Chapter 20: Lymphatic System, Lymphoid Organs, and Tissues

Introduction to the Lymphatic System and Lymphoid Structures

• The lymphatic system, organs, and tissues are essential to physiological survival; if they ceased functioning, the cardiovascular system would stop and the immune system would become non-functional.

• The primary function of the lymphatic system is to clean and return lymph back to the cardiovascular system.

  • Lymph originates as plasma, serum, or interstitial fluid.

  • Once returned, it becomes part of the blood plasma again.

• The system consists of three main components:

  • A network of lymphatic vessels.

  • Lymph (the fluid being transported).

  • Lymph nodes, which clean the lymph as it passes through the network.

• Lymphoid organs and tissues provide the structural basis for the immune system and include:

  • Spleen.

  • Thymus.

  • Tonsils.

  • Lymph nodes (which belong to both the lymphatic system and lymphoid organ categories).

  • Other lymphoid tissues distributed throughout the body.

Lymphatic Capillaries and Permeability

• Lymphatic capillaries are highly permeable vessels that weave through tissues and surround blood capillaries.

• They function to return interstitial fluids and any leaked plasma proteins to the cardiovascular system.

• Lymph flow is strictly unidirectional, moving only toward the heart.

• The extreme permeability of lymphatic capillaries is due to two anatomical features:

  • Endothelial cells are not tightly joined; they overlap loosely to form flap-like, easy-to-open mini-valves.

  • Collagen filaments anchor these endothelial cells to surrounding structures. An increase in interstitial fluid volume pushes the flaps open, allowing fluid to enter.

• These capillaries can easily take up proteins and large particles, such as cell debris during inflammation.

• Lacteals are specialized lymphatic capillaries located in the fingerlike villi of the intestinal mucosa.

  • They transfer absorbed fat into the bloodstream.

  • Chyle is the special, milky lymph that drains from these lacteals.

  • Chylothorax is a medical condition involving the abnormal accumulation of chyle in the pleural space surrounding the lung, typically caused by a disruption in thoracic duct drainage.

Lymphatic Vessels, Trunks, and Ducts

• Collecting lymphatic vessels possess the same three tunics as veins but differ in having thinner walls, more internal valves, and more frequent anastomoses - connections between lymphatic vessels and veins.

• Superficial lymphatic vessels generally follow the pathways of veins, while deep lymphatic vessels follow the pathways of arteries.

• There is significant individual variation in the pattern of lymphatic vessels, even more so than in the venous system.

• The hierarchy of lymph drainage flows from collecting vessels into lymphatic trunks.

• Lymphatic trunks are named for the specific regions they drain:

  • Right and Left Jugular trunks.

  • Right and Left Subclavian trunks.

  • Right and Left Bronchomediastinal trunks.

  • Right and Left Lumbar trunks.

  • Intestinal trunk.

• Lymphatic trunks eventually lead to one of two major lymphatic ducts:

  • Right Lymphatic Duct: Drains lymph from the right upper arm and the right side of the head and thorax.

  • Thoracic Duct: Drains lymph from the remainder of the body. It begins as an enlarged sac called the Cisterna chyli.

• Both ducts return lymph to the venous system at the junction of the internal jugular and subclavian veins on their respective sides of the body.

Mechanisms of Lymph Transport

• Lymph transport is slow and occurs through mechanisms similar to venous blood flow:

  • One-way valves to prevent backflow.

  • Skeletal muscle contractions (the "muscle pump").

  • Pressure changes in the thoracic and abdominal cavities caused by breathing.

  • Pulsations of nearby arteries.

  • Contractions of smooth muscle within the walls of larger lymphatic vessels.

Clinical Application: Venomous Snake Bites

• Because the lymphatic system accepts proteins from interstitial fluid and vessels run superficially, venom proteins often travel through this system.

• Medically sound first aid for a venomous bite involves:

  • Keeping the patient calm.

  • Transporting the patient immediately to a healthcare facility.

  • Applying a lymphatic constriction band (e.g., a wide rubber band) proximal to the bite if located on a limb.

  • This band is designed to slow the travel of venom proteins through the lymphatic system rather than cutting off deep arterial circulation.

Lymphoid Cells

• Lymphoid cells include four primary types:

  • Lymphocytes: The main warriors of the immune system.

  • T cells: Manage the immune response and directly attack/destroy infected cells.

  • B cells: Produce plasma cells, which in turn secrete antibodies.

  • Lymphocytes are on a constant circuit through the bloodstream, loose connective tissues, interstitial fluid, and lymphoid tissues.

  • Macrophages: Derived from monocytes; they phagocytize foreign substances and help activate T cells.

  • Dendritic cells: Specialized, "octopus-like" cells that capture antigens and transport them back to lymph nodes.

  • Reticular cells: Fibroblast-like cells that produce reticular fibers (stroma). This stroma provides the "soft skeleton" supporting most lymphoid organs.

Lymphoid Tissues

• Lymphoid tissue serves as a housing site for lymphocytes and a place for them to proliferate. It also provides a surveillance point for lymphocytes and macrophages.

• Most lymphoid tissue (except for the thymus) is composed of reticular connective tissue.

• There are two main types of lymphoid tissue:

  • Diffuse lymphoid tissue: A loose arrangement of lymphoid cells and reticular fibers found throughout the body, particularly in the lamina propria of mucous membranes.

  • Lymphoid follicles (lymphoid nodules): Spherical structures of tightly packed lymphoid cells and reticular fibers.

  • These contain light germinal centers which are active sites of B cell production.

  • They are found as part of larger lymphoid organs (nodes, appendix) or in intestinal walls as Peyer’s patches.

Classification of Lymphoid Organs

• Primary Lymphoid Organs: Sites where lymphocytes originate and/or mature.

  • Red Bone Marrow: Origin of both B and T cells; site of B cell maturation.

  • Thymus: Site of T cell maturation.

• Secondary Lymphoid Organs: Sites where mature lymphocytes first encounter antigens and become activated.

  • Includes lymph nodes, spleen, tonsils, Peyer’s patches, appendix, and diffuse lymphoid tissues.

Lymph Nodes

• Lymph nodes occur along lymphatic vessels and are clustered near the body surface in the axillary, inguinal, and cervical regions.

• Internal Structures:

  • Stroma: The internal framework composed of reticular fibers.

  • Hilum: An indentation on the concave side where efferent vessels exit.

  • Trabeculae: Extensions of the external capsule that extend into the cortex and divide the node into compartments.

• Functions:

  • Cleansing (Filtration): Macrophages within the nodes remove and destroy microorganisms and debris.

  • Immune System Activation: Lymphocytes encounter antigens and become activated.

• Dynamics of flow: There are more afferent vessels entering the node than efferent vessels exiting at the hilum.

  • This difference in vessel number causes the lymph flow rate to slow down within the node.

  • The slow flow provides necessary time for macrophages to clean the lymph and for the immune system to recognize antigens.

• Clinical Note: Metastasizing cancer cells frequently invade lymph nodes, as they are easily taken up by the highly permeable lymphatic capillaries in inflamed tissues. Nodes then become secondary cancer sites.

The Spleen

• The spleen is roughly the size of a fist, located on the left side of the abdominal cavity just below the diaphragm.

• Anatomy: Served by the splenic artery and splenic vein, which enter and exit through the hilum.

• Functions:

  • Site for lymphocyte proliferation, surveillance, and immune response.

  • Blood cleaning: Macrophages remove debris and foreign matter; the spleen weeds out aged or damaged red blood cells (RBCs).

  • Recycling: Breaks down RBCs and stores the materials for re-use (e.g., iron).

  • Storage: Acts as a reservoir for thrombocytes (platelets) and monocytes.

  • Erythropoiesis: Occurs in the spleen only during the fetal stage of development.

• Internal Divisions:

  • White pulp: Composed mostly of lymphocytes on reticular fibers; this is where immune functions occur.

  • Red pulp: Contains splenic cords and splenic sinusoids; rich in macrophages; this is where bloodborne pathogens and old RBCs are destroyed.

Mucosa-Associated Lymphoid Tissue (MALT)

• MALT refers to lymphoid tissues scattered throughout the body that protect mucous membranes from pathogens.

• The largest accumulations are found in three regions:

  • Tonsils: Remove pathogens from food and inhaled air. They contain follicles with germinal centers and possess tonsillar crypts (breaks in the capsule).

  • Crypts trap pathogens and invite them deep into the tissue to interact with lymphocytes, creating long-lasting immune memory.

  • Peyer’s patches: Also known as aggregated lymphoid nodules, these are structurally similar to tonsils and located in the wall of the distal small intestine.

  • Appendix: A tubular offshoot of the cecum (where the small and large intestines meet). It contains lymphoid tissue similar to Peyer's patches; it prevents pathogens from crossing the digestive tract wall and generates immune memory.

The Thymus

• Located in the inferior neck, extending onto the superior surface of the heart.

• Lifespan and size: It is large in newborns and continues to grow through childhood. Starting at puberty, it begins to atrophy (shrink) slowly; by old age, it is hardly perceptible and composed mostly of fibrous and fatty tissue.

• Specific characteristics differentiating the thymus from secondary lymphoid organs:

  • It lacks follicles because it lacks B cells.

  • It does not directly fight antigens. The blood-thymus barrier keeps bloodborne antigens away from developing T cells to prevent premature activation.

  • The stroma is made of epithelial tissue rather than reticular connective tissue; this environment is necessary for T cell maturation.

• Endocrine Function: The thymus produces peptide hormones (like thymosin) that play a vital, though not fully understood, role in T cell development.