Chapter Notes for Lecture: E.N. Marieb, HUMAN ANATOMY & PHYSIOLOGY,5^TH 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