{"id":5275,"date":"2019-06-24T17:38:28","date_gmt":"2019-06-24T17:38:28","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/19-3-lymphatic-system-3\/"},"modified":"2023-11-30T23:14:46","modified_gmt":"2023-11-30T23:14:46","slug":"19-3-lymphatic-system-3","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/acchumanbio\/chapter\/19-3-lymphatic-system-3\/","title":{"raw":"17.3\u00a0Lymphatic System","rendered":"17.3\u00a0Lymphatic System"},"content":{"raw":" \r\n\r\n[caption id=\"attachment_4840\" align=\"aligncenter\" width=\"400\"]\"17.3.1 Figure 17.3.1 Ouch! Tonsillitis can be very painful.\u00a0<\/em>[\/caption]\r\n\r\n
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Tonsillitis<\/h1>\r\n<\/div>\r\nThe white patches on either side of the throat in Figure 17.3.1 are signs of tonsillitis. The tonsils are small structures in the throat that are very common sites of infection. The white spots on the tonsils pictured here are evidence of infection. The patches consist of large amounts of dead bacteria, cellular debris, and white blood cells \u2014 in a word: pus. Children with recurrent tonsillitis may have their tonsils removed surgically to eliminate this type of infection. The tonsils are organs of the lymphatic system.\r\n
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What Is the Lymphatic System?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"2911\"]lymphatic system[\/pb_glossary]<\/strong>\u00a0is a collection of organs involved in the production, maturation, and harboring of white\u00a0blood\u00a0cells\u00a0called lymphocytes. It also includes a network of vessels that transport or filter the fluid known as\u00a0[pb_glossary id=\"4552\"]lymph[\/pb_glossary]<\/strong> in which lymphocytes circulate. Figure 17.3.2 shows major lymphatic vessels and other structures that make up the lymphatic system. Besides the tonsils, organs of the lymphatic system include the thymus, the spleen, and hundreds of lymph nodes distributed along the lymphatic vessels.\r\n\r\n[caption id=\"attachment_4841\" align=\"aligncenter\" width=\"1024\"]\"17.3.2 Figure 17.3.2 The lymphatic system includes organs such as the thymus and spleen, as well as a body-wide network of vessels that transport lymph.<\/em>[\/caption]\r\n\r\nThe lymphatic vessels form a transportation network similar in many respects to the\u00a0[pb_glossary id=\"5835\"]blood vessels<\/span>[\/pb_glossary]<\/span>\u00a0of the\u00a0[pb_glossary id=\"5927\"]cardiovascular system[\/pb_glossary]. However, unlike the cardiovascular system, the lymphatic system is not a closed system. Instead, lymphatic vessels carry lymph in a single<\/em> direction \u2014 always toward the upper chest, where the lymph empties from lymphatic vessels into blood vessels.<\/span>\r\n
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Cardiovascular Function of the Lymphatic System<\/h1>\r\n<\/div>\r\nThe return of lymph to the bloodstream is one of the major functions of the lymphatic system. When blood travels through [pb_glossary id=\"5923\"]capillaries[\/pb_glossary] of the cardiovascular system, it is under pressure, which forces some of the components of blood (such as water, oxygen, and nutrients) through the walls of the capillaries and into the tissue spaces between cells, forming tissue fluid, also called interstitial fluid (see Figure 17.3.3). Interstitial fluid bathes and nourishes cells, and also absorbs their waste products. Much of the water from interstitial fluid is reabsorbed into the capillary blood by osmosis. Most of the remaining fluid is absorbed by tiny lymphatic vessels called lymph capillaries. Once interstitial fluid enters the lymphatic vessels, it is called lymph. Lymph is very similar in composition to blood plasma. Besides water, lymph may contain proteins, waste products, cellular debris, and pathogens. It also contains numerous white blood cells, especially the subset of white blood cells known as lymphocytes. In fact, lymphocytes are the main cellular components of lymph.\r\n\r\n[caption id=\"attachment_4842\" align=\"aligncenter\" width=\"482\"]\"17.3.3 Figure 17.3.3 Fluid and other substances in blood are forced by blood pressure through the walls of capillaries and into the surrounding tissue spaces. Some of the tissue fluid is absorbed by tiny lymphatic vessels, forming lymph. The arrows show the direction of lymph through the\u00a0 lymphatic vessels.<\/em>[\/caption]\r\n\r\nThe lymph that enters lymph capillaries in tissues is transported through the lymphatic vessel network to two large lymphatic ducts in the upper chest. From there, the lymph flows into two major veins (called subclavian veins) of the cardiovascular system. Unlike blood, lymph is not pumped through its network of vessels. Instead, lymph moves through lymphatic vessels via a combination of contractions of the vessels themselves and the forces applied to the vessels externally by skeletal muscles, similarly to how blood moves through veins. Lymphatic vessels also contain numerous valves that keep lymph flowing in just one direction, thereby preventing backflow.\r\n
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Digestive Function of the Lymphatic System<\/h1>\r\n<\/div>\r\n\r\n[caption id=\"attachment_4843\" align=\"alignright\" width=\"253\"]\"17.3.4 Figure 17.3.4 Vessels called lacteals in the villi lining the small intestine are the main way that fatty acids from digestion are absorbed from the gastrointestinal tract. These nutrients eventually reach the blood circulation after traveling through the network of lymphatic vessels.<\/em>[\/caption]\r\n\r\nLymphatic vessels called [pb_glossary id=\"4607\"]lacteals<\/span>[\/pb_glossary]<\/span> (see Figure 17.3.4) are present in the lining of the gastrointestinal tract, mainly in the small intestine. Each tiny [pb_glossary id=\"4543\"]villus[\/pb_glossary] in the lining of the small intestine has an internal bed of capillaries and lacteals. The capillaries absorb most nutrients from the digestion of food into the blood. The lacteals absorb mainly fatty acids from lipid digestion into the lymph, forming a fatty-acid-enriched fluid called [pb_glossary id=\"4844\"]chyle[\/pb_glossary]. Vessels of the lymphatic network then transport chyle from the [pb_glossary id=\"4559\"]small intestine[\/pb_glossary] to the main lymphatic ducts in the chest, from which it drains into the blood circulation. The nutrients in chyle then circulate in the blood to the liver, where they are processed along with the other nutrients that reach the liver directly via the bloodstream.<\/span>\r\n
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Immune Function of the Lymphatic System<\/h1>\r\n<\/div>\r\nThe primary immune function of the lymphatic system is to protect the body against pathogens and cancerous cells. This function of the lymphatic system is centred on the production, maturation, and circulation of lymphocytes. [pb_glossary id=\"4832\"]Lymphocyte<\/strong>s<\/strong>[\/pb_glossary]<\/strong>\u00a0are leukocytes that are involved in the\u00a0[pb_glossary id=\"4822\"]adaptive immune system[\/pb_glossary]. They are responsible for the recognition of \u2014 and tailored defense against \u2014 specific pathogens or tumor cells. Lymphocytes may also create a lasting memory of pathogens, so they can be attacked quickly and strongly if they ever invade the body again. In this way, lymphocytes bring about long-lasting\u00a0immunity\u00a0to specific pathogens.<\/span>\r\n\r\nThere are two major types of lymphocytes, called B cells and T cells. Both B cells and T cells are involved in the adaptive immune response, but they play different roles.\r\n

Production and Maturation of Lymphocytes<\/h2>\r\nLike all other types of blood cells (including erythrocytes), both B cells and T cells are produced from stem cells in the red marrow inside bones. After lymphocytes first form, they must go through a complicated maturation process before they are ready to search for pathogens. In this maturation process, they \u201clearn\u201d to distinguish self from non-self. Only those lymphocytes that successfully complete this maturation process go on to actually fight infections by pathogens.\r\n\r\nB cells mature in the [pb_glossary id=\"4021\"]bone marrow[\/pb_glossary], which is why they are called B cells. After they mature and leave the bone marrow, they travel first to the\u00a0circulatory system\u00a0and then enter the lymphatic system to search for pathogens. T cells, on the other hand, mature in the [pb_glossary id=\"4829\"]thymus[\/pb_glossary], which is why they are called T cells. The\u00a0[pb_glossary id=\"4829\"]thymus[\/pb_glossary]<\/strong> is illustrated in Figure 17.3.5. It is a small lymphatic organ in the chest that consists of an outer cortex and inner medulla, all surrounded by a fibrous capsule. After maturing in the thymus, T cells enter the rest of the lymphatic system to join B cells in the hunt for pathogens. The bone marrow and thymus are called [pb_glossary id=\"4845\"]primary lymphoid organs<\/span>[\/pb_glossary]<\/span>\u00a0because of their role in the production and\/or maturation of lymphocytes.<\/span>\r\n\r\n[caption id=\"attachment_4846\" align=\"alignnone\" width=\"1024\"]\"17.3.5 Figure 17.3.5 The thymus is an important organ of the lymphatic system because it is the location of T cell maturation.<\/em>[\/caption]\r\n

Lymphocytes in Secondary Lymphoid Organs<\/h2>\r\nThe [pb_glossary id=\"4830\"]tonsils<\/span>[\/pb_glossary]<\/span>, [pb_glossary id=\"4497\"]spleen[\/pb_glossary], and [pb_glossary id=\"4831\"]lymph node<\/span>s<\/span>[\/pb_glossary]<\/span>\u00a0are referred to as [pb_glossary id=\"4847\"]secondary lymphoid organs[\/pb_glossary]. These organs do not produce or mature lymphocytes. Instead, they filter lymph and store lymphocytes. It is in these secondary lymphoid organs that pathogens (or their antigens) activate lymphocytes and initiate adaptive immune responses. Activation leads to\u00a0cloning\u00a0of pathogen-specific lymphocytes, which then circulate between the lymphatic system and the blood, searching for and destroying their specific pathogens by producing antibodies against them.<\/span>\r\n

Tonsils<\/h3>\r\nThere are four pairs of human [pb_glossary id=\"4830\"]tonsil<\/strong>s<\/strong>[\/pb_glossary].<\/strong> Three of the four are shown in Figure 17.3.6. The fourth pair, called tubal tonsils, is located at the back of the nasopharynx. The palatine tonsils are the tonsils that are visible on either side of the throat. All four pairs of tonsils encircle a part of the anatomy where the respiratory and gastrointestinal tracts intersect, and where pathogens have ready access to the body. This ring of tonsils is called Waldeyer's ring.<\/span>\r\n\r\n[caption id=\"attachment_4849\" align=\"aligncenter\" width=\"488\"]\"17.3.6 Figure 17.3.6 Three of four pairs of human tonsils are shown in this figure.<\/em>[\/caption]\r\n

Spleen<\/h3>\r\nThe\u00a0[pb_glossary id=\"4497\"]spleen[\/pb_glossary]<\/strong> (Figure 17.3.7) is the largest of the secondary lymphoid organs, and is centrally located in the body. Besides harboring [pb_glossary id=\"4832\"]lymphocytes<\/span>[\/pb_glossary]<\/span>\u00a0and filtering [pb_glossary id=\"4552\"]lymph[\/pb_glossary], the spleen also filters [pb_glossary id=\"2702\"]blood[\/pb_glossary]. Most dead or aged erythrocytes are removed from the blood in the red pulp of the spleen. Lymph is filtered in the white pulp of the spleen. In the fetus, the spleen has the additional function of producing red blood cells. This function is taken over by bone marrow after birth.<\/span>\r\n\r\n[caption id=\"attachment_4850\" align=\"aligncenter\" width=\"544\"]\"17.3.7 Figure 17.3.7 The spleen is a secondary lymphoid organ, where pathogens are likely to encounter lymphocytes and trigger an adaptive immune response.<\/em>[\/caption]\r\n

Lymph Nodes<\/h3>\r\nEach\u00a0[pb_glossary id=\"4831\"]lymph node[\/pb_glossary]<\/strong> is a small, but organized collection of lymphoid tissue (see Figure 17.3.8) that contains many lymphocytes. Lymph nodes are located at intervals along the lymphatic vessels, and lymph passes through them on its way back to the blood.\r\n\r\n[caption id=\"attachment_4851\" align=\"aligncenter\" width=\"465\"]\"17.3.8 Figure 17.3.8 Lymph flows through lymph nodes like this one before returning to the blood.<\/em>[\/caption]\r\n\r\nThere are at least 500 lymph nodes in the human body. Many of them are clustered at the base of the limbs and in the neck. Figure 17.3.9 shows the major lymph node concentrations, and includes the spleen and the region named Waldeyer\u2019s ring, which consists of the tonsils.\r\n\r\n[caption id=\"attachment_4852\" align=\"aligncenter\" width=\"464\"]\"17.3.9 Figure 17.3.9 In this diagram, lymph node regions are shown for the left side of the body only. The same regions are also found on the right side of the body.<\/em>[\/caption]\r\n\r\n
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Feature: Myth vs. Reality<\/h1>\r\n<\/div>\r\nWhen lymph nodes become enlarged and tender to the touch, they are obvious signs of immune system activity. Because it is easy to see and feel swollen lymph nodes, they are one way an individual can monitor his or her own health. To be useful in this way, it is important to know the myths and realities about swollen lymph nodes.\r\n\r\n \r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
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Myth<\/span><\/h2>\r\n <\/td>\r\n

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Reality<\/span><\/h2>\r\n <\/td>\r\n<\/tr>\r\n

\"You should see a doctor immediately whenever you have swollen lymph nodes.\"<\/span><\/em><\/td>\r\nLymph nodes are constantly filtering lymph, so it is expected that they will change in size with varying amounts of debris or pathogens that may be present. A minor, unnoticed infection may cause swollen lymph nodes that may last for a few weeks. Generally, lymph nodes that return to their normal size within two or three weeks are not a cause for concern.<\/span><\/strong><\/td>\r\n<\/tr>\r\n
\"Swollen lymph nodes mean you have a bacterial infection.\"<\/span><\/em><\/td>\r\nAlthough an infection is the most common cause of swollen lymph nodes, not\u00a0<\/span>all<\/em>\u00a0infections are caused by\u00a0bacteria. Mononucleosis, for example, commonly causes swollen lymph nodes, and it is caused by viruses. There are also other causes of swollen lymph nodes besides infections, such as\u00a0cancer\u00a0and certain medications.<\/span><\/strong><\/td>\r\n<\/tr>\r\n
\"A swollen lymph node means you have cancer.\"<\/span><\/em><\/td>\r\nCancer\u00a0is far less likely to be the cause of a swollen lymph node than is an infection. However, if a lymph node remains swollen longer than a few weeks \u2014 especially in the absence of an apparent infection \u2014 you should have your doctor check it.<\/span><\/strong><\/td>\r\n<\/tr>\r\n
\"Cancer in a lymph node always originates somewhere else. There is no cancer of the lymph nodes.\"<\/span><\/em><\/td>\r\nCancers do commonly spread from their site of origin to nearby lymph nodes and then to other organs, but cancer may also originate in the lymph nodes. This type of cancer is called lymphoma.<\/span><\/strong><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n \r\n
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17.3 Summary<\/span><\/h1>\r\n<\/header>\r\n
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