The lymphatic system consists of lymphatic vessels, a fluid called lymph, lymph nodes, the thymus, and the spleen. This system supplements and extends the cardiovascular system in the following ways:
-
The lymphatic system collects excess fluids and plasma proteins from surrounding tissues (interstitial fluids) and returns them to the blood circulation. Because lymphatic capillaries are more porous than blood capillaries, they are able to collect fluids, plasma proteins, and blood cells that have escaped from the blood. Within lymphatic vessels, this collected material forms a usually colorless fluid called lymph, which is transported to the right and left subclavian veins of the circulatory system.
-
The lymphatic system absorbs lipids and fat‐soluble materials from the digestive tract.
-
The lymphatic system filters the lymph by destroying pathogens, inactivating toxins, and removing particulate matter. Lymph nodes, small bodies interspersed along lymphatic vessels, act as cleaning filters and as immune response centers that defend against infection.
The movement of lymph through lymphatic vessels is slow (3 liters/day) compared to blood flow (about 5 liters/minute). Lymph does not circulate like blood, but moves in one direction from its collection in tissues to its return in the blood. There are no lymphatic pumps. Instead, lymph, much like blood in veins, is propelled forward by the action of the nearby skeletal muscles, the expansion and contraction of the lungs, and the contraction of the smooth muscle fibers in the walls of the lymphatic vessels. Valves in the lymphatic vessels prevent the backward movement of lymph.