1. 20
The Lymphatic System and Lymphoid Organs and Tissues

2. Returns fluids that leaked from blood vessels back to blood
Lymphatic System
Returns fluids that leaked from blood vessels back to blood
Consists of three parts
Network of lymphatic vessels (lymphatics)
Lymph – fluid in vessels
Lymph nodes – cleanse lymph
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3. Lymphoid Organs and Tissues
Provide structural basis of immune system
House phagocytic cells and lymphocytes
Structures include spleen, thymus, tonsils, lymph nodes, other lymphoid tissues
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4. Lymphatic System: Functions
Lymphatic vessels (lymphatics)
Return interstitial fluid and leaked plasma proteins back to blood
~ 3L / day
Once interstitial fluid enters lymphatics, called lymph
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5. Figure 20.1 Distribution and special features of lymphatic capillaries.
Venous system
Arterial system
Heart
Lymphatic system:
Lymph duct
Lymph trunk
Lymph node
Collecting lymphatic
vessels, with valves
Lymphatic capillary
Blood capillaries
Tissue
fluid
Tissue cell
Blood
capillaries
Lymphatic
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 capillaries are blind-ended tubes in which
adjacent endothelial cells overlap each other,
forming flaplike minivalves.
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6. Lymphatic Vessels: Distribution and Structure
One-way system; lymph flows toward heart
Lymph vessels (lymphatics) include:
Lymphatic capillaries
Collecting lymphatic vessels
Lymphatic trunks and ducts
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7. Lymphatic Capillaries
Similar to blood capillaries, except
Very permeable (take up proteins, cell debris, pathogens, and cancer cells)
Endothelial cells overlap loosely to form one-way minivalves
Anchored by collagen filaments, preventing collapse of capillaries; increased ECF volume opens minivalves
Pathogens travel throughout body via lymphatics
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8. Lymphatic Capillaries
Absent from bones, teeth, bone marrow, and CNS
Lacteals: specialized lymph capillaries present in intestinal mucosa
Absorb digested fat and deliver fatty lymph (chyle) to the blood
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9. Endothelial cell Flaplike minivalve
Figure 20.1b Distribution and special features of lymphatic capillaries.
Filaments anchored
to connective tissue
Endothelial cell
Flaplike minivalve
Fibroblast in loose
connective tissue
Lymphatic capillaries are blind-ended tubes in which
adjacent endothelial cells overlap each other,
forming flaplike minivalves.
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10. Lymphatic Collecting Vessels
Similar to veins, except
Have thinner walls, with more internal valves
Anastomose more frequently
Collecting vessels in skin travel with superficial veins
Deep vessels travel with arteries
Nutrients supplied from branching vasa vasorum
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11. Formed by union of largest collecting ducts
Lymphatic Trunks
Formed by union of largest collecting ducts
Paired lumbar
Paired bronchomediastinal
Paired subclavian
Paired jugular trunks
Single intestinal trunk
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12. Lymph delivered into one of two large ducts
Lymphatic Ducts
Lymph delivered into one of two large ducts
Right lymphatic duct drains right upper arm and right side of head and thorax
Thoracic duct arises as cisterna chyli; drains rest of body
Each empties lymph into venous circulation at junction of internal jugular and subclavian veins on its own side of body
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13. General distribution of collecting lymphatic
Figure 20.2a The lymphatic system.
Regional
lymph
nodes:
Internal
jugular vein
Entrance of
right lymphatic
duct into vein
Cervical
nodes
Entrance of
thoracic duct
into vein
Axillary
nodes
Thoracic
duct
Cisterna
chyli
Aorta
Inguinal
nodes
Collecting
lymphatic
vessels
Drained by the right lymphatic duct
Drained by the thoracic duct
General distribution of collecting lymphatic
vessels and regional lymph nodes.
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14. Internal jugular veins Esophagus Trachea
Figure 20.2b The lymphatic system.
Right jugular trunk
Internal
jugular veins
Esophagus
Right lymphatic
duct
Trachea
Right subclavian
trunk
Left jugular
trunk
Left subclavian
trunk
Right subclavian
vein
Left subclavian
vein
Right broncho-
mediastinal trunk
Entrance of
thoracic duct
into vein
Brachiocephalic
veins
Superior
vena cava
Left broncho-
mediastinal
trunk
Azygos vein
Ribs
Thoracic duct
Hemiazygos
vein
Cisterna chyli
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.
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15. Lymph Transport Lymph propelled by Milking action of skeletal muscle
Pressure changes in thorax during breathing
Valves to prevent backflow
Pulsations of nearby arteries
Contractions of smooth muscle in walls of lymphatics
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16. Lymphocytes main warriors of immune system
Lymphoid Cells
Lymphocytes main warriors of immune system
Arise in red bone marrow
Mature into one of two main varieties
T cells (T lymphocytes)
B cells (B lymphocytes)
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17. T cells and B cells protect against antigens
Lymphocytes
T cells and B cells protect against antigens
Anything body perceives as foreign
Bacteria and bacterial toxins, viruses, mismatched RBCs, cancer cells
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18. Lymphocytes T cells B cells Manage immune response
Attack and destroy infected cells
B cells
Produce plasma cells, which secrete antibodies
Antibodies mark antigens for destruction by phagocytosis or other means
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19. Macrophages phagocytize foreign substances; help activate T cells
Other Lymphoid Cells
Macrophages phagocytize foreign substances; help activate T cells
Dendritic cells capture antigens and deliver them to lymph nodes; activate T cells
Reticular cells produce reticular fiber stroma that supports other cells in lymphoid organs
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20. Macrophage Reticular cells on reticular fibers Lymphocytes
Figure Reticular connective tissue in a human lymph node.
Macrophage
Reticular cells on
reticular fibers
Lymphocytes
Medullary sinus
Reticular fiber
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21. Houses, and provides proliferation site for, lymphocytes
Lymphoid Tissue
Houses, and provides proliferation site for, lymphocytes
Surveillance vantage point for lymphocytes and macrophages
Largely reticular connective tissue – type of loose connective tissue
Two main types
Diffuse lymphoid tissue; Lymphoid follicles
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22. Lymphoid Tissue
Diffuse lymphoid tissue of lymphoid cells and reticular fibers in ~ every body organ
Larger collections in lamina propria of mucous membranes
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23. Lymphoid Tissue
Lymphoid follicles (nodules) are solid, spherical bodies of tightly packed lymphoid cells and reticular fibers
Germinal centers of proliferating B cells
May form part of larger lymphoid organs
Isolated aggregations of Peyer's patches and in appendix
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24. Tonsils (in pharyngeal region)
Figure Lymphoid organs.
Tonsils (in pharyngeal
region)
Thymus (in thorax; most
active during youth)
Spleen (curves around
left side of stomach)
Peyer’s patches
(aggregated lymphoid
nodules in small
intestine)
Appendix
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25. Principal lymphoid organs of body
Lymph Nodes
Principal lymphoid organs of body
Embedded in connective tissue, in clusters along lymphatic vessels
Near body surface in inguinal, axillary, and cervical regions of body
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26. General distribution of collecting lymphatic
Figure 20.2a The lymphatic system.
Regional
lymph
nodes:
Internal
jugular vein
Entrance of
right lymphatic
duct into vein
Cervical
nodes
Entrance of
thoracic duct
into vein
Axillary
nodes
Thoracic
duct
Cisterna
chyli
Aorta
Inguinal
nodes
Collecting
lymphatic
vessels
Drained by the right lymphatic duct
Drained by the thoracic duct
General distribution of collecting lymphatic
vessels and regional lymph nodes.
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27. Lymph Nodes
Functions
Filter lymph—macrophages destroy microorganisms and debris
Immune system activation—lymphocytes activated and mount attack against antigens
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28. Structure of a Lymph Node
Vary in shape and size but most bean shaped
External fibrous capsule
Trabeculae extend inward and divide node into compartments
Two histologically distinct regions
Cortex
Medulla
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29. Structure of a Lymph Node
Cortex contains follicles with germinal centers, heavy with dividing B cells
Dendritic cells nearly encapsulate follicles
Deep cortex houses T cells in transit
T cells circulate continuously among blood, lymph nodes, and lymph
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30. Longitudinal view of the internal structure of a lymph
Figure 20.4a Lymph node.
Afferent
lymphatic
vessels
Cortex
• Lymphoid follicle
• Germinal center
• Subcapsular sinus
Efferent
lymphatic
vessels
Hilum
Medulla
• Medullary
cord
• Medullary
sinus
Trabeculae
Capsule
Longitudinal view of the internal structure of a lymph
node and associated lymphatics
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31. Structure of a Lymph Node
Medullary cords extend inward from cortex and contain B cells, T cells, and plasma cells
Lymph sinuses contain macrophages
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32. Photomicrograph of part of a lymph node (72x)
Figure 20.4b Lymph node.
Follicles
Trabecula
Subcapsular
sinus
Capsule
Medullary
cords
Medullary
sinuses
Photomicrograph of part of a lymph
node (72x)
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33. Circulation in the Lymph Nodes
Enters convex side via afferent lymphatic vessels; travels through large subcapsular sinus and smaller sinuses to medullary sinuses; exits concave side at hilum via efferent vessels
Fewer efferent vessels so flow somewhat stagnate; allows lymphocytes and macrophages time to function
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34. Figure 20.4 Lymph node. Afferent lymphatic vessels Cortex
• Lymphoid follicle
• Germinal center
• Subcapsular sinus
Efferent
lymphatic
vessels
Follicles
Trabecula
Subcapsular
sinus
Hilum
Capsule
Medulla
• Medullary
cord
• Medullary
sinus
Medullary
cords
Trabeculae
Medullary
sinuses
Capsule
Longitudinal view of the internal structure of a lymph
node and associated lymphatics
Photomicrograph of part of a lymph
node (72x)
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35. Largest lymphoid organ
Spleen
Largest lymphoid organ
Served by splenic artery and vein, which enter and exit at the hilum
Functions
Site of lymphocyte proliferation and immune surveillance and response
Cleanses blood of aged cells and platelets, macrophages remove debris
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36. Photograph of the spleen in its normal position in
Figure 20.6c The spleen.
Diaphragm
Spleen
Adrenal
gland
Left
kidney
Splenic
artery
Pancreas
Photograph of the spleen in its normal position in
the abdominal cavity, anterior view.
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37. Spleen: Additional Functions
Stores breakdown products of RBCs (e.g., iron) for later reuse
Stores blood platelets and monocytes
May be site of fetal erythrocyte production (normally ceases before birth)
Encased by fibrous capsule; has trabeculae
Contains lymphocytes, macrophages, and huge numbers of erythrocytes
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38. Structure of the Spleen
Two distinct areas
White pulp around central arteries
Mostly lymphocytes on reticular fibers; involved in immune functions
Red pulp in venous sinuses and splenic cords
Rich in RBCs and macrophages for disposal of worn-out RBCs and bloodborne pathogens
Composed of splenic cords and sinusoids
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39. Diagram of spleen histology
Figure 20.6a–b The spleen.
Capsule
Trabecula
Splenic cords
Splenic
sinusoids
Arterioles
and
capillaries
Splenic
artery
Red pulp
Splenic
vein
White pulp
Hilum
Central artery
Diagram of the spleen,
anterior view
Splenic artery
Splenic vein
Diagram of spleen histology
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40. Important functions early in life
Thymus
Important functions early in life
Found in inferior neck; extends into mediastinum; partially overlies heart
Increases in size and most active during childhood
Stops growing during adolescence, then gradually atrophies
Still produces immunocompetent cells, though slowly
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41. Thymic lobules contain outer cortex and inner medulla
Thymus
Thymic lobules contain outer cortex and inner medulla
Most thymic cells are lymphocytes
Cortex contains rapidly dividing lymphocytes and scattered macrophages
Medulla contains fewer lymphocytes and thymic corpuscles involved in regulatory T cell development (prevent autoimmunity)
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42. Capsule Cortex Medulla Thymic corpuscles Figure 20.7 The thymus.
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43. Differs from other lymphoid organs in important ways
Thymus
Differs from other lymphoid organs in important ways
Has no follicles because it lacks B cells
Does not directly fight antigens
Functions strictly in T lymphocyte maturation
Keeps isolated via blood thymus barrier
Stroma of epithelial cells (not reticular fibers)
Provide environment in which T lymphocytes become immunocompetent
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44. Mucosa-associated Lymphoid Tissue (MALT)
Lymphoid tissues in mucous membranes throughout body
Protects from pathogens trying to enter body
Largest collections of MALT in tonsils, Peyer's patches, appendix
Also in mucosa of respiratory and genitourinary organs; rest of digestive tract
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45. Tonsils Simplest lymphoid organs
Form ring of lymphatic tissue around pharynx
Palatine tonsils—at posterior end of oral cavity
Lingual tonsil—grouped at base of tongue
Pharyngeal tonsil—in posterior wall of nasopharynx
Tubal tonsils—surrounding openings of auditory tubes into pharynx
Gather and remove pathogens in food or air
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46. Contain follicles with germinal centers Are not fully encapsulated
Tonsils
Contain follicles with germinal centers
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
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47. Pharyngeal tonsil Palatine tonsil Lingual tonsil Germinal centers
Figure Histology of the palatine tonsil.
Pharyngeal tonsil
Palatine tonsil
Lingual tonsil
Germinal centers
in lymphoid follicles
Tonsillar
crypt
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48. Aggregates of Lymphoid Follicles
Peyer's patches
Clusters of lymphoid follicles
In wall of distal portion of small intestine
Similar structures are also found in the appendix
Peyer's patches and appendix
Destroy bacteria, preventing them from breaching intestinal wall
Generate "memory" lymphocytes
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49. Smooth muscle in the intestinal wall
Figure Peyer’s patch (aggregated lymphoid nodules).
Follicles of a
Peyer’s patch
(aggregated
lymphoid nodules)
Smooth muscle in
the intestinal wall
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50. Developmental Aspects
Beginnings of lymphatic vessels and main clusters of lymph nodes by 5th week of embryonic development
Arise as lymph sacs from developing veins
Jugular lymph sacs arise  right lymphatic duct and thoracic duct
Lymphatic organs (except thymus) arise from mesoderm
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51. Developmental Aspects
Lymphoid organs (except thymus) develop from mesodermal mesenchymal cells
Thymus (endodermal origin) forms as an outgrowth of pharynx
Except for spleen and tonsils, lymphoid organs poorly developed at birth
After birth high numbers of lymphocytes; their development parallels maturation of immune system
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