Lymphatic SystemLymphatic Vessels transport only in the direction toward the heart .Pick up the extra tissue fluid that capillaries leaveLymph the interstitial fluid carried by lymphaticsLymphatic Organs filtration devices staged at various important junction points in the body

Lymph VesselsLymphatic capillaries diffuse, highly permeable due to presence of:Minivalves loosely overlapping endothelial cellsCollagen fiber attachment to surrounding structures prevents collapse of capillaryLymphatic collecting vessels like veins but thinner walled w/ more valves and anastomosesLymphatic trunks lumbar, bronchomediastinal, subclavian, jugular and intestinalLymphatic ducts Right Lymphatic Duct & Thoracic Duct

Lymphatic CapillariesFigure 20.1b

Lymphatic CapillariesDuring inflammation, lymph capillaries can absorb:Cell debrisPathogensCancer cells Cells in the lymph nodes cleanse and examine this debris Lacteals specialized lymph capillaries present in intestinal mucosaAbsorb digested fat and deliver chyle to the blood

Lymphatic Collecting VesselsHave the same three tunics as veinsHave thinner walls, with more internal valvesAnastomose more frequentlyCollecting vessels in the skin travel with superficial veinsDeep vessels travel with arteriesNutrients are supplied from branching vasa vasorum

Lymphatic TrunksLymphatic trunks are formed by the union of the largest collecting ductsMajor trunks include:Paired lumbar, bronchomediastinal, subclavian, and jugular trunksA single intestinal trunk

Lymphatic TrunksLymph is delivered into one of two large trunksRight lymphatic duct drains the right upper arm and the right side of the head and thoraxThoracic duct arises from the cisterna chyli and drains the rest of the body

Lymph TransportThe lymphatic system lacks a pumping organ Vessels are low-pressure conduitsUses the same methods as veins to propel lymph:Pulsations of nearby arteriesContractions of smooth muscle in the walls of the lymphatics

Lymphoid CellsLymphocytes are the main cells involved in the immune response Two main varieties:T cellsB cells

LymphocytesT cells and B cells protect the body against antigensAntigen anything the body perceives as foreignBacteria and their toxins; virusesMismatched RBCs or cancer cells

LymphocytesT cells Manage the immune responseAttack and destroy foreign cellsB cells Produce plasma cells, which secrete antibodiesAntibodies immobilize antigens

Other Lymphoid CellsMacrophages phagocytize foreign substances and help activate T cellsDendritic cells spiny-looking cells with functions similar to macrophagesReticular cells fibroblastlike cells that produce a stroma, or network, that supports other cell types in lymphoid organs

Lymphoid TissueDiffuse lymphatic tissue scattered reticular tissue elements in every body organLarger collections appear in the lamina propria of mucous membranes and lymphoid organsLymphatic follicles (nodules) solid, spherical bodies consisting of tightly packed reticular elements and cellsGerminal center composed of dendritic and B cellsFound in isolation and as part of larger lymphoid organs

Lymph NodesPrincipal lymphoid organs of the bodyEmbedded in connective tissue and clustered along lymphatic vesselsAggregations of these nodes occur near the body surface in inguinal, axillary, and cervical regions of the bodyFigure 20.4a

Lymph NodesTwo basic functions:Filtration macrophages destroy microorganisms and debrisImmune system activation monitor for antigens and mount an attack against them

Structure of a Lymph NodeNodes are bean shaped and surrounded by a fibrous capsuleTrabeculae extended inward from the capsule and divide the node into compartmentsNodes have two histologically distinct regions: a cortex and a medullaFigure 20.4a

Structure of a Lymph NodeCortex contains follicles with germinal centers, heavy with dividing B cellsDendritic cells nearly encapsulate the folliclesDeep cortex houses T cells in transitT cells circulate continuously among the blood, lymph nodes, and lymphatic stream

Structure of a Lymph NodeMedullary cords extend from the cortex and contain B cells, T cells, and plasma cellsThroughout the node are lymph sinuses crisscrossed by reticular fibersMacrophages reside on these fibers and phagocytize foreign matter

Structure of a Lymph NodeFigure 20.4a, b

Circulation in the Lymph NodesLymph enters via afferent lymphatic vesselsIt then enters a large subcapsular sinus and travels into smaller sinusesIt meanders through these sinuses and exits the node at the hilus via efferent vesselsBecause there are fewer efferent vessels, lymph stagnates somewhat in the nodeThis allows lymphocytes and macrophages time to carry out protective functions

Lymph Vessel & Transport DisordersLymphangitis inflammation of the larger lymphatics vasa vasorum which congest with bloodLymphedema collection of fluid in the interstitial, abdominal, and pleural spacesblockage of drainage or loss after cancer surgeryhypoproteinemia

Other Lymphoid OrgansThe spleen, thymus gland, and tonsilsPeyers patches and bits of lymphatic tissue scattered in connective tissue All are composed of reticular connective tissue All help protect the bodyOnly lymph nodes filter lymph

SpleenLargest lymphoid organ, located on the left side of the abdominal cavity beneath the diaphragm It is served by the splenic artery and vein, which enter and exit at the hilusFunctions:Site of lymphocyte proliferationImmune surveillance and responseCleanses the blood

Additional Spleen FunctionsStores breakdown products of RBCs for later reuseSpleen macrophages salvage and store iron for later use by bone marrowSite of fetal erythrocyte production (normally ceases after birth)Stores blood platelets

Structure of the SpleenSurrounded by a fibrous capsule, it has trabeculae that extend inward and contains lymphocytes, macrophages, and huge numbers of erythrocytesTwo distinct areas:White pulp containing mostly lymphocytes suspended on reticular fibers and involved in immune functionsRed pulp remaining splenic tissue concerned with disposing of worn-out RBCs and bloodborne pathogens

ThymusYoung individualsAging reduces it to fibrous/fatty tissueCreates immunocompetent T-cellsUnique since it DOESNT get involved in the immune reaction to antigensThymocytes reside in epithelial tissue rather than reticulocytes

ThymusA bilobed organ that secretes hormones (thymosin and thymopoietin) that cause T lymphocytes to become immunocompetentSize of the thymus varies with age:In infants, it is found in the inferior neck and extends into the mediastinum where it partially overlies the heartIt increases in size and is most active during childhood It stops growing during adolescence and then gradually atrophies

Internal Anatomy of the ThymusThymic lobes contain an outer cortex and inner medullaCortex contains densely packed lymphocytes and scattered macrophagesMedulla contains fewer lymphocytes and thymic (Hassalls) corpuscles

ThymusThe thymus differs from other lymphoid organs in important waysIt functions strictly in T lymphocyte maturationIt does not directly fight antigensThe stroma of the thymus consists of star-shaped epithelial cells (not reticular fibers)These thymocytes secrete the hormones that stimulate lymphocytes to become immunocompetent

TonsilsSimplest lymphoid organs; form a ring of lymphatic tissue around the pharynxLocation:Palatine tonsils either side of the posterior end of the oral cavityLingual tonsils lie at the base of the tonguePharyngeal tonsil posterior wall of the nasopharynxTubal tonsils surround the openings of the auditory tubes into the pharynx

TonsilsLymphoid tissue of tonsils contains follicles with germinal centersTonsil masses are not fully encapsulatedEpithelial tissue overlying tonsil masses invaginates, forming blind-ended cryptsCrypts trap and destroy bacteria and particulate matter

Aggregates of Lymphoid FolliclesPeyers patches isolated clusters of lymphoid tissue, similar to tonsilsFound in the wall of the distal portion of the small intestineSimilar structures are found in the appendixPeyers patches and the appendix:Destroy bacteria, preventing them from breaching the intestinal wallGenerate memory lymphocytes for long-term immunity

MALTMALT mucosa-associated lymphatic tissue:Peyers patches, tonsils, and the appendix (digestive tract)Lymphoid nodules in the walls of the bronchi (respiratory tract)MALT protects the digestive and respiratory systems from foreign matter