Lymphatic System

 

Lymphatic System

•            Consists of three parts

•           A network of lymphatic vessels (lymphatics)

•           Lymph

•           Lymph nodes

Functions of the Lymphatic System

•   Fluid balance

•     Excess interstitial fluid enters lymphatic capillaries and becomes lymph (30L from capillaries into interstitial fluid, 27L return leaving 3L).

•   Fat absorption

•     Absorption of fat and other substances from digestive tract via lacteals.  Fluid called chyme

•   Defense

•     Microorganisms and other foreign substances are filtered from lymph by lymph nodes and from blood by spleen

Lymphatic System: Functions

•   Returns interstitial fluid and leaked plasma proteins back to the blood

•    Once interstitial fluid enters lymphatics, it is called lymph

•   Together with lymphoid organs and tissues, provide the structural basis of the immune system

Lymph

•   Water plus solutes from two sources

•    Plasma: ions, nutrients, gases, some proteins

•    Cells: hormones, enzymes, waste products

•   Returns to circulatory system via veins; essential for fluid balance.

 

 

Lymphatic Vessels

•   One-way system, lymph flows toward the heart

•   Lymph vessels (lymphatics) include:

•    Lymphatic capillaries

•    Lymphatic collecting vessels

•    Lymphatic trunks and ducts

Lymphatic Capillaries

•   Similar to blood capillaries, except

•    Very permeable (take up cell debris, pathogens, and cancer cells)

•    Endothelial cells overlap to form one-way minivalves, and are anchored by collagen filaments, preventing collapse of capillaries

Lymphatic Capillaries

•   Absent from bones, teeth, bone marrow and the CNS

•   Lacteals: specialized lymph capillaries present in intestinal mucosa

•    Absorb digested fat and deliver fatty lymph (chyle) to the blood

 

Lymphatic Collecting Vessels

•   Similar to veins, except

•    Have thinner walls, with more internal valves

•    Anastomose more frequently

•   Collecting vessels in the skin travel with superficial veins

•   Deep vessels travel with arteries

•   Nutrients are supplied from branching vasa vasorum

Lymphatic Trunks

•   Formed by the union of the largest collecting ducts

•    Paired lumbar

•    Paired bronchomediastinal

•    Paired subclavian

•    Paired jugular trunks

•    A single intestinal trunk

Lymphatic Ducts

•   Lymph is delivered into one of two large ducts

•    Right lymphatic duct drains the right upper arm and the right side of the head and thorax

•    Thoracic duct arises from the cisterna chyli and drains the rest of the body

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

 

 

Lymph Transport

•   Lymph is propelled by

•    Pulsations of nearby arteries

•    Contractions of smooth muscle in the walls of the lymphatics

 

Lymphoid Cells

•   Lymphocytes the main warriors of the immune system

•   Two main varieties

•    T cells (T lymphocytes)

•    B cells (B lymphocytes)

Lymphocytes

•   T cells and B cells protect against antigens

•    Anything the body perceives as foreign

•   Bacteria and their toxins; viruses

•   Mismatched RBCs or cancer cells

Lymphocytes

•   T cells

•    Manage the immune response

•    Attack and destroy foreign cells

•   B cells

•    Produce plasma cells, which secrete antibodies

Other Lymphoid Cells

•   Macrophages phagocytize foreign substances and help activate T cells

•   Dendritic cells capture antigens and deliver them to lymph nodes

•   Reticular cells produce stroma that supports other cells in lymphoid organs

Lymphoid Tissue

•   Houses and provides a proliferation site for lymphocytes

•   Furnishes a surveillance vantage point

•   Two main types

•    Diffuse lymphatic tissue

•    Lymphatic follicles

 

Lymphoid Tissue

•   Diffuse lymphatic tissue comprises scattered reticular tissue elements in every body organ

•    Larger collections in the lamina propria of mucous membranes and lymphoid organs

Lymphoid Tissue

•   Lymphatic follicles (nodules) are solid, spherical bodies of tightly packed reticular elements and cells

•    Germinal center composed of dendritic and B cells

•    May form part of larger lymphoid organs

Lymph Nodes

•   Principal lymphoid organs of the body

•   Embedded in connective tissue, in clusters along lymphatic vessels

•   Near the body surface in inguinal, axillary, and cervical regions of the body

 

Lymph Nodes

•            Functions

•           Filter lymph—macrophages destroy microorganisms and debris

•           Immune system—lymphocytes are activated and mount an attack against antigens

Structure of a Lymph Node

•   Bean shaped

•   External fibrous capsule

•   Trabeculae extend inward and divide the node into compartments

•   Two histologically distinct regions

•    Cortex

•    Medulla

Structure of a Lymph Node

•   Cortex contains follicles with germinal centers, heavy with dividing B cells

•   Dendritic cells nearly encapsulate the follicles

•   Deep cortex houses T cells in transit

•   T cells circulate continuously among the blood, lymph nodes, and lymphatic stream

 

Structure of a Lymph Node

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

•   Lymph sinuses contain macrophages

 

Circulation in the Lymph Nodes

•   Lymph

•    Enters via afferent lymphatic vessels

•    Travels through large subcapsular sinus and smaller sinuses

•    Exits the node at the hilus via efferent vessels

•   Fewer efferent vessels, causing flow of lymph to stagnate, allowing lymphocytes and macrophages time to carry out functions

 

Spleen

•   Largest lymphoid organ

•   Served by splenic artery and vein, which enter and exit at the hilus

•   Functions

•    Site of lymphocyte proliferation and immune surveillance and response

•    Cleanses the blood of aged cells and platelets and debris

 

 Spleen

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

•   Stores blood platelets

•   Site of fetal erythrocyte production (normally ceases after birth)

•   Has a fibrous capsule and trabeculae

•   Contains lymphocytes, macrophages, and huge numbers of erythrocytes

Structure of the Spleen

•   Two distinct areas

•    White pulp around central arteries

•    Mostly lymphocytes on reticular fibers and involved in immune functions

•    Red pulp in venous sinuses and splenic cords

•   Rich in macrophages for disposal of worn-out RBCs and bloodborne pathogens

 

Splenectomy

Thymus

•   Size with age

•    In infants, it is found in the inferior neck and extends into the mediastinum, where it partially overlies the heart

•    Increases in size and is most active during childhood

•    Stops growing during adolescence and then gradually atrophies

Thymus

•   Thymic lobes contain an outer cortex and inner medulla

•   Cortex contains densely packed lymphocytes and scattered macrophages

•   Medulla contains fewer lymphocytes and thymic (Hassall’s) corpuscles involved in regulatory T cell development

 

 

Thymus

•   Differs from other lymphoid organs in important ways

•    It functions strictly in T lymphocyte maturation

•    It does not directly fight antigens

•   The stroma of the thymus consists of star-shaped epithelial cells (not reticular fibers)

•   These thymocytes provide the environment in which T lymphocytes become immunocompetent

Tonsils

•   Simplest lymphoid organs

•   Form a ring of lymphatic tissue around the pharynx

•     Palatine tonsils—at posterior end of the oral cavity

•     Lingual tonsils—grouped at the base of the tongue

•     Pharyngeal tonsil—in posterior wall of the nasopharynx

•     Tubal tonsils—surrounding the openings of the auditory tubes into the pharynx

Tonsils

•   Contain follicles with germinal centers

•   Are not fully encapsulated

•   Epithelial tissue overlying tonsil masses invaginates, forming tonsillar crypts

•   Crypts trap and destroy bacteria and particulate matter

 

 

Aggregates of Lymphoid Follicles

•   Peyer’s patches

•     Clusters of lymphoid follicles

•     In the wall of the distal portion of the small intestine

•     Similar structures are also found in the appendix

•   Peyer’s patches and the appendix

•     Destroy bacteria, preventing them from breaching the intestinal wall

•     Generate “memory” lymphocytes

 

MALT

•   Mucosa-associated lymphatic tissue, including

•    Peyer’s patches, tonsils, and the appendix (digestive tract)

•    Lymphoid nodules in the walls of the bronchi (respiratory tract)

•   Protects the digestive and respiratory systems from foreign matter

Overview of the
Lymphatic system

Developmental Aspects

•   Beginnings of the lymphatic vessels and main clusters of lymph nodes are apparent by the 5th week of embryonic development

•    These arise from the budding of lymph sacs from developing veins

•   Lymphatic organs (except the thymus) arise from mesoderm

Developmental Aspects

•   The thymus (endodermal origin) forms as an outgrowth of the pharynx

•   Except for the spleen and tonsils, lymphoid organs are poorly developed at birth