and lymph nodes contain valves that prevent backflow, so that movements of the body and the normal pressure of the exuding lymph from the arterial system tends to force the lymph on a one-way trip toward the heart. The main lymph channels coalesce into two large channels and empty into the main venous circulation at the left and right subclavian veins in the upper chest. In adults about a gallon of lymph is recycled this way per day.