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Lymph is a clear, colorless fluid that bathes the body cells. Lymph is constantly being formed from the fluid part of the blood. This fluid part passes out through the capillary walls as blood flows through the capiDaries. Large elements, such as cells and proteins, cannot get through the walls and remain in the blood. Lymph is therefore similar in composition to blood, except that it has no red blood cells or platelets and contains considerably less protein than blood.
Lymph brings nutrients and oxygen to body cells and carries away wastes. After picking up wastes from the cells, some of the lymph passes directly back into the blood through the capillary walls. The rest of the lymph remains in the tissues, eventually returning to the bloodstream through the lymphatic system.
The lymphatic system spreads in a network throughout the body. It consists of a series of vessels, called lymphatics, that run together like streams flowing into a river, becoming progressively larger and larger. The smallest lymphatics, called lymph capillaries, lie next to the blood capillaries. Because the lymph capillaries have extremely thin walls, lymph, together with large particles of waste matter, can easily enter. The lymph then drains through the lymphatics until it reaches the largest vessels, the lymph ducts. There are two lymph ducts: the right lymphatic duct and the thoracic duct. They empty the lymph into large veins in the neck.
Along the course of the lymphatics are accumulations of lymphatic tissue, called lymph nodes or lymph glands. Lymph nodes act as filters and remove abnormal material, microorganisms, and harmful substances from the lymph. They also produce lymphocytes, one of the kinds of white blood cells. Numerous lymph nodes can be felt in the neck, under the jaw, in the armpits, and in the groin. In the course of an infection the lymph nodes often become enlarged and tender.