The Lymphatic System

Lymphatic System Overview

• Comprises lymphatic vessels, lymph fluid, and lymph nodes
• Lymphoid organs and tissues provide the structural basis for the immune system
• Houses phagocytic cells and lymphocytes
• Structures include the spleen, thymus, tonsils, and other lymphoid tissues

Lymphatic Vessel Functions

• Lymphatic vessels form an elaborate drainage network
• They return interstitial fluid and leaked plasma proteins back to the blood
• About 3 liters of fluid are returned per day
• Interstitial fluid that enters lymphatics is called lymph

Lymphatic Vessel Structure and Distribution

• The lymphatic system is a one-way system, with lymph flowing toward the heart
• Lymph vessels include lymphatic capillaries, collecting lymphatic vessels, and lymphatic trunks and ducts

Lymphatic Capillaries Details

• Lymphatic capillaries are similar to blood capillaries but are more permeable
• They can take up proteins, cell debris, pathogens, and cancer cells
• Endothelial cells overlap loosely, forming one-way minivalves
• Anchored by collagen filaments, preventing collapse; increased ECF volume opens minivalves
• Absent from bones, teeth, bone, and the central nervous system (CNS)
• Lacteals are specialized lymph capillaries in the intestinal mucosa
• Lacteals absorb digested fat and deliver fatty lymph (chyle) to the blood

Lymphatic Collecting Vessels Details

• Lymphatic collecting vessels are similar to veins but have thinner walls and more internal valves
• They 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 Details

• Lymph moves through vessels of increasing size: lymphatic capillaries, lymphatic vessels, lymphatic trunks, and lymphatic ducts
• Fluid is ultimately returned to blood circulation
• Trunks are formed by the union of the largest collecting ducts
• paired lumbar trunks
• paired bronchomediastinal trunks
• paired subclavian trunks
• paired jugular trunks
• single intestinal trunk.

Lymphatic Ducts Details

• Lymph is delivered into one of two large ducts
• Right lymphatic duct drains the right upper arm and right side of the head and thorax
  • This includes the right side of the head and neck, right upper limb, and right side of the thorax
• The thoracic duct arises at the cisterna chyli and drains the rest of the body
• Each duct empties lymph into venous circulation at the junction of the internal jugular and subclavian veins on its respective side

Lymph Transport Mechanisms

• Lymph is propelled by the milking action of skeletal muscle
• Pressure changes in the thorax during breathing
• Valves prevent backflow
• Pulsations of nearby arteries
• Contractions of smooth muscle in the walls of lymphatics
• Physical activity increases lymph flow; immobilization keeps inflammatory material in the area for faster healing

Lymphoid Cells and Lymphocytes

• Lymphocytes are the main warriors of the immune system
• All lymphocytes Arise in red bone marrow
• They mature into T cells (T lymphocytes) or B cells (B lymphocytes)
• T cells and B cells protect against antigens (anything the body perceives as foreign), including bacteria, bacterial toxins, viruses, mismatched RBCs, and cancer cells

T Cells

• Manage the immune response
• 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 Cells

• Macrophages phagocytize foreign substances and help activate T cells
• Dendritic cells capture antigens and deliver them to lymph nodes
• Reticular cells produce reticular fiber stroma that supports other cells in lymphoid organs

Lymphoid Tissue Details

• Lymphoid tissue houses lymphocytes and provides proliferation sites
• It serves as a surveillance vantage point for lymphocytes and macrophages
• Largely composed of reticular connective tissue (loose connective tissue)
• There are two main types:
  • Diffuse lymphoid tissue: lymphoid cells and reticular fibers found in almost every body organ
  • Larger collections in the lamina propria of mucous membranes
• Lymphoid follicles are solid, spherical bodies of tightly packed lymphoid cells and reticular fibers
• Germinal centers are areas of proliferating B cells
• Lymphoid tissue may form part of larger lymphoid organs
• Isolated aggregations are found in Peyer's patches and the appendix

Lymph Nodes Details

• Lymph nodes are the principal secondary lymphoid organs of the body
• They are embedded in connective tissue, in clusters along lymphatic vessels
• Located near the body surface in inguinal, axillary, and cervical regions
• Functions to:
  • Filter lymph where macrophages destroy microorganisms and debris
  • Activate the Immune system where lymphocytes are activated and mount attacks against antigens

Lymph Node Structure

• Lymph nodes vary in shape and size but are mostly bean-shaped
• They have an external fibrous capsule
• Trabeculae extend inward and divide the node into compartments
• There are two histologically distinct regions, The cortex and the medulla

Lymph Node Cortex

• Contains follicles with germinal centers, which are heavy with dividing B cells
• Dendritic cells nearly encapsulate follicles
• The deep cortex houses T cells in transit
• T cells circulate continuously among blood, lymph nodes, and lymph

Lymph Node Medulla

• Medullary cords extend inward from the cortex
• The cords contain B cells, T cells, and plasma cells
• Lymph sinuses contain macrophages

Circulation in Lymph Nodes

• Lymph enters the convex side via afferent lymphatic vessels
• Travels through the large subcapsular sinus and smaller sinuses to medullary sinuses
• Exits the concave side at the hilum via efferent vessels
• Fewer efferent vessels make flow somewhat stagnant
• Slower flow allows lymphocytes and macrophages time to function

Spleen Details

• The spleen is the largest lymphoid organ
• Served by the splenic artery and vein, which enter and exit at the hilum
• Functions include lymphocyte proliferation, it is a site for immune surveillance and response
• Cleanses blood of aged cells and platelets and macrophages remove debris

Additional Spleen Functions

• Stores breakdown products of RBCs (e.g., iron) for later reuse
• Stores blood platelets and monocytes
• May be a site of fetal erythrocyte production (normally ceases before birth)
• Encased by a fibrous capsule and has trabeculae
• Contains lymphocytes, macrophages, and many erythrocytes

Spleen Structure

• Characterized by two distinct areas:
  • White pulp around central arteries, which mostly contains lymphocytes on reticular fibers and is involved in immune functions
  • Red pulp in venous sinuses and splenic cords, which is rich in RBCs and macrophages for disposal of worn-out RBCs and bloodborne pathogens and is composed of splenic cords and sinusoids

Thymus Details

• The Thymus is important early in life
• Located in the inferior neck, extends into the mediastinum, and partially overlies the heart
• Increases in size and is most active during childhood
• Stops growing during adolescence and then gradually atrophies
• Still produces immunocompetent cells, though slowly

Thymus Structure and Function

• Thymic lobules contain an outer cortex and inner medulla
• Most thymic cells are lymphocytes
• The cortex contains rapidly dividing lymphocytes and scattered macrophages
• The medulla contains fewer lymphocytes and thymic corpuscles involved in regulatory T cell development (prevent autoimmunity)
• It differs from other lymphoid organs as it has no follicles because it lacks B cells
• It does not directly fight antigens; instead, it functions strictly in T lymphocyte maturation
• Keeps cells isolated via a blood-thymus barrier
• The stroma is composed of epithelial cells not reticular fibers, creates an environment in which T lymphocytes become immunocompetent

Mucosa-Associated Lymphoid Tissue (MALT)

• Lymphoid tissues in mucous membranes throughout the body
• MALT Protects from pathogens trying to enter the body
• MALT is located in Largest collections in tonsils, Peyer's patches, and the appendix
• Also found in the mucosa of respiratory and genitourinary organs, and the rest of the digestive tract

Tonsils Details

• The simplest lymphoid organs
• Form a ring of lymphatic tissue around the pharynx, including:
  • Palatine tonsils at the posterior end of the oral cavity
  • Lingual tonsil grouped at the base of the tongue
  • Pharyngeal tonsil (adenoids) in the posterior wall of the nasopharynx
  • Tubal tonsils surrounding openings of auditory tubes into the pharynx
• Gather and remove pathogens in food or air
• Contain follicles with germinal centers
• These are not fully encapsulated
• The overlying epithelium invaginates, forming tonsillar crypts
• Trap and destroy bacteria and particulate matter
• Allow immune cells to build memory for pathogens

Aggregates of Lymphoid Follicles

• Peyer's patches are clusters of lymphoid follicles
• They are found in the wall of the distal portion of the small intestine
• Similar structures are also found in the appendix
• These destroy bacteria, preventing them from breaching the intestinal wall
• They also generate "memory" lymphocytes