Lymphatic System PDF

Lymphatic System Lymphatic System/Lymphoid Tissues Overview Lymphatic System & the Lymphoid Tissues/Organs are two overlapping systems that work to maintain cardiac function protect the body from infection...

Lymphatic System Lymphatic System/Lymphoid Tissues Overview Lymphatic System & the Lymphoid Tissues/Organs are two overlapping systems that work to maintain cardiac function protect the body from infection Lymphatic System: returns fluids that have leaked from vasculature back to circulation Lymphatic vessels Lymph, a fluid Lymph nodes that cleanse lymph as it passes through Lymphoid Tissues/Organs: structural basis of the immune system Spleen Thymus Tonsils Lymph nodes Lymphatic Vessels Fluid is pushed out of circulation by hydrostatic and colloid osmotic pressures at capillary beds Much of the fluid gets reabsorbed ~3 L daily becomes part of interstitial fluid and eventually, with any additional leaked plasma, gets absorbed by the lymphatic system Lymphatics return fluid to the circulatory system to ensure maintenance of appropriate blood volume Absorbed fluid: lymph  Lymphatic Capillaries First step in transport of lymph Weave between cells and blood capillaries in the loose connective tissues of the body Exceptionally permeable endothelial cells are not joined by tight junctions; instead adjacent cells overlap each other like shingles to form minivalves When interstitial fluid volume increases, the valves are pushed open: one-way door When tissues become inflamed, capillaries will open even more to allow uptake of proteins and large particles (pathogens) Lacteal: specialized lymphatic capillaries that absorb fat in small intestine  Larger Lymphatic Vessels Lymphatic Capillaries < Vessels < Trunks < Ducts Vessels: same three tunics as the veins thinner walls, more anastomoses in the skin: travel along superficial veins in the trunk: travel with deep arteries distribution has significant variability between individuals Trunks: drain fairly large areas of the body, with names corresponding to drainage regions lumbar bronchomediastinal subclavian pairs jugular intestinal Ducts: both in thoracic region Right Lymphatic Duct: drains lymph from the right upper limb and right side of head and thorax Thoracic Duct: drains lymph from rest of the body Both ducts empty at the junction of the internal jugular vein and the subclavian vein  Lymph Transport Lymphatics have no pump mechanism Low-pressure conduits Mechanisms that contribute to lymph movement skeletal muscle movements pressure changes in thorax during breathing valves that prevent backflow pulsation of nearby arteries lymph vessel smooth muscle contracts rhythmically Even with these mechanisms in place, lymph moves sporadically and slowly  Lymphoid Cells All the immune system cells found in lymphoid tissues + supporting cells that forms structures of those tissues Lymphocytes: main warriors of the immune system and derive from bone marrow T cells (T lymphocytes): managers of the immune response B cells (B lymphocytes): may mature to plasma cells and produce antibodies Macrophages (myeloid cell type) phagocytose foreign materials activate T cells in a process called antigen presentation Dendritic cells: superior to macrophages at their overlapping functions Reticular cells fibroblast-like cells produce stroma: network of materials/cells that support other cell types in lymphoid organs  Lymphoid Tissues & Organs Two major functions of lymphoid tissues 1. proliferation site for lymphocytes 2. surveillance vantage point for both lymphocytes and macrophages/DCs Two organizations of lymphoid tissues Diffuse lymphoid tissue loose arrangement found in virtually all organs Lymphoid follicles (nodules) solid spherical bodies of reticular fibers + lymphoid cells centers may contain proliferating B cells may be part of larger lymphoid organs OR aggregates within non-lymphoid tissues ex: Peyer’s Patches Lymph Nodes: Principal Lymphoid Organs Hundreds cluster around the lymphatic vessels Two basic functions: Filtration: macrophages phagocytose debris and microorganisms Immune System Activation: Lymphocyte activation upon antigen recognition Structure: external dense fibrous capsule trabeculae connective tissue strands that divide the node into compartments cortex: outer region of the node numerous follicles dense with B cells in germinal centers deeper in cortex T cells that circulate between blood and lymph nodes performing surveillance medulla: inner region of the node lymphocytes (B & T) + macrophages sinuses: large lymph capillary structures; often where patrolling macrophages will uptake material afferent lymphatic vessels: entrances into lymph nodes hilum: indented region of the node efferent lymphatic vessels: exits from the lymph nodes NOTE: more afferent than efferent vessels to maintain lymph in the node for contact time with leukocytes Additional Lymphoid Organs Features of Lymphoid Organs: All composed of reticular connective tissue (except thymus) All help to protect the organism from infection Only lymph nodes filter lymph Spleen Thymus MALT: Tonsils & Peyer’s Patches Appendix  Spleen Spleen largest lymphoid organ lymphocyte proliferation, immune surveillance, response Blood-cleansing functions: ‘Erythrocyte graveyard’: filtration of old/defective erythrocytes and platelets from blood Macrophages remove debris and foreign matter Stores break-down products of erythrocytes for re-use (iron salvage) Platelet and monocyte storage for release into blood as needed Possibly a site of fetal erythrocyte production Splenic Structures: White pulp: immune functions mostly lymphocytes associated with reticular fibers form cuffs around central arteries in stained vs fresh tissue samples, white pulp is often darker than red Red pulp: erythrocyte and pathogen destruction Thymus Site of T lymphocyte (T cell) maturation Activity decreases with age prominent in newborns and increases through ~ 1 yr begins to gradually atrophy after puberty by old age is largely replaced by fibrous and fatty tissue less capable of producing T cells as age progresses Differs from other lymphoid organs in that thymus has no follicles (no B cells) does not directly recognize antigen/fight infection stroma is epithelial cells, not reticular fibers Structures: cortical region contains rapidly dividing T cells medullary region contains thymic corpuscles previously thought to play major role in T cell destruction currently believed to play role in development of Tregs (regulatory T cells) that prevent autoimmune responses and/or immune over-activation MALT: Mucosa-Associated Lymphoid Tissue Tonsils capture and remove pathogens entering the pharynx in food or air via crypts Peyer’s Patches aggregated lymphoid nodules located in the small intestine Appendix offshoot of the large intestine destroys pathogens before they can breach the intestinal mucosa generate a number of memory lymphocytes