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Chapter Notes for Lecture: E.N. Marieb, HUMAN ANATOMY & PHYSIOLOGY,5TH Edition, , Benjamine Cummings Publisher, 2001 Prepare from : V.A. Austin’s PowerPpoint Presentation (ISBN: 0-8053-5469-7), CD ROM: Pearson Education, Inc. , 2003.ter

 

The Lymphatic System

 

Lymphatic System: Overview

•      Consists of two semi-independent parts

•    A meandering network of lymphatic vessels

•    Lymphoid tissues and organs scattered throughout the body

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

•      Lymph – interstitial fluid once it has entered lymphatic vessels

Lymphatic Vessels

•      A one-way system in which lymph flows toward the heart

•      Lymph vessels include:

•    Microscopic, permeable, blind-ended capillaries

•    Lymphatic collecting vessels

•    Trunks and ducts

Lymphatic Capillaries

•      Similar to blood capillaries, with modifications

•    Remarkably permeable

•    Loosely joined endothelial minivalves

•    Withstand interstitial pressure and remain open

•      The minivalves function as one-way gates that:

•    Allow interstitial fluid to enter lymph capillaries

•    Do not allow lymph to escape from the capillaries

•      During inflammation, lymph capillaries can absorb:

•    Cell debris

•    Pathogens

•    Cancer cells

•      Cells in the lymph nodes:

•    Cleanse and “examine” this debris

•      Lacteals – specialized lymph capillaries present in intestinal mucosa

•    Absorb digested fat and deliver chyle to the blood

Lymphatic Collecting Vessels

•      Have the same three tunics as veins

•      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

•      Lymphatic trunks are formed by the union of the largest collecting ducts

•      Major trunks include:

•    Paired lumbar, bronchomediastinal, subclavian, and jugular trunks

•    A single intestinal trunk

•      Lymph is delivered into one of two large trunks

•    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

Lymphatic Transport

•      The lymphatic system lacks an organ that acts as a pump

•      Vessels are low pressure conduits

•      Uses the same methods as veins to propel lymph

•    Pulsations of nearby arteries

•    Contractions of smooth muscle in the walls of the lymphatics

Lymphoid Cells

•      Lymphocytes are the main cells involved in the immune response

•      The two main varieties are T cells B cells

Lymphocytes

•      T cells and B cells protect the body against antigens

•      Antigen – anything the body perceives as foreign

•    Bacteria and their toxins, and viruses

•    Mismatched RBCs or cancer cells

•      T cells

•    Manage the immune response

•    Attack and destroy foreign cells

•      B cells

•    Produce plasma cells, which secrete antibodies

•    Antibodies immobilize antigens

Other Lymphoid Cells

•      Macrophages – phagocytize foreign substances and help activate T cells

•      Dendritic cells – spiny-looking cells with functions similar to macrophages

•      Reticular cells – fibroblastlike cells that produce a stroma, or network, that supports other cell types in lymphoid organs

Lymphoid Tissue

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

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

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

•    Have a germinal center composed of dendritic and B cells

•    Found in isolation and as part of larger lymphoid organs

Lymphoid Organs

•      Lymphoid organs – discrete, encapsulated collections of diffuse lymphoid tissue and follicles

•      Examples include the lymph nodes, spleen, and thymus

Lymph Nodes

•      Nodes are imbedded in connective tissue and clustered along lymphatic vessels

•      Aggregations of these nodes occur near the body surface in inguinal, axillary, and cervical regions of the body

•      Their two basic functions are:

•    Filtration – macrophages destroy microorganisms and debris

•    Immune system activation – monitor for antigens and mount an attack against them

Structure of a Lymph Node

•      Nodes are bean shaped and surrounded by a fibrous capsule

•      Trabeculae extended inward from the capsule and divide the node into compartments

•      Nodes have two histologically distinct regions: a cortex and a medulla

•      The cortex contains follicles with germinal centers, heavy with dividing B cells

•      Dendritic cells nearly encapsulate the follicles

•      The deep cortex houses T cells in transit

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

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

•      Throughout the node are lymph sinuses crisscrossed by reticular fibers

•      Macrophages reside on these fibers and phagocytize foreign matter

Circulation in the Lymph Nodes

•      Lymph enters via a number of afferent lymphatic vessels

•      It then enters a large subcapsular sinus and travels into a number of smaller sinuses

•      It meanders through these sinuses and exits the node at the hilus via efferent vessels

•      Because there are fewer efferent vessels, lymph stagnates somewhat in the node

•      This allows lymphocytes and macrophages time to carry out their protective functions

Homeostatic Imbalances of the Lymph Nodes

•      If lymph nodes are overwhelmed by large numbers of antigen:

•    They become inflamed and tender to the touch

•   Such nodes are called buboes (or erroneously, swollen glands)

•      Nodes can also become secondary cancer sites

•    Such nodes are swollen, but are not painful

•   This distinguishes cancerous nodes from infected ones

Other Lymphoid Organs

•      The spleen, thymus gland, and tonsils

•      Peyer’s patches and bits of lymphatic tissue scattered in connective tissue

•      All are composed of reticular connective tissue and all help protect the body

•      Only lymph nodes filter lymph

Spleen

•      Largest lymphoid organ, located on the left side of the abdominal cavity beneath the diaphragm

•      It extends to curl around the anterior aspect of the stomach

•      It is served by the splenic artery and vein, which enter and exit at the hilus

•      Functions

•    Site of lymphocyte proliferation

•    Immune surveillance and response

•    Cleanses the blood

Additional Spleen Functions

•      Stores breakdown products of RBCs 

•    Spleen macrophages salvage and store iron for later use by bone marrow

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

•      Stores blood platelets

Structure of the Spleen

•      Surrounded by a fibrous capsule, it has trabeculae that extend inward and contains lymphocytes, macrophages, and huge numbers of erythrocytes

•      Two distinct areas of the spleen are:

•    White pulp – area containing mostly lymphocytes suspended on reticular fibers and involved in immune functions

•    Red pulp – remaining splenic tissue concerned with disposing of worn-out RBCs and bloodborne pathogens

Thymus

•      A bilobed organ that secrets hormones (thymosin and thymopoietin) that cause T lymphocytes to become immunocompetent

•      The size of the thymus varies with age

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

•    It increases in size and is most active during childhood

•    It stops growing during adolescence and then gradually atrophies

Internal Anatomy of the Thymus

•      Thymic lobes contain an outer cortex and inner medulla

•      The cortex contains densely packed lymphocytes and scattered macrophages

•      The medulla contains fewer lymphocytes and thymic (Hassall’s) corpuscles

•      The 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 star-shaped thymocytes secret thymosins and thymopoietins that stimulate lymphocytes to become immunocompetent

Tonsils

•      Simplest lymphoid organs; form a ring of lymphatic tissue around the pharynx

•      Location of the tonsils

•    Palatine tonsils – either side of the posterior end of the oral cavity

•    Lingual tonsil – lies at the base of the tongue

•    Pharyngeal tonsil – posterior wall of the nasopharynx

•    Tubal tonsils – surround the openings of the auditory tubes into the pharynx

•      Lymphoid tissue of tonsils contains follicles with germinal centers

•      Tonsil masses are not fully encapsulated

•      Epithelial tissue overlying tonsil masses invaginates, forming blind-ended crypts

•      Crypts trap and destroy bacteria and particulate matter

Aggregates of Lymphoid Follicles

•      Peyer’s patches – isolated clusters of lymphoid tissue, similar to tonsils

•    Found in the wall of the distal portion of the small intestine

•    Similar structures are found in the appendix

•      Peyer’s patches and the appendix:

•    Destroy bacteria, preventing them from breaching the intestinal wall

•    Generate “memory” lymphocytes for long-term immunity

MALT

•      MALT – mucosa-associated lymphatic tissue, composed of:

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

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

•      MALT protects the digestive and respiratory systems from foreign matter

Developmental Aspects

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

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

•      Lymphatic organs (except the thymus) arise from mesoderm

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

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