Lymphatic System: Exam 2

Question 1

Lymphatic System & Lymphoid Organs and Tissues

Answer 1

• lymphatic system returns fluids leaked from blood vessels back to blood; has three parts
  1) network of lymphatic vessels (lymphatics)
  2) lymph: fluid in vessels
  3) lymph nodes: cleanse lymph
• lymphoid organs and tissues provide structural basis of immune system by housing phagocytic cells and lymphocytes
  
  • spleen, thymus, tonsils, lymph nodes, and other lymphoid tissues

Question 2

Lymphatic System

Answer 2

lymphatic system returns interstitial fluid and leaked plasma proteins back to blood via:
- lymphatic vessels (lymphatics): elaborate network of drainage vessels
- circulates about 3 L interstitial fluid per day
- once interstitial fluid enters lymphatic, it is called lymph

Question 3

Distribution & Structure of Lymphatic Vessels

Answer 3

• lymphatic vessels offer a 1-way system, ensuring lymph flows only toward heart
• lymph vessels (lymphatics) include lymphatic capillaries and larger lymphatic vessels
• lymph is delivered from trunks into one of two large lymphatic ducts
  
  • right lymphatic duct drains right upper arm and right side of head and thorax
  • thoracic duct drains rest of body
    - half of individuals start out as an enlarged sac, cisterna chyli
• each empties lymph into venous circulation at junction of internal jugular and subclavian veins on its own side of the body

Question 4

Lymphatic Capillaries

Answer 4

• blind-ended vessels that weave between tissue cells and blood capillaries
  
  • absent from bones, teeth and bone marrow
  • once thought to be absent from CNS, now found to be present but limited to locations in meninges where they help drain interstitial fluid and CSF
• similar to blood capillaries, more permeable
• take up larger molecules and particles that blood capillaries cannot
  
  • ex: proteins, cell debris, pathogens and cancer cells
  • can act as route for pathogens or cancer cells to travel throughout body
• increased permeability due to two specialized structures
  1) endothelial cells overlap lossely to form one-way minivalves
  2) minivalves are anchored by collagen filaments to matrix, so increases in ECF volume opens minivalves even more
  - decreases in ECF cause minivalves to close
• Lacteals: specialized lymph capillaries present in intestinal muscosa
  
  • absorb digested fat and deliver fatty lymph (chyle) to blood

Question 5

Larger Lymphatic Vessels

Answer 5

• lymph capillaries drain into increasinglt larger vessels called collecting lymphatic vessels
• consists of collecting vessels, trunks, and ducts
• have structures and tunics similar to veins, except:
  
  • thinner walls, more internal valves
  • anastomose more frequently
• collecting vessels in skin travel with superficial veins but deep vessels travel with arteries
• Lymphatic Trunks: formed by union of largest collecting vessels, drain large areas of body
  
  • named for regions of body they drain:
    - paired lumbar
    - paired bronchomediastinal
    - paired subclavian
    - paired jugular trunks
    - single intestinal trunk

Question 6

(CLINICAL) Lymphangitis

Answer 6

condition in which lymphatic vessels appear as painful red lines under the skin
- caused by inflammation of larger lymphatic vessels that contain vaso vasora
- vaso vasoea become congested with blood
- larger lymphatics, like blood vessels, receive nutrients from branching vasa vasorum

Question 7

Lymph Transport

Answer 7

• lymph system is a low pressure system like venous systemous
• lymph is propelled by same mechanisms:
  
  • milking action of skeletal muscle
  • pressure changes in thorax during breathibg
  • vavles to prevent backflow
  • pulsations of nearby arteries
  • contractions of smooth mucle in walls of lymphatics
• physical activity increases flow of lymph, immobilization of area keeps needed inflammatory material in area for faster healing

Question 8

(CLINICAL) Lymphedema

Answer 8

Severe localized edema
- caused by anything that prevents normal return of lymph to blood
- ex: tumors blocking lymphatics of removal of lymphatics during cancer surgery
- lymphedema may improve if some lymphatic pathways remain and enlarge

Question 9

Lymphoid Cells

Answer 9

• lymphoid cells have immune system cells found in lymphoid tissue and supporting cells that form lymphoid tissue structures
  1) Immune System Cells
  
  • lymphocytes: cells of adaptive immune system; mature into one of two main types
    - T cells (T lymphocytes)
    - B cells (B lymphocytes)
  • T cells and B cells protect against antigens (anything the body perceives as foreign)
    - ex: bacteria, toxins, virus, mismatched RBC, cancer cells
  • T cells: manage immune response, some also attack and destroy infected cells
  • B cells: produce plasma cells, which secrete antibodies
    - antibodies mark antigens for destruction by phagocytes or other means
  • other lymphoid immune cells
    - macrophages phagocytize foreign substances and help activate T cells
    - dendritic cells capture antigens and deliver them to lymph nodes; also help activate T cells
    2) Supporting Lymphoid Cell
  • reticular cells produce reticular fibers called stroma in lymphoid organs
    - stroma: network like support that acts as scaffolding for immune cells

Question 10

Lymphoid Tissue

Answer 10

• main functions:
  
  • houses and provides proliferation sites for lymphocytes
  • offer survillience vantage points for lymphocytes and macrophages as they filter through lymph
• largely composed of reticular connective tissue, type of loose connective tissue
  
  • macrophages live on reticular fibers
  • spaces between fibers offer a place for lymphocytes to occupy when they return from patrolling body

Question 11

2 main types of lymphoid tissues

Answer 11

• Diffuse Lymphoid Tissue: loose arrangement of lymphoid cells and some reticular fibers
  
  • found in every body organ
  • larger collections in lamina propria of mucous membranes
• Lymphoid Follicles (nodules): solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers
  
  • contain germinal centers of proliferating B cells
  • may form part of larger lymphoid organs (nodes)
  • isolated aggregations of Peyers patches and in appendix

Question 12

Lymphoid organs are grouped into 2 functional categories

Answer 12

• primary lymphoid organs: areas where T and B cells mature-red bone marrow and thymus
  
  • T and B cells originate in bone marrow, but only B cells mature there; T cells mature in thymus
• secondary lymphoid organs: areas where mature lymphocytes first encounter their antigen and become activated
  
  • nodes, spleen, MALT (mucosa-associated lymphoid tissue) and diffuse lymphoid tissues

Question 13

Lymph Nodes

Answer 13

• lymph nodes: principal secondary lymphoid organs of body
• 100s of nodes found throughout body
  
  • most are embedded deep in connective tissue in clusters along lymphatic vessels
  • some are near to body surface in the inguinal, axillary and cervical regions of body where collecting vessels converge into trunks
• main functions of lymph nodes
  1) cleansing the lymph: act as lymph “filters”
  - macrophages remove and destroy microorganisms and debris that enter lymph (prevent unwanted substance from being delivered to blood)
  2) immune system activation: offer place for lymphocytes to become activated and become mount an attack against antigens

Question 14

Structure of a Lymph node

Answer 14

• vary in shape and size, most are bean shaped
  
  • small, less than 2.5 cm (about 1 inch)
• surrounded by external fibrous capsule
• capsule fibers extend inward as trabeculate that divide node into compartments
• lymph sinuses are found throughout node
  
  • conisist of large lymphatic capillaries spanned by crisscrossing reticular fibers
  • macrophages reside on ribers, checking for and phagocytizing foreign matter
• two histologically distinct regions of node:
  1) cortex
• superficial area of cortex contains follicles with germinal centers that are heavy with dividing B cells
• deep cortex houses T cells in transit
  
  • T cells circulate continuously among blood, lymph nodes and lymph
• abundant numbers of dendritic cells are closely associated with T and B cells
  
  • play role in activating both lymphocytes
    2) medulla
• medullary cords extend inward from cortex and contain B cells, T cells and plasma cells

Question 15

Circulation in Lymph Nodes

Answer 15

• lymph enters convex side of node by afferent lymphatic vessels
• travels through large subcapsular sinus and then into 2 smaller sinuses found throughout the cortex and medulla
• lymph then enters medullary sinus
• finally exits concave side at hilum by efferent lymphatic vessels
  
  • presence of fewer efferent vessels cause flow to somewhat stagnate, allows lymphocytes and macrophages time to function
  • lymph travels through several nodes

Question 16

(CLINICAL) Buboes

Answer 16

inflamed, swollen, tender lymph nodes that result when nodes are overwhelmed by what they are trying to destroy
- condition often referred as swollen “glands”
- sometimes pus filled
- bubonic plague was named after chief clinical feature of this disease

Question 17

(CLINICAL) Lymph nodes can become secondary cancer cites if metastasizing cancer cells become trapped in node

Answer 17

• cancer infiltrated lymph nodes are swollen but not usually painful; fact that helps distinguish cancerous nodes from those infected by microorganisms

Question 18

Spleen

Answer 18

• a blood-rich organ about size of fist, located in left side of abdominal cavity, below stomach
• largest lymph organ
• served as splenic artery and vein, which exit and enter at the hilum
• spleen has a thin capsule, so direct blow or severe infection can cause it to rupture, spilling blood into peritoneal cavity
• functions
  
  • site of lymphocyte proliferation & immune surveillance and response
  • cleanses blood of aged blood cells and platelets, macrophages remove debris
  • stores breakdown products of RBC (iron) for later reuse
  • stores blood platelets and monocytes for release into blood when needed
  • maybe site of fetal erythrocyte production
• spleen is encased by fibrous capsule and has trabeculae
• histologically, consist of two components
  1) White Pulp: site where immune function occurs
  - contains most lymphocytes on reticular fibers
  - clusters are found around central arteries; appear as islands of white in a sea of red pulp
  2) Red Pulp: site where old blood cells and bloodborne pathogens are destroyed
  - rich in RBC and macrophages that engulf them
  - composed of splenic cords (reticular tissue) that separate blood filled splenic sinusoids (venous sinuses)

Question 19

(CLINICAL) Splenectomy

Answer 19

a surgical removal of ruptured spleen
- once standard treatment to prevent hemorrhage and shock, but has been discovered spleen can often repair itself (need of emergency splenectomies has decreased)
- if spleen must be removed; liver and bone marrow take over most of its functions (in kids younger than 12, spleen with regenerate if small part is left)

Question 20

MALT

Answer 20

• known as mucosa-associated lymphoid tissue
  
  • lymphoid tissues in mucous membranes throughout body
• protects from pathogens trying to enter body
• function: gather and remove pathogens in food or air
• contain follicles with germinal centers and scattered lymphocytes
• not fully encapsulated
• overlying epithelium invaginates, forming tonsillar crypts
  
  • bacteria or particulate matter enter crypts, where they are trapped and destroyed
    - risky to lure bacteria into tissues, but allows immune cells to become activated and build memory cells against potential pathogens
• found in mucosa of respiratory tract, genitourinary organs and digestive tract; largest collections of MALT found in:
  1) Tonsils
  
  • simplest lymphoid organs
  • form ring of lymphatic tissue around pharynx, appear as swellings of mucosa
  • name according to location
    - palatine tonsils: located at posterior end of oral cavity (largest of tonsils and most infected)
    - lingual tonsil: lumpy collection of follicles at base of tongue
    - pharyngeal tonsil (adenoids): located in posterior wall of nasopharynx
    - tubal tonsils: surround opening of auditory tubes into pharynx
    2) Peyers Patches
  • clusters of lymphoid follicles in wall of distal portion of small intestine (also called aggregated lymphoid nodules)
  • structurally similar to tonsils
  • location aides in functions:
    - destroy bacteria, prevent them from breaching into intestinal wall
    - generate “memory” lymphocytes
    3) Appendix
  • an offshoot of first part of large intestine
  • contains large number of lymphoid follicles
  • location aides in functions (like peyers patch):
    - destroy bacteria, prevent them from breaching intestinal wall
    - generate “memory” lymphocyte

Question 21

Thymus

Answer 21

• thymus: bilobed lymphoid organ found in inferior neck
  
  • extends into mediastinum and partially overlies heart
• functions as lymphoid organ where T cells mature
  
  • most active and largest size during childhood
  • stops growing during adolescence then gradually atrophies
    -produces immunocompetent cells more slowly
• thymus is broken into lobules that contain outer cortex and inner medulla
  
  • cortex contains rapidly dividing lymphocytes (bulk of thymic cells) and scattered macrophages
• medulla contains fewer lymphocytes and thymic corpuscles
  
  • thymic corpuscles are where regulatory T cells develop
    regulatory T cells: type of T cell that helps prevent autoimmunity
• thymus differs from other lymphoid organs in important ways
  1) no follicles because lacks B cells
  2) does not directly fight antigens
  - functions strictly in T lymphocyte maturation (contains blood thymus barrier: keeps immature T lymphocytes isolated from any antigens to prevent premature activation)
  3) stroma is made up of epithelial cells, not reticular fibers
  - provide environment in which T lymphocytes become immunocompetent