Lymphatic System • Returns Fluids That Leaked from Blood Vessels Back To

Home , Capillary, Cisterna chyli, Lymph, Lymphatic system, Lymph duct, Lymph sacs

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Lymphatic System Lymphoid Organs and Tissues

• Returns fluids that leaked from blood • Provide structural basis of immune system vessels back to blood • House phagocytic cells and lymphocytes • Consists of three parts • Structures include spleen, thymus, tonsils, 1. Network of lymphatic vessels (lymphatics) lymph nodes, other lymphoid tissues 2. Lymph – fluid in vessels 3. Lymph nodes – cleanse lymph

Figure 20.1 Distribution and special features of lymphatic capillaries. Venous system Arterial system Lymphatic System: Functions Heart Lymphatic system: Lymph duct Lymph trunk Lymph node

Collecting lymphatic • Lymphatic vessels (lymphatics) vessels, with valves Lymphatic capillary Blood capillaries – Return interstitial fluid and leaked plasma proteins back to blood – ~ 3L / day

– Once interstitial fluid enters lymphatics, called Tissue fluid

Tissue cell Blood Lymphatic lymph capillaries capillaries

Structural relationship between a capillary bed of the blood vascular system and lymphatic capillaries.

Filaments anchored to connective tissue

Endothelial cell

Flaplike minivalve

Fibroblast in loose connective tissue

Lymphatic Vessels: Distribution and Lymphatic Capillaries Structure • One-way system; lymph flows toward • Similar to blood capillaries, except heart – Very permeable (take up proteins, cell debris, • Lymph vessels (lymphatics) include: pathogens, and cancer cells) • Endothelial cells overlap loosely to form one-way – Lymphatic capillaries minivalves – Collecting lymphatic vessels • Anchored by collagen filaments, preventing – Lymphatic trunks and ducts collapse of capillaries; increased ECF volume opens minivalves – Pathogens travel throughout body via lymphatics

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Lymphatic Capillaries Figure 20.1b Distribution and special features of lymphatic capillaries.

Filaments anchored • Absent from bones, teeth, bone marrow, to connective tissue and CNS • Lacteals: specialized lymph capillaries Endothelial cell present in intestinal mucosa – Absorb digested fat and deliver fatty lymph Flaplike minivalve (chyle) to the blood

Fibroblast in loose connective tissue

Lymphatic Collecting Vessels Lymphatic Trunks

• Similar to veins, except • Formed by union of largest collecting – Have thinner walls, with more internal valves ducts – Anastomose more frequently – Paired lumbar • Collecting vessels in skin travel with – Paired bronchomediastinal superficial veins – Paired subclavian • Deep vessels travel with arteries – Paired jugular trunks • Nutrients supplied from branching vasa – Single intestinal trunk vasorum

Figure 20.2a The lymphatic system. Lymphatic Ducts Regional Internal lymph jugular vein nodes: Entrance of Cervical • Lymph delivered into one of two large right lymphatic nodes duct into vein ducts Entrance of thoracic duct – Right lymphatic duct drains right upper arm into vein Axillary nodes and right side of head and thorax Thoracic duct – Thoracic duct arises as cisterna chyli; Cisterna Aorta drains rest of body chyli Inguinal Collecting nodes • Each empties lymph into venous lymphatic vessels circulation at junction of internal jugular and subclavian veins on its own side of Drained by the right lymphatic duct body Drained by the thoracic duct General distribution of collecting lymphatic vessels and regional lymph nodes. © 2013 Pearson Education, Inc. © 2013 Pearson Education, Inc.

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Figure 20.2b The lymphatic system. Right jugular trunk Internal Lymph Transport jugular veins Right lymphatic Esophagus duct Trachea Right subclavian Left jugular trunk trunk Right subclavian Left subclavian vein trunk • Lymph propelled by Left subclavian Right broncho- vein mediastinal trunk Entrance of – Milking action of skeletal muscle Brachiocephalic thoracic duct veins into vein – Pressure changes in thorax during breathing Superior Left broncho- vena cava mediastinal Azygos vein trunk – Valves to prevent backflow Ribs – Pulsations of nearby arteries Thoracic duct

Hemiazygos – Contractions of smooth muscle in walls of Cisterna chyli vein lymphatics Right lumbar trunk Left lumbar trunk Inferior vena cava

Intestinal trunk Major lymphatic trunks and ducts in relation to veins and surrounding structures. Anterior view of thoracic and abdominal wall. © 2013 Pearson Education, Inc. © 2013 Pearson Education, Inc.

Lymphoid Cells Lymphocytes

• Lymphocytes main warriors of immune • T cells and B cells protect against system antigens – Arise in red bone marrow – Anything body perceives as foreign • Mature into one of two main varieties • Bacteria and bacterial toxins, viruses, mismatched RBCs, cancer cells – T cells (T lymphocytes) – B cells (B lymphocytes)

Lymphocytes Other Lymphoid Cells

• T cells • Macrophages phagocytize foreign – Manage immune response substances; help activate T cells – Attack and destroy infected cells • Dendritic cells capture antigens and • B cells deliver them to lymph nodes; activate – Produce plasma cells, which secrete T cells antibodies • Reticular cells produce reticular fiber • Antibodies mark antigens for destruction by stroma that supports other cells in phagocytosis or other means lymphoid organs

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Figure 20.3 Reticular connective tissue in a human lymph node. Lymphoid Tissue

Macrophage • Houses, and provides proliferation site for,

Reticular cells on lymphocytes reticular fibers • Surveillance vantage point for

Lymphocytes lymphocytes and macrophages • Largely reticular connective tissue – Medullary sinus type of loose connective tissue Reticular fiber • Two main types – Diffuse lymphoid tissue; Lymphoid follicles

Lymphoid Tissue Lymphoid Tissue

• Diffuse lymphoid tissue of lymphoid cells • Lymphoid follicles (nodules) are solid, and reticular fibers in ~ every body organ spherical bodies of tightly packed – Larger collections in lamina propria of mucous lymphoid cells and reticular fibers membranes – Germinal centers of proliferating B cells – May form part of larger lymphoid organs – Isolated aggregations of Peyer's patches and in appendix

Figure 20.5 Lymphoid organs. Lymph Nodes Tonsils (in pharyngeal region)

Thymus (in thorax; most • Principal lymphoid organs of body active during youth) • Embedded in connective tissue, in clusters along lymphatic vessels • Near body surface in inguinal, axillary, and Spleen (curves around cervical regions of body left side of stomach)

Peyer’s patches (aggregated lymphoid nodules in small intestine)

Appendix

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Figure 20.2a The lymphatic system. Regional Lymph Nodes Internal lymph jugular vein nodes: Entrance of Cervical right lymphatic nodes • Functions duct into vein Entrance of 1. Filter lymph—macrophages destroy thoracic duct into vein Axillary microorganisms and debris nodes Thoracic duct 2. Immune system activation—lymphocytes activated and mount attack against antigens Cisterna Aorta chyli Inguinal Collecting nodes lymphatic vessels

Drained by the right lymphatic duct Drained by the thoracic duct General distribution of collecting lymphatic vessels and regional lymph nodes. © 2013 Pearson Education, Inc. © 2013 Pearson Education, Inc.

Structure of a Lymph Node Structure of a Lymph Node

• Vary in shape and size but most bean • Cortex contains follicles with germinal shaped centers, heavy with dividing B cells • External fibrous capsule • Dendritic cells nearly encapsulate follicles • Trabeculae extend inward and divide • Deep cortex houses T cells in transit node into compartments • T cells circulate continuously among • Two histologically distinct regions blood, lymph nodes, and lymph – Cortex – Medulla

Figure 20.4a Lymph node. Afferent Cortex Structure of a Lymph Node lymphatic vessels • Lymphoid follicle • Germinal center • Subcapsular sinus • Medullary cords extend inward from cortex Efferent lymphatic vessels and contain B cells, T cells, and plasma cells • Lymph sinuses contain macrophages

Hilum Medulla • Medullary cord • Medullary sinus

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Figure 20.4b Lymph node. Circulation in the Lymph Nodes

Follicles • Lymph Trabecula – Enters convex side via afferent lymphatic Subcapsular vessels; travels through large subcapsular sinus sinus and smaller sinuses to medullary sinuses; exits concave side at hilum via Capsule efferent vessels • Fewer efferent vessels so flow somewhat

Medullary stagnate; allows lymphocytes and cords macrophages time to function Medullary sinuses

Figure 20.4 Lymph node. Spleen

Afferent Cortex lymphatic vessels • Lymphoid follicle • Germinal center • Subcapsular sinus • Largest lymphoid organ

Efferent lymphatic vessels Follicles • Served by splenic artery and vein, which Trabecula enter and exit at the hilum

Subcapsular sinus • Functions

Hilum – Site of lymphocyte proliferation and immune Capsule Medulla • Medullary surveillance and response cord • Medullary sinus – Cleanses blood of aged cells and platelets, Medullary cords macrophages remove debris Trabeculae Medullary Capsule sinuses

Longitudinal view of the internal structure of a lymph Photomicrograph of part of a lymph node and associated lymphatics node (72x)

Figure 20.6c The spleen. Spleen: Additional Functions Diaphragm

Spleen • Stores breakdown products of RBCs (e.g., iron) for later reuse Adrenal gland • Stores blood platelets and monocytes • May be site of fetal erythrocyte production Left kidney (normally ceases before birth) • Encased by fibrous capsule; has Splenic artery trabeculae

Pancreas • Contains lymphocytes, macrophages, and huge numbers of erythrocytes Photograph of the spleen in its normal position in the abdominal cavity, anterior view. © 2013 Pearson Education, Inc. © 2013 Pearson Education, Inc.

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Structure of the Spleen Figure 20.6a–b The spleen.

• Two distinct areas Capsule – White pulp around central arteries Trabecula Splenic cords • Mostly lymphocytes on reticular fibers; involved in Splenic immune functions sinusoids Arterioles and capillaries – Red pulp in venous sinuses and splenic Splenic artery Red pulp cords Splenic vein Hilum White pulp Central artery • Rich in RBCs and macrophages for disposal of Diagram of the spleen, worn-out RBCs and bloodborne pathogens anterior view Splenic artery Splenic vein • Composed of splenic cords and sinusoids Diagram of spleen histology

Thymus Thymus

• Important functions early in life • Thymic lobules contain outer cortex and • Found in inferior neck; extends into inner medulla mediastinum; partially overlies heart • Most thymic cells are lymphocytes • Increases in size and most active during – Cortex contains rapidly dividing lymphocytes childhood and scattered macrophages • Stops growing during adolescence, then • Medulla contains fewer lymphocytes and gradually atrophies thymic corpuscles involved in regulatory – Still produces immunocompetent cells, though T cell development (prevent autoimmunity) slowly

Figure 20.7 The thymus. Thymus

• Differs from other lymphoid organs in important ways – Has no follicles because it lacks B cells – Does not directly fight antigens

Capsule • Functions strictly in T lymphocyte maturation – Keeps isolated via blood thymus barrier Cortex • Stroma of epithelial cells (not reticular Medulla fibers)

Thymic corpuscles – Provide environment in which T lymphocytes become immunocompetent

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Mucosa-associated Lymphoid Tissue Tonsils (MALT) • Lymphoid tissues in mucous membranes • Simplest lymphoid organs throughout body • Form ring of lymphatic tissue around pharynx • Protects from pathogens trying to enter – Palatine tonsils—at posterior end of oral cavity body – Lingual tonsil—grouped at base of tongue – Pharyngeal tonsil—in posterior wall of nasopharynx • Largest collections of MALT in tonsils, – Tubal tonsils—surrounding openings of auditory Peyer's patches, appendix tubes into pharynx • Also in mucosa of respiratory and • Gather and remove pathogens in food or air genitourinary organs; rest of digestive tract

Tonsils Figure 20.8 Histology of the palatine tonsil.

Pharyngeal tonsil Palatine tonsil • Contain follicles with germinal centers Lingual tonsil • Are not fully encapsulated • Overlying epithelium invaginates forming tonsillar crypts – Trap and destroy bacteria and particulate matter – Allow immune cells to build memory for pathogens

Aggregates of Lymphoid Follicles Figure 20.9 Peyer’s patch (aggregated lymphoid nodules).

• Peyer's patches – Clusters of lymphoid follicles – In wall of distal portion of small intestine – Similar structures are also found in the appendix Follicles of a • Peyer's patches and appendix Peyer’s patch (aggregated – Destroy bacteria, preventing them from lymphoid nodules)

breaching intestinal wall Smooth muscle in – Generate "memory" lymphocytes the intestinal wall

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Developmental Aspects Developmental Aspects

• Beginnings of lymphatic vessels and main • Lymphoid organs (except thymus) develop clusters of lymph nodes by 5th week of from mesodermal mesenchymal cells embryonic development • Thymus (endodermal origin) forms as an – Arise as lymph sacs from developing veins outgrowth of pharynx – Jugular lymph sacs arise  right lymphatic • Except for spleen and tonsils, lymphoid duct and thoracic duct organs poorly developed at birth • Lymphatic organs (except thymus) arise • After birth high numbers of lymphocytes; from mesoderm their development parallels maturation of immune system