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Chapter 22. The lymphatic system. The lymphatic system. Consists of a network of tissues, vessels and organs that help maintain body fluid balance. Closely tied to the immune system in protecting the body against foreign organisms. Organs and tissues of the lymphatic system: Lymph
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Chapter 22 The lymphatic system
The lymphatic system • Consists of a network of tissues, vessels and organs that help maintain body fluid balance. • Closely tied to the immune system in protecting the body against foreign organisms. • Organs and tissues of the lymphatic system: • Lymph • Lymphatic vessels • Lymphatic cells • Lymph nodes • Spleen • Tonsils • Peyer’s patches • Thymus
The lymphatic system • Functions: • (a). fluid recovery: reabsorbs 2-4 l/day; prevents edema. • (b). Immunity: lymph nodes and lymph organs monitor lymph and blood for pathogens and cancer cells. • (c). lipid absorption through the lacteals in small intestine.
Distribution of lymph nodes in body. Lymph flows toward the heart with the aid of small valves
Lymphatic fluid • How Formed: produced by filtration of plasma through the capillaries and is based on hydrostatic and osmotic pressures in capillaries and interstitial space. • Pathway of Lymph Flow: Flow is always toward the heart. Begins in dead ended lymphatic capillaries that occur everywhere capillaries exist (except: brain, teeth, bones and bone marrow).
Lymphatic capillaries • lymph capillaries –smallest of lymph vessels; are permeable to lymph fluid and proteins. Lymph capillaries drain into lymph collecting vessels. • lacteals: ‑ highly specialized lymphatic capillaries located in intestinal mucosa - transport absorbed fat from intestines and empty into great thoracic duct
Lymphatic collecting vessels • Lymphatic collecting vessels contain valves that act as smooth muscle pumps to move lymph toward heart. • Travel with veins in superficial tissues/arteries in deeper tissues. • Have the same three tunics as blood vessels but walls are much thinner and lymph pressure is very low. • They also have lymph nodes scattered along their length to filter the lymph.
Lymphatic cells • Natural killer cells (NK cells) large lymphocytes that attack and lyse bacteria, foreign tissue cells and infected host cells. • T lymphocytes mature in the thymus (4 types Tc, Ts, Th, Tm). • B lymphocytes mature in red bone marrow and produce antibodies when activated. • Macrophages develop from monocytes and are phagocytic. • Dendritic cells (branched macrophages) found in skin. • Reticular cells branched cells in stroma of lymphatic organs.
Lymph nodes • Bean shaped organs that cleanse the lymph of pathogens • ~500 lymph nodes in the body ~ 1mm to 25mm. • Node is surrounded by a fibrous capsule from which trabeculae extend inward to split the node into compartments called sinuses. • Lymph enters node via afferent lymphatic vessel and exits through efferent lymphatic vessel back into collecting vessels. • As lymph percolates thru the node macrophages cleanse the lymph of all pathogens.
Lymph Trunk • Formed by the convergence of several lymph collecting vessels that drain large areas of the body. • Six major trunks in body: • Lumbar trunks- lymph from lower limb, pelvic region and anterior abdominal wall. • Intestinal trunk- lymph from stomach, intestines and other digestive organs (chyle). • Bronchomediastinal trunk- lymph from thoracic viscera • Subclavian trunks- lymph from upper limbs, inferior neck and superior thoracic wall. • Jugular trunks- lymph from head and neck. • Intercostal trunk- lymph from intercostal region
Lymphatic ducts • The lymphatic trunks drain into the largest lymphatic vessels: the lymphatic ducts. Most people (80%) have 2 lymphatic ducts: (1) Thoracic duct (present in all people) and (2) the Right lymphatic duct. • The thoracic duct empties into the venous circulation at the junction of the internal jugular and left subclavian veins. Lymph is then incorporated into the circulating blood, eventually cleansed of foreign and dead matter and excreted via the liver.
Thoracic duct and right lymphatic duct
Thoracic duct drains right side of face and most of left thorax and lower body. Right lymphatic drains left side of head and upper body.
Lymphoid tissue • The most important tissue of the immune system as it is responsible for the production and activation of the “B” and “T” lymphocytes. Found in two primary sites: • (1) In the infected mucous membranes of the digestive, respiratory, urinary and reproductive tracts. This tissue is called MALT (mucosa associated lymphoid tissue). • (2) In all lymphoid organs except the thymus. • Lymphoid tissue is a framework of reticular fibers and reticular cells in a network of “B” and “T” lymphocytes as well as macrophages. This network consists of a network of lymphoid follicles with germinal centers of dividing lymphocytes
Lymphoid organs • Primary lymphoid organs are thymus and bone marrow • Secondary lymphoid organs include the lymph nodes, spleen, tonsils, aggregated lymphoid nodules in the intestine and appendix. • The thymus and bone marrow produce “T” and “B” lymphocytes and the secondary organs serve to collect and destroy the infectious microorganisms where they are removed by the liver and spleen.
Red bone marrow • Second largest organ in the body next to skin. • Two types red and yellow; red actively produces all blood cells; yellow is dormant and produces blood cells in extreme emergencies. • Composed of a network of reticular fibers forming cave like structures with reticular cells lining the walls. Within the caves are blood cells in various stages of development and fat cells. • Capillaries run throughout the network and carry the developed blood cells into the circulation.
Spleen • Located in the upper left quadrant of abdominal cavity with a gastric and renal area. • Functions to remove blood borne antigens and aged or defective blood cells. • It is a site for hematopoiesis in fetus and storage of blood throughout life. • Consists of red and white pulp. (seen in fresh tissue). • Red pulp is sinuses engorged with rbc’s. • White pulp is lymphocytes and macrophages collected like sleeves along the splenic artery
Tonsils • Located in the oropharynx, they trap bacteria and foreign pathogens entering the mouth and nose. • Palatine tonsils – either side of and posterior end of oral cavity • Lingual tonsils – base of tongue • Pharyngeal tonsils (adenoids)– posterior wall of nasopharynx
Peyer’s patches • Defined: large isolated clusters of lymphatic nodules found intermingled around the ileum of the small intestines. • Function: destroy bacteria and generates memory lymphocytes for long term immunity against infection.
Thymus • Site where immature lymphocytes develop into “T” lymphocytes • Secretes thymosin and thymopoietin which stimulate “T” cells to become immunocompetent. • Most prominent in childhood and by age 40 is ~ 5% active.