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50702649-The System - Lymphatic System A n important...

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Unformatted text preview: Lymphatic System A n important supplement to the cardiovascular system in helping to remove toxins from the body, the lymphatic system is also a crucial support of the immune system. Unlike blood, lymph only moves one way through your body, propelled by the action of nearby skeletal muscles. The lymph is pushed into the bloodstream for elimination. Appreciating the importance of the lymphatic system in filtering, recycling, and producing blood as well as filtering lymph, collecting excess fluids, and absorbing fat-soluble materials is important in the understanding of human physiology. The lymphatic system consists of lymphatic vessels, a fluid called lymph, lymph nodes, the thymus, and the spleen . This system supplements and extends the cardiovascular system in the following ways: y The lymphatic system collects excess fluids and plasma proteins from surrounding tissues (interstitial fluids) and returns them to the blood circulation. Because lymphatic capillaries are more porous than blood capillaries, they are able to collect fluids, plasma proteins, and blood cells that have escaped from the blood. Within lymphatic vessels, this collected material forms a usually colorless fluid called lymph, which is transported to the neck, where it empties into the circulatory system. y The lymphatic system absorbs lipids and fat-soluble materials from the digestive tract. y The lymphatic system filters the lymph by destroying pathogens, inactivating toxins, and removing particulate matter. Lymph nodes, small bodies interspersed along lymphatic vessels, act as cleaning filters and as immune response centers that defend against infection. The movement of lymph through lymphatic vessels is slow (3 liters/day) compared to blood flow (about 5 liters/minutes). Lymph does not circulate like blood, but moves in one direction from its collection in tissues to its return in the blood. There are no lymphatic pumps. Instead, lymph, much like blood in veins, is propelled forward by the action of the nearby skeletal muscles, the expansion and contraction of the lungs, and the contraction of the smooth muscle fibers in the walls of the lymphatic vessels. Valves in the lymphatic vessels prevent the backward movement of lymph. Lymphatic Vessels Lymphatic vessels occur throughout the body alongside arteries (in the viscera) or veins (in the subcutaneous tissue). They are absent from the central nervous system, bone marrow, teeth, and avascular tissues. y Lymph capillaries, the smallest lymphatic vessels, begin as dead-end vessels. They resemble blood capillaries, but are much more porous to surrounding fluids due to the following two features: y V alvelike openings form at the juncture of adjacent endothelial cells. Unlike the tightly joined endothelial cells that make up the walls of blood capillaries, those of lymph capillaries loosely overlap. When fluid pressure increases in surrounding regions, the overlapped cells separate, allowing fluids to enter the lymph capillary. When pressure inside the capillary exceeds the pressure outside, the spaces between the endothelial cells close, holding fluids inside the capillary. y A nchoring filaments attach the endothelial cells of the lymphatic vessels to surrounding collagen. When interstitial fluid pressure increases, the anchoring filaments prevent the endothelial cells from collapsing, keeping the spaces between the endothelial cells open. Lacteals are specialized lymph capillaries that occur in the fingerlike projections (villi) that extend into the small intestine. Lacteals absorb lipids from the intestinal tract. The lymph within these capillaries, called chyle, has a creamy white color (rather than clear) due to the presence of fats. Lymphatic collecting vessels form as lymph capillaries merge. Collecting vessels have the following characteristics: y V alves are present to prevent the backward flow of lymph (as in veins). y The walls of collecting vessels consist of the same three tunics (layers) that characterize veins, but the layers are thinner and poorly defined. Lymphatic trunks form from the union of collecting vessels. The nine major trunks, draining lymph from regions for which they are named, are the lumbar, jugular, subclavian, and bronchomediastinal trunks, each of which occurs in pairs (left and right, for each side of the body), and a single intestinal trunk. Lymphatic ducts are the largest lymphatic vessels. These two ducts drain lymph into veins in the neck (the right and left subclavian veins at their junctures with the internal jugular veins). Valves in the lymphatic ducts at their junctures with the veins prevent the entrance of blood into the lymphatic vessels. y The thoracic duct (left thoracic duct) collects lymph from the left side of the body and regions of the right side of body below the thorax. It begins at the cisterna chili, an enlarged region of the lymphatic vessel that forms following the union of the intestinal trunk and right and left lumbar trunks. y The right thoracic duct collects lymph from the upper right side of the body (right arm and right regions of thorax, neck, and head), a much smaller area than that serviced by the thoracic duct. Lymphoid Cells Lymphatic (lymphoid) tissue is a kind of connective tissue. It consists of the following types of cells: y Lymphocytes are white blood cells (leukocytes) that provide an immune response that attacks specific kinds of nonself cells and foreign substances (antigens). There are two major classes of lymphocytes: y T cells (T lymphocytes) originate in the bone marrow but mature in the thymus gland. T cells attack self cells that have been invaded by pathogens, abnormal self cells (such as cancerous cells), or nonself cells (such as those that might be introduced in an organ transplant). y B cells (B lymphocytes) originate and mature in the bone marrow. When B cells encounter an antigen (a toxin, virus, or bacterium), they produce plasma cells and memory cells. Plasma cells release antibodies that bind to the antigen and inactivate it. Memory cells circulate in the lymph and blood with the capacity to produce additional antigens for future encounters with the same antigen. Macrophages are enlarged monocytes (white blood cells) that engulf microbes and cellular debris. R eticular cells and their reticular fibers made from collagen and glycoproteins provide a network within which the lymphocytes and other cells reside. Lymphatic Tissues and Organs Lymphatic cells are organized into tissues and organs based upon how tightly the lymphatic cells are arranged and whether the tissue is encapsulated by a layer of connective tissue. Three general categories exist: y D iffuse, unencapsulated bundles of l ymphati c cell s. Thi s kind of l ymphati c ti ssue consi sts of l ymphocytes and macrophages associated w ith a r eti cular fiber network. It occurs i n the l amina propria ( middle l ayer) of the mucus membranes ( mucosae) that l ine the r espi ratory and gastrointestinal tr acts. y D i screte, unencapsulated bundles of l ymphati c cell s, called l ymphati c nodules ( folli cles) . These bundles have clear boundaries t hat separate t hem f rom nei ghboring cell s. N odules occur w i thin t he l amina propria of t he mucus membranes that l ine t he gastrointestinal , respiratory, reproducti ve, and urinary t racts. They are referred t o as mucosa- associated lymphoid t i ssue (MALT). The nodules contain l ymphocytes and macrophages t hat protect against bacteria and other pathogens that may enter t hese passages wi th f ood, air, or urine. N odules occur as soli tary nodules, or t hey cluster as patches or aggregates. H ere are t he maj or clusters of nodules: y Peyer's patches are clusters of lymphatic nodules that occur in the mucosa that lines the ileum of the small intestine. y The tonsils are aggregates of lymphatic nodules that occur in the mucosa that lines the pharynx (throat). Each of the seven tonsils that form a ring around the pharynx are named for their specific region: A single pharyngeal tonsil (adenoid) in the rear wall of the nasopharynx, two palatine tonsils on each side wall of the oral cavity at its entrance in the throat, two lingual tonsils at the base of the tongue, and two small tubal tonsils in the pharynx at the entrance to the auditory tubes. y The appendix, a small fingerlike attachment to the beginning of the large intestine, is lined with aggregates of nodules. Encapsulated organs contain lymphatic nodules and diffuse lymphatic cells surrounded by a capsule of dense connective tissue. The three lymphatic organs are discussed in the following sections. Lymph nodes Lymph nodes are small, oval, or bean-shaped bodies that occur along lymphatic vessels. They are abundant where lymphatic vessels merge to form trunks, especially in the inguinal (groin), axillary (armpit), and mammary gland areas. Lymph flows into a node through afferent lymphatic vessels that enter the convex side of a node. It exits the node at the hilus, the indented region on the opposite, concave side of the node, through efferent lymphatic vessels. Efferent vessels contain valves that restrict lymph to movement in one direction out of the lymph node. The number of efferent vessels leaving the lymph node is fewer than the number of afferent vessels entering, slowing the flow of lymph through the node. Lymph nodes perform three functions: y They filter the lymph, preventing the spread of microorganisms and toxins that enter interstitial fluids. y They destroy bacteria, toxins, and particulate matter through the phagocytic action of macrophages. y They produce antibodies through the activity of B cells. The structure of a lymph node is characterized by the following features: y A capsule of dense connective tissue surrounds the lymph node. y Trabeculae are projections of the capsule that extend into the node forming compartments. The trabeculae support reticular fibers that form a network that supports lymphocytes. y The cortex is the dense, outer region of the node. It contains lymphatic nodules where B cells and macrophages proliferate. y The medulla is the center of the node. Less dense than the surrounding cortex, the medulla primarily contains T cells. y Medullary cords are strands of reticular fibers with lymphocytes and macrophages that extend from the cortex toward the hilus. y Sinuses are passageways through the cortex and medulla through which lymph moves toward the hilus. Thymus The thymus is a bilobed organ located in the upper chest region between the lungs. It grows during childhood and reaches its maximum size of 40 g at puberty. It then slowly decreases in size as it is replaced by adipose and areolar connective tissue. By age 65, it weighs about 6 g. Each lobe of the thymus is surrounded by a capsule of connective tissue. Lobules produced by trabeculae (inward extensions of the capsule) are characterized by an outer cortex and inner medulla. The following cells are present: y Lymphocytes consist almost entirely of T cells. y Epithelial-reticular cells resemble reticular cells, but do not form reticular fibers. Instead, these star-shaped cells form a reticular network by interlocking their slender cellular processes (extensions). These processes are held together by desmosomes, cell junctions formed by protein fibers. y Epithelial-reticular cells produce thymosin and other hormones believed to promote the maturation of T cells. y Thymic (Hassall's) corpuscles are dense, concentric layers of epithelial-reticular cells. Their function is unknown. The function of the thymus is to promote the maturation of T lymphocytes. Immature T cells migrate through the blood from the red bone marrow to the thymus. Within the thymus, the immature T cells concentrate in the cortex where they continue their development. Mature T cells leave the thymus by way of blood vessels or efferent lymphatic vessels, migrating to other lymphatic tissues and organs where they become active (immunocompetent) in immune responses. The thymus does not provide a filtering function similar to lymph nodes (there are no afferent lymphatic vessels leading into the thymus), and unlike all other centers of lymphatic tissues, the thymus does not play a direct role in immune responses. Blood vessels that permeate the thymus are surrounded by epithelial-reticular cells. These cells establish a protective blood-thymus barrier that prevents the entrance of antigens from the blood and into the thymus where T cells are maturing. Thus, an antigenfree environment is maintained for the development of T cells. Spleen Measuring about 12 cm (5 in) in length, the spleen is the largest lymphatic organ. It is located on the left side of the body between the diaphragm and stomach. Like other lymphatic organs, the spleen is surrounded by a capsule whose extensions into the spleen form trabeculae. The splenic artery, splenic vein, nerves, and efferent lymphatic vessels pass through the hilus of the spleen located on its slightly concave, upper surface. There are two distinct areas within the spleen: y White pulp consists of reticular fibers and lymphocytes in nodules that resemble the nodules of lymph nodes. y R ed pulp consists of venous sinuses filled with blood. Splenic cords consisting of reticular connective tissue, macrophages, and lymphocytes form a mesh between the venous sinuses and act as a filter as blood passes between arterial vessels and the sinuses. The functions of the spleen include the following: y The spleen f il ters t he blood. Macrophages i n t he spleen remove bacteri a and other pathogens, cell ul ar debri s, and aged blood cell s. There are no afferent l ymphati c vessel s and, unli ke l ymph nodes, t he spleen does not filter l ymph. y The spleen destroys old red blood cell s and recycles t heir parts. It removes t he i ron f rom heme groups and binds t he iron t o t he st orage protein. y The spleen provides a reservoir of blood. The diff use nature of t he red pulp retains l arge quanti ties of blood, w hi ch can be directed t o t he circulation w hen necessary. O ne t hird of t he blood platelets are st ored i n t he spleen. y The spleen i s acti ve i n immune responses. T cell s proli f erate i n t he w hite pulp bef ore returning t o t he blood t o attack nonsel f cell s w hen necessary. B cell s prolif erate i n t he w hi te pulp, producing plasma cell s and anti bodies t hat return t o the blood t o i nacti vate anti gens. y The spleen produces blood cell s. R ed and white blood cell s are produced i n t he spleen duri ng f etal development. ...
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