Lymphatic System (ch.20)

Lymphatic system returns fluids leaked from blood vessels back to blood.

• Lymphoid organs and tissues provide structural basis of immune system by housing phagocytic cells and lymphocytes

  • Structures include:

    • SPLEEN

    • THYMUS

    • TONSILS

    • LYMPH NODES

    • Other lymphoid tissues

Distribution and Structure of Lymphatic Vessels

• Lymphatic vessels offer a one-way system, ensuring LYMPH flows only toward heart

• LYMPHATIC CAPILLARIES

  • Blind-ended vessels that weave between tissue cells and blood capillaries

    • Absent from bones, teeth, and bone marrow

    • CNS - in meninges where they help to drain interstitial fluid and CSF (cerebrospinal fluid)

    • Similar to blood capillaries, but more PERMEABLE

    • Can take up larger molecules and particles that blood capillaries cannot

      • Example: proteins, cell debris, pathogens, and cancer cells

      • Can act as route for pathogens or cancer cells to travel throughout body

The lymph system filter 3 L / Day

Distribution and Structure of Lymphatic Vessels

• Increased permeability due to two specialized structures

  • Endothelial cells overlap loosely to form one-way MINIVALVES

  • 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 mucosa

  • Absorb digested FAT and deliver fatty lymph (CHYLE) to the blood

• Larger lymphatic vessels

  • Lymph capillaries drain into increasingly larger vessels called COLLECTING LYMPHATIC VESSELS

  • Consist of collecting vessels, trunks, and ducts

  • Have structures similar to veins, except:

    • Have thinner walls, with more internal VALVES

  • Collecting vessels in skin travel with superficial veins, but deep vessels travel with arteries

LYMPHATIC TRUNKS, which are formed by union of largest collecting vessels, drain large areas of body

• Named for regions of body they drain:

  • Paired LUMBAR

  • Paired BRONCHOMEDISTINAL

  • Paired SUBCLAVIAN

  • Paired JUGULAR TRUNKS

  • SINGLE INTESTINAL TRUNK

Lymph is delivered from trunks into one of two large LYMPHATIC DUCTS

• RIGHT LYMPHATIC DUCT drains and right side of head and thorax

• THORACIC DUCT drains rest of body

• Each empties lymph into VENOUS circulation at junction of internal jugular and subclavian veins on its own side of body

Lymph Transport

• Lymph system is a LOW PRESSURE SYSTEM

• Lymph is propelled by:

1. MILKING ACTION of skeletal muscles

2. PRESSURE CHANGES in thorax during breathing

3. Valves to PREVENT BACKFLOW

4. PULSATIONS of nearby arteries

5. CONTRACTIONS OF SMOOTH MUSCLE in walls of lymphatics

• Physical activity INCREASES flow of lymph

• IMMOBILIZATION of area keeps needed inflammatory material in area for faster healing

Clinical – Homeostatic Imbalance 20.2

• LYMPHEDEMA: severe localized edema

• Caused by anything that prevents normal return of lymph to blood

  • Examples: TUMORS blocking lymphatics or removal of lymphatics during cancer surgery

  • Lymphedema may improve if some lymphatic pathways remain and enlarge

20.2 Lymphoid Cells, Tissues, and Organs

• Lymphoid cells consist of

(1) IMMUNE SYSTEM CELLS found in lymphoid tissue

(2) supporting cells that form lymphoid tissue structures

• Immune system cells

  1. LYMPHOCYTES: Formed in the RED bone marrow; mature into one of two main types:

     • T CELLS ( T lymphocytes )

     • B CELLS ( B lymphocytes )

2. T cells and B cells protect against ANTIGENS

   • Anything the body perceives as FOREIGN

     • Bacteria, toxins, viruses, mismatched RBCs, cancer cells

• T CELLS: manage immune response, and some also ATTACK and DESTROY infected cells

• B CELLS: produce plasma cells, which secrete ANTIBODIES 

  • Antibodies mark antigens for destruction by phagocytosis or other means

• Other lymphoid immune cells

  • MACROPHAGES phagocytize foreign substances and help ACTIVATE T CELLS

  • DENDRITIC CELLS capture antigens and DELIVER to lymph nodes; also help activate T cells

Supporting Lymphoid Cell

• RETICULAR CELLS produce reticular fibers called STROMA in lymphoid organs

  • STROMA: network-like support that acts as scaffolding for immune cells

Lymphoid Tissue

• Main functions of LYMPHOID TISSUE

  • HOUSES and provides proliferation sites for lymphocytes

  • Offers surveillance vantage points for lymphocytes and macrophages as they filter through lymph

• Largely composed of RETICULAR CONNECTIVE TISSUE, a 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

Lymphoid Organs

• LYMPHOID ORGANS are grouped into two functional categories

  • PRIMARY LYMPHOID ORGANS: areas where T and B cells mature— RED BONE MARROW and THYMUS

    • T and B cells originate in BONE MARROW

      • Only B cells matures 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):

      • TONSILS

      • PEYER’S PATCHES

      • APPENDIX

20.3 Lymph Nodes

• Principal secondary lymphoid organs of body

• Hundreds of nodes are found throughout body

  • Most are embedded deep in connective tissue in clusters along lymphatic vessels

  • Some are nearer to body surface:

    • INGUINAL

    • AXILLARY

    • CERVICAL regions of body where collecting vessels converge into trunks

Two 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 substances from being delivered to blood

2. Immune system activation: offer a place for lymphocytes to become activated and mount an attack against antigens

   • DENDRITIC CELLS capture antigens and deliver them to lymph nodes; also help activate T cells

Structure of a Lymph Node

• Vary in shape and size, but most are bean shaped

  • Small, less than 2.5 cm (~1 inch)

• Surrounded by EXTERNAL FIBROUS CAPSULE

• Capsule fibers extend inward as TRABECULAE that divide node into compartments

• Two histologically distinct regions of node:

  • CORTEX

  • MEDULLA

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 both T and B cells

     • Play a role in activating both lymphocytes

2. Medulla

   • Medullary cords extend inward from cortex and contain B cells, T cells, and plasma cells

• LYMPH SINUSES are found throughout node

  • Consist of large lymphatic capillaries spanned by crisscrossing reticular fibers

  • Macrophages reside on fibers, checking for and phagocytizing any foreign matter

Circulation in the Lymph Nodes

• Lymph enters convex side of node via AFFERENT LYMPHATIC VESSELS

• Travels through large SUBSCAPULAR SINUS and then into smaller sinuses found throughout cortex and medulla

• Lymph then enters MEDULLARY SINUSES

• Finally exits concave side at HILUM via EFFERENT LYMPHATIC VESSELS

  • Presence of FEWER efferent vessels causes flow to somewhat stagnate; allows lymphocytes and macrophages time to function

  • Lymph travels through several nodes

Clinical – Homeostatic Imbalance 20.3

• BUBOES: inflamed, swollen, tender lymph nodes that result when nodes are overwhelmed by what they are trying to destroy

  • Condition often referred to as SWOLLEN GLANDS

  • Buboes are sometimes pus-filled

  • Bubonic plague was named after chief clinical feature of this disease

• Lymph nodes can become secondary cancer sites if metastasizing cancer cells become trapped in node

  • Cancer-infiltrated lymph nodes are swollen but usually not painful, a fact that helps distinguish cancerous nodes from those infected by microorganisms

20.4 Spleen

• SPLEEN is blood-rich organ about size of fist, located in LEFT SIDE of abdominal cavity, just below stomach

• 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 blood cells and platelets; macrophages remove debris

• Three additional functions of spleen:

  • Stores breakdown products of RBCs (e.g., IRON) for later reuse

  • Stores blood PLATELETS and MONOCYTES for release into blood when needed

  • May be site of FETAL ERYTHROCYTE production

Spleen is encased by fibrous capsule and also has TRABECULAE

• Histologically, consists of two components

  • White pulp

  • Red pulp

1. WHITE PULP: site where IMMUNE FUNCTION OCCURS

   • Contains mostly lymphocytes on reticular fibers

   • White pulp 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 RBCs and macrophages that engulf them

   • Composed of SPLENIC CORDS (reticular tissue) that separate blood-filled SPLENIC SINUSOIDS (venous sinuses)

Clinical – Homeostatic Imbalance 20.4

• The spleen has a thin capsule, so direct blow or severe infection may cause it to rupture, spilling blood into peritoneal cavity

• SPLENECTOMY: surgical removal of ruptured spleen

  • Once standard treatment to prevent hemorrhage and shock, but has been discovered spleen can often repair itself

    • Frequency of emergency splenectomies has decreased dramatically

  • If spleen must be removed, liver and bone marrow take over most of its functions

  • In children younger than 12, spleen will regenerate if a small part is left

20.5 MALT - MUCOSA--associated lymphoid tissue

• Lymphoid tissues in mucous membranes throughout body

• Protects from pathogens trying to ENTER THE BODY

• Found in mucosa of RESPIRATORY TRACT, GENITOURINARY ORGANS, and DIGESTIVE TRACT; largest collections of MALT found in:

  • TONSILS

  • PEYER’S PATCHES

  • APPENDIX

Tonsils

• Simplest lymphoid organs

• Form ring of lymphatic tissue around pharynx; appear as swellings of MUCOSA

• Named according to location:

  • PALATINE TONSILS: at posterior end of oral cavity

    • Largest of tonsils and most often infected

  • LINGUAL TONSIL: lumpy collection of follicles at base of TONGUE

  • PHARYNGEAL TONSIL: also called ADENOIDS;

    • located in posterior wall of nasopharynx

  • TUBAL TONSILS: surround openings of auditory tubes into pharynx

• Tonsils function is to gather and remove pathogens in FOOD or AIR

• Contain follicles with germinal centers and scattered lymphocytes

• Are not fully ENCAPSULATED

• Overlying epithelium invaginates, forming TONSILLAR CRYPTS

  • Bacteria or particulate matter enters 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 these potential pathogens

PEYER’S PATCHES: clusters of lymphoid follicles in wall of distal portion of SMALL INTESTINE

• Also called aggregated lymphoid nodules

• Structurally similar to TONSILS

• Location aids in functions:

  • Destroy bacteria, preventing them from breaching intestinal wall

  • Generate “MEMORY” lymphocytes

APPENDIX: offshoot of first part of LARGE INTESTINE

• Contains large number of lymphoid follicles

• Location aids in functions (like Peyer’s patches)

  • Destroy bacteria, preventing them from breaching intestinal wall

  • Generate “memory” lymphocytes

20.6 Thymus

• 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 in size during childhood

  • Stops growing during adolescence, then gradually ATROPHIES

  • Still produces immunocompetent cells, though more slowly

• Thymus is broken into lobules that contain outer cortex and inner medulla

  • Cortex contains RAPIDLY DIVIDING LYMPHOCYTES (the bulk of thymic cells) and scattered macrophages

  • Medulla contains fewer lymphocytes and thymic corpuscles

    • Thymic corpuscles are where regulatory T cells develop

      • Type of T cell that helps to prevent AUTOIMMUNITY

• 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

      • Contains BLOOD THYMUS BARRIER: keeps immature T lymphocytes isolated from any antigens to prevent PREMATURE ACTIVATION

  • STROMA is made up of epithelial cells, not reticular fibers

    • Provide environment in which T lymphocytes become IMMUNOCOMPETENT

Developmental Aspects of the Lymphatic System

• Beginnings of lymphatic vessels and main clusters of lymph nodes seen by week 5 of embryonic development

  • Arise as LYMPH SACS from developing veins

  • Jugular lymph sacs arise to form right lymphatic duct and thoracic duct

• 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 are POORLY DEVELOPED at birth

• After birth, high numbers of lymphocytes appear; their development parallels maturation of immune system