Lymphatic system

Blood pressure pushes a little fluid out through the thin walls of your capillaries all the time. This fluid bathes your cells, delivers nutrients, and picks up wastes, but most of it must be collected and returned. About 3 liters a day are left behind. The lymphatic system is the drainage network that collects the extra fluid and returns it to the blood, and it also plays a central part in your defenses.

The fluid collected is called lymph. It enters tiny lymph capillaries, which merge into larger lymph vessels, and eventually the lymph empties into large veins near the base of the neck. Unlike the circulatory system, the lymphatic system has no heart. Lymph moves because skeletal muscles squeeze the vessels when you move, because breathing changes pressure in the chest, and because one-way valves, like those in veins, keep it from flowing backward. When lymph cannot drain properly, fluid builds up and causes swelling.

Along the lymph vessels are small bean-shaped lymph nodes, which are clustered in the neck, armpits, and groin. Lymph flows slowly through the nodes, which act as filters. They are packed with lymphocytes and phagocytes that trap and destroy pathogens in the lymph. When you have an infection, immune cells multiply in the nodes and make them swell and feel tender, which is why a doctor feels your neck when you have a sore throat.

Other lymphatic organs have special jobs. The spleen, on the left side of the abdomen, filters blood, removes old red blood cells, and stores platelets and immune cells. The thymus, in the chest, is where T cells mature; it is most active in childhood and shrinks with age. The tonsils at the back of the throat trap germs entering the mouth and nose. Red bone marrow, which you studied with the skeleton, is where lymphocytes are made.

The lymphatic system has a third job: absorbing fats. The lacteals in the villi of the small intestine take up digested fats, which then travel in lymph to the blood.

Common mistake: thinking the lymphatic system is separate from the circulatory system. The two are closely linked: lymph comes from blood and returns to it. Another mistake is thinking swollen nodes mean something is wrong with the nodes themselves; most often they mean the nodes are working. Next lesson: the reproductive system.