Lymphatic System Anatomy and Function

Functions of the Lymphatic System

• Returns fluids leaked from the vascular system to ensure sufficient blood volume in the cardiovascular system, approximately 3 liters.
• Transports dietary fats from the intestine to the bloodstream via lacteals; fatty lymph is called chyle.
• Protects against cancer and infection.

Lymph Formation and Flow

• Interstitial fluid that enters lymphatic capillaries is called lymph.
• Lymph flows from lymph capillaries to lymph vessels, then to lymph trunks, and finally to lymph ducts.

Lymphatic Capillaries

• Found in nearly all tissues, excluding bone and teeth.
• Display permeability due to endothelial cells not tightly joined, forming easily openable minivalves.
• Collagen filaments anchor endothelial cells, and increases in interstitial fluid volume open the minivalves allowing fluid to enter.
• Fluid can move into but not out of lymphatic capillaries.
• Function as blind-ended tubes where adjacent endothelial cells overlap.
• Proteins in interstitial space can easily enter lymphatic capillaries.
• When tissues are inflamed, lymphatic capillaries enlarge to allow uptake of cell debris, pathogens, and cancer cells, which are then removed by immune system cells in lymph nodes.
• Lymphatics in the skin travel with superficial veins, while those in the trunk and digestive viscera travel with deep arteries.

Lymphatic Trunks and Ducts

• The five lymphatic trunks include: subclavian (2), bronchomediastinal (2), lumbar (2), jugular (2), and intestinal (1).
• The right lymphatic duct drains lymph from the right side of the head and right arm.
• The left lymphatic duct (thoracic duct) receives lymph from the rest of the body, beginning at the cisterna chyli.
• Each duct empties lymph into venous circulation at the junction of the internal jugular and subclavian veins on its respective side of the body.
• Lymphatics function as low-pressure conduits, and the movement of lymph is aided by the milking action of active skeletal muscles, pressure changes during breathing, and valves.

Lymphoid Cells

• There are five types of lymphoid cells: B and T lymphocytes, dendritic cells (APCs), macrophages, and reticular cells.
• Reticular cells produce reticular fiber stroma, which supports other cells in lymphoid organs.

Lymphoid Tissue

• This exists in two types: diffuse lymphoid tissue and lymphoid follicles (nodules).
• Diffuse lymphoid tissue has a loose arrangement of lymphoid cells and reticular fibers and can be found in almost every body organ (except the thymus), with large collections in the mucous membranes of the digestive tract.
• Lymphoid follicles (nodules) are solid, spherical bodies consisting of tightly packed lymphoid cells and reticular fibers.
• Follicles contain light-staining germinal centers where rapidly dividing B lymphocytes are located.
• Follicles are part of large organs like lymph nodes, and isolated aggregations can be found in the small intestine as Peyer's patches and in the appendix.

Lymphoid Organs

• Primary lymphoid organs are where lymphocytes mature.
• Secondary lymphoid organs are where lymphocytes activate and proliferate.
• The primary lymphoid organs are the red bone marrow, where B cells mature, and the thymus, where T cells mature.
• Secondary lymphoid organs include the spleen, lymph nodes, collections of mucosa-associated lymphoid tissue (tonsils), and Peyer's patches (aggregated lymphoid nodules) in the small intestine and appendix.

Lymph Nodes

• These are the most important secondary lymphoid organs, clustering along lymphatic vessels throughout the body with large clusters in inguinal, axillary, and cervical regions.
• Lymph nodes are covered by a capsule of dense connective tissue, and capsular extensions called trabeculae divide the node into compartments.
• The capsule, trabeculae, reticular fibers, and fibroblasts constitute the stroma (supporting network) of the lymph node.
• The functioning part (parenchyma) of the lymph node consists of the cortex and medulla.
• The cortex of the lymph node contains germinal centers with proliferating B cells, while the outer edge of the follicle contains T cells and dendritic cells.
• Medullary cords in the medulla contain B cells, plasma cells, macrophages, and T cells.
• Two primary functions of lymph nodes include cleansing the lymph and activating the immune system.
• Buboes are infected, pus-filled lymph nodes.
• Lymphadenopathy refers to swollen, inflamed lymph nodes.
• Cancer-infiltrated lymph nodes are swollen but not painful.

Spleen

• It is the largest lymphoid organ.
• It provides a site for B and T cell activation and proliferation and immune surveillance, as well as cleansing the blood through macrophages that remove debris and foreign matter.
• Additional functions include recycling breakdown products of red blood cells, storing blood platelets and monocytes for release when needed, and serving as a site for erythrocyte production in the fetus.
• Spleen components include the fibrous capsule, trabeculae, red pulp, and white pulp.
• The white pulp of the spleen functions in immunity and contains lymphocytes.
• Red pulp contains erythrocytes and macrophages, where worn-out red blood cells and pathogens are destroyed.

Mucosa-Associated Lymphoid Tissue (MALT)

• Examples include tonsils, Peyer's patches, and the appendix.
• Tonsils are rings of lymphoid tissue that gather and remove pathogens entering the pharynx (throat) via food and inhaled air.
• Lymphoid tissue of tonsils contains germinal centers and scattered lymphocytes, they lack a capsule and form tonsillar crypts.
• Peyer's patches are aggregated lymphoid nodules and are found in the distal part of the small intestine.
• The appendix contains aggregations of lymphoid nodules.

Thymus

• This is a primary lymphoid organ that is prominent in newborns and continues to grow during the first year.
• It stops growing during adolescence and then atrophies.
• It continues to produce immunocompetent cells.
• The thymus is divided into lobules with an outer cortex and inner medulla.
• The cortex contains rapidly dividing lymphocytes and scattered macrophages, while the medulla contains fewer lymphocytes and thymic corpuscles.
• Thymic corpuscles are where regulatory T cells develop, which help prevent autoimmunity.
• The thymus differs from other lymphoid organs by lacking B cells and follicles, not directly fighting antigens, and having a stroma made of epithelial cells.

Lymphadenopathy

• This is the enlargement of lymph nodes in response to infection and is also known as swollen glands.

Lymphoma

• These are cancers of the lymphoid organs, especially the lymph nodes.
• The two main types are Hodgkin lymphoma and non-Hodgkin lymphoma.
• Hodgkin lymphoma is characterized by painless enlargement of lymph nodes and can be cured in 90-95% of cases if diagnosed early.
• Burkitt lymphoma is a form of B cell non-Hodgkin lymphoma.

Explore the anatomy and function of the lymphatic system. Topics covered include the spleen, thymus, lymphatic capillaries, and tonsils.