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Structure and Function
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While blood circulates throughout the body, fluid leaks out from the blood vessels. The lymphatic system includes an intricate network of vessels that collect the excess tissue fluid, called lymph, and return it to the circulation (Fig. 7-1). Lymph is a clear, colorless, alkaline fluid made up mostly of water, along with some protein, salts, fats, white blood cells, and urea (a waste product of protein metabolism). Lymphatic vessels are found throughout the body alongside arteries, veins, and capillaries.
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Flashpoint
While blood vessels rely on the pumping action of the heart, there is no pump for lymphatic vessels; instead, lymph flow is facilitated by the pumping action of skeletal muscles.
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Learning Style Tip
Read twice—slowly and aloud—any key sections of the text that you are struggling to grasp. This sometimes helps to slow down your brain so you can focus on each word and think about what it means before you continue.
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The lymphatic system includes lymph nodes, commonly called glands, which are rich in specialized white blood cells called phagocytes. The phagocytes clean debris from lymph through a process called phagocytosis. In this process, the white blood cells remove microorganisms, cell debris, and blood cells that are damaged, old, or abnormal by engulfing them and literally gobbling them up. Because of these functions, the lymphatic system is also considered part of the immune system. When infection and inflammation occur, the body can respond by increasing the production of phagocytes.
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Lymphatic vessels are located throughout the body and are connected to the superior vena cava, which is where lymph enters the circulatory system and is combined with blood. Lymph nodes are distributed along lymphatic vessels with higher numbers in the neck, axillae (armpits), groin, and abdomen (Fig. 7-2). There are two sets of lymph nodes in the throat, commonly known as the tonsils and adenoids. These nodes may become tender and swollen when you have a cold or sore throat; this occurs when the lymph nodes, which have been working to filter lymph in that area, become overwhelmed and inflamed. An inflamed gland may be referred to as lymphadenitis or lymphadenopathy....