The Lymphatic System Chapter 20

Figure 20.1 The Lymphatic System Lymphatic System - Series of vessels, tissues and organs performing 2 major functions: 1.Vessels return to the blood any fluids and proteins that leaked into tissues 2.Tissues and organs house cells for immune functioning

Figure 20.1 Lymphatic Vessels Lymphatic vessels – the series of vessels draining lymph, returning it to the blood in a unidirectional flow Lymph – the protein-rich, interstitial fluid that has entered the lymphatic vessels Lymphatic capillaries – smallest vessels near capillary beds that intake lymph Minivalves – flap-like openings between endothelial cells making lymphatic capillaries highly permeable Collagen filaments – anchor endothelial cells to surrounding connective tissue, encourages minivalve opening

Figure 20.1 Lacteals – specialized lymphatic capillaries of the small intestine that absorb lymph that is high in fat content, called chyle Other Lymphatic Vessels: Lymphatic capillaries merge to form lymphatic collecting vessels, which merge to form lymphatic trunks, which merge to form lymphatic ducts, which drain into venous circulation The larger vessels have tunics and valves similar to veins Lymphatic Vessels

Figure 20.2a Lymphatic Vessels Lymph Transport A unidirectional, low-pressure flow returning protein-rich fluid and absorbed fats to the blood (about 3L/day) No pump – lymph transport relies on body movements and the respiratory and muscular ‘pumps’ seen in veins

Figure 20.3 Lymphatic Tissue Lymphoid cells: Lymphocytes – the T cells and B cells that fight disease-causing pathogens Plasma cells – specialized B cells that produce antibodies Macrophages – phagocytize any foreign cells or debris Dendritic cells – specialized phagocytes with cellular projections Reticular cells – produce the web of reticular fibers (stroma) that support the lymphoid cells

Figure 20.3 Lymphatic Tissue Lymphatic Tissue: Lymphatic tissue - largely reticular connective tissue, it houses large populations of the lymphoid cells on a stroma of reticular fibers Diffuse lymphatic tissue – scattered reticular tissue found in and around all body organs Lymphatic follicles (nodules) – dense masses of reticular tissue, often with a lighter germinal center of dendritic and B cells

Figure 20.4 Lymph Nodes Lymph nodes – the most abundant lymphoid organs, scattered amongst the lymphatic vessels, and clustered in the inguinal, axillary, and cervical regions Filter lymph of foreign cells and debris via phagocytes Activate the immune system by stimulating lymphocytes

Figure 20.4a Lymph Nodes Lymph node structure: bean shaped, 2.5 cm in length Capsule – dense fibrous connective tissue surrounding for shape/support Trabeculae – connective tissue walls dividing the node into compartments Cortex – outer layer of lymphoid follicles Medulla – central region with medullary cord filled with B and T cells Sinuses – spaces allowing lymph movement through the node Afferent vessels – bring lymph into the node Efferent vessels – drain lymph from the node

Figure 20.5 Lymphatic Organs Lymph nodes – most abundant lymphoid organs, and the only ones to filter lymph Other lymphoid organs do NOT filter lymph, include the tonsils, thymus, spleen, appendix, and Peyer’s patches

Figure 20.6 The Spleen Spleen Largest lymphoid organ, in left side of abdomen Capsule – dense fibrous surrounding, prone to rupture White pulp – regions performing immune surveillance via lymphocytes and macrophages Red pulp – area of enlarged veins and macrophages performing erythrocyte breakdown Also stores platelets and salvages iron

Figure 20.7 The Thymus Thymus – a glandular structures in the lower neck that does not contain reticular tissue, but still functions as a lymphoid organ Produces hormones used in the activation of T lymphocytes, peak activity in childhood, then deteriorates after puberty Thymic corpuscles – specialized areas for T cell destruction

Figure 20.8 Tonsils Tonsils – a group of simple lymphoid organs encircling the throat that gather and remove potential pathogens that entered the mouth or nose Tonsillar crypts – invaginations that trap bacteria and other debris, allowing lymphoid cells in the nearby follicles to destroy them

Figure 20.9 M.A.L.T. Peyer’s patches – clusters of lymphoid follicles in the wall of the small intestine Appendix – a cluster of follicles hanging from the start of the large intestine M.A.L.T (mucosa-associated lymphatic tissue) – the appendix, Peyer’s patches, and tonsils all line the mucosa of the digestive tract, trapping and destroying bacteria and generating immunological memory