Anatomi og fysiologi Lymfesystemet PDF

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Anatomi og fysiologi Lymfesystemet KAP 20 The lymphatic system returns fluids that have leaked from the vascular system back to the blood. It consists of three parts: - A meandering network of lymphatic vessels - Lymph, the fluid contained in those vessels - Lymph nodes that cleanse the lymph as it passes through them The lymphoid organs and tissues provide the structural basis of the immune system. These organs and tissues play essential roles in body`s defense mechanisms and its resistance to disease. These structures include the spleen, thymus, tonsils, and other lymphoid tissues scattered throughout the body. The lymph nodes are also part of this system, and like a keystone, they have important roles to play in both the lymphoid organs and tissues and the lymphatic system. 20.1 Once interstitial fluid enters the lymphatic vessels, it is called lymph. The lymphatic vessels form a one-way system in which lymph flows only toward the heart. When fluid pressure in the interstitial space is greater than the pressure in the lymphatic capillary, the minivalve flaps gape open, allowing fluid to enter the lymphatic capillary. However, when the pressure is greater inside the lymphatic capillary, it forces the endothelial minivalve flaps shut, preventing lymph from leaking out as the pressure moves it along the vessel. Proteins in the interstitial space are unable to enter blood capillaries, but they enter lymphatic capillaries easily. In addition, when tissues become inflamed, lymphatic capillaries develop openings that permit uptake of even larger particles such as cell debris, pathogens, and cancer cells. The pathogens can then use the lymphatics to travel throughput the body. This threat to the body is partly offset by the lymph traveling through the lymph nodes, where it is cleansed of debris and “examined” by cells of the immune system. The collecting lymphatic vessels have the same three tunics as veins, but the collecting vessels have thinner walls and more internal valves, and they anastomose more. In general, lymphatics in the skin travel along with superficial veins, while the deep lymphatic vessels of the trunk and digestive system travel with the deep arteries. The lymphatic system lacks an organ that acts as a pump. Under normal conditions, lymphatic vessels are low pressure conduits, and the same mechanisms that promote venous return in blood vessels act here as well- the milking action of active skeletal muscles, pressure changes in the thorax during breathing and valves to prevent backflow. Lymphatic vessels are usually bundled together in connective tissue sheaths along with blood vessels, and pulsations of nearby arteries also promote lymph flow. In addition to these mechanisms, smooth muscle in the walls of all but the smallest lymphatic vessels contracts rhythmically, helping to pump the lymph along. 20.2 Lymphocytes are the main warriors of the immune system. There are two main varieties of lymphocytes- T cells and B cells- that protect the body against antigens. Activated T cells manage the immune response, and some of the, directly attack and destroy infected cells. B

cells protect the body by producing plasma cells, daughter cells that secrete antibodies into the blood. Antibodies mark antigens for destructions. Macrophages play a crucial role in body protection and the immune response by phagocytizing foreign substances and by helping to activate T cells. So too do dendritic cells that capture antigens and bring then back to the lymph nodes. Last but not least are the reticular cells, fibroblast like cells that produce the reticular fiber stroma, which is the network that supports the other cell types in lymphoid organs and tissues. Lymphoid tissue is an important component of the immune system, mainly because it: - Houses and provides a proliferation site for lymphocytes - Furnishes an ideal surveillance vantage point for lymphocytes and macrophages The lymphoid organs are grouped into two functional categories: - The primary lymphoid organs are where B and T cells mature- the red bone marrow and the thymus. While both B and T cells originate in the red bone marrow, B cells mature in the red bone marrow and T cells mature in the thymus. - The secondary lymphoid organs are where mature lymphocytes first encounter their antigens and are activated. They include the lymph nodes, the spleen, and the collections of mucosa-associated lymphoid tissue that forms the tonsils, Peyer`s patches in the small intestine, and the appendix. Lymphocytes also encounter antigens and are activated in the diffuse lymphoid tissues. Although all lymphoid organs help protect the body, only the lymph nodes filter lymph. The other secondary lymphoid organs typically have efferent lymphatics draining them, but lack afferent lymphatics. 20.3 Large clusters of lymph nodes occur near the body surface in the inguinal, axillary and cervical regions, places where the collecting lymphatic vessels converge to form trunks. Lymph nodes have two basic protective functions: - Cleansing the lymph. As lymph is transported back to the bloodstream, the lymph nodes act as lymph filters. Macrophages in the nodes remove and destroy microorganisms another debris that enter the lymph from the loose connective tissues, preventing them from being delivered to the blood and spreading to other parts of the body. - Immune system activation. Lymph nodes and other lymphoid organs are strategically located sites where lymphocytes encounter antigens and are activated to mount an attack against them 20.4 The spleen is the largest lymphoid organ. The spleen provides a site for lymphocyte proliferation and immune surveillance and response. But perhaps even more important are its blood cleansing functions. Besides extracting aged and defective blood cells and platelets from the blood, its macrophages remove debris and foreign matter. The spleen also performs three additional, and related, functions: - It recycles the breakdown products of red blood cells for later reuse. It releases the breakdown products to the blood for processing by the lover and stores some of the iron salvaged from hemoglobin

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Stores blood platelets and monocytes for release into the blood when needed May be a site of erythrocyte production in the fetus

20.5 Mucosa- associated lymphoid tissues (MALT) are a set of distributed lymphoid tissues strategically located in mucous membranes throughout the body. MALT help protect us from the never-ending onslaught of pathogens that seek to enter our bodies. The largest collections of MALT= the tonsils, Peyer´s patches and appendix. In addition to these large named collections, MALT also occurs in the mucosa of the respiratory system, and genitourinary organs as well as the rest of the digestive tract. The tonsils form a ring of lymphoid tissue around the entrance to the pharynx (throat), where they appear as swellings of the mucosa. The tonsils gather and remove many of the pathogens entering the pharynx in food or in inhaled air. The tonsils are not fully encapsulated, and the epithelium overlying them invaginates deep into their interior, forming blind ended tonsillar crypts. The crypts trap bacteria and particular matter, and the bacteria work their way through the mucosal epithelium into the lymphoid tissue where most are destroyed. Peyer`s patches, or aggregated lymphoid nodules, are large clusters of lymphoid follicles, structurally similar to the tonsils. They are located in the wall of the distal portion of the small intestine. The appendix contains a high concentration of lymphoid follicles. Like Peyer`s patches, the appendix is in an ideal position (1) to prevent bacteria from breaching the intestinal wall, and (2) to generate many “memory” lymphocytes for long term immunity. 20.6 The thymus has important functions primarily during early years of life. In the thymus, T lymphocyte precursors mature to become immunocompetent lymphocytes. Most thymic cells are lymphocytes. The thymus is a primary lymphoid organ and differs from secondary lymphoid organs in three important ways: - The thymus has no follicles because it lacks B cells - The thymus does not directly fight antigens. Instead the thymus functions strictly as a maturation site for T lymphocyte precursors. These precursors must be kept isolated from foreign antigens to prevent their premature activation. In fact, there is a blood thymus barrier that keeps bloodborne antigens out of the thymus - The stroma of the thymus consists of epithelial cells rather than reticular fibers. These epithelial cells provide the physical and chemical environment in which T lymphocytes mature....