Lymphatic System Functions and Components

• Understanding lymph node structure and function aids in evaluating patients for infection.

Lymphatic Systems and Lymphoid Organ and Tissues

• The lymphatic system returns fluids leaked from blood vessels back to the blood.
• It is comprised of lymphatic vessels, lymph, and lymph nodes.
• The lymphatic system's vessels form a network called lymphatics.
• Lymph is the fluid contained within these vessels.
• Lymph nodes are the structures that cleanse the lymph.
• Lymphoid organs and tissues provide the structural basis for the immune system.
• They house phagocytic cells and lymphocytes.
• These include the spleen, thymus, tonsils, lymph nodes, and other lymphoid tissues.

20.1 Lymphatic System

• The lymphatic system returns interstitial fluid and leaked plasma proteins to the blood.
• Lymphatic vessels act as drainage vessels.
• Approximately 3L of interstitial fluid is circulated per day.
• Interstitial fluid that enters the lymphatics becomes lymph.

Distribution and Structure of Lymphatic Vessels

• Lymphatic vessels ensure lymph flows only toward the heart.
• Lymph vessels include both lymphatic capillaries and larger lymphatic vessels.

Lymphatic Capillaries

• These are blind-ended vessels weaving between tissue cells and blood capillaries.
• They are absent from bones, teeth, and bone marrow.
• Lymphatic capillaries are more permeable than blood capillaries.
• They pick up larger molecules and particles such as proteins, cell debris, pathogens, and cancer cells.
• They allow pathogens or cancer cells to travel throughout the body.
• Increased permeability is due to one-way minivalves that endothelial cells loosely overlap to form.
• Lacteals are lymphatic capillaries present in intestinal mucosa and absorb digested fat, delivering fatty lymph (chyle) to the blood.

Larger Lymphatic Vessels

• Lymph capillaries drain into progressively larger vessels called collecting lymphatic vessels.
• Lymphatic trunks are formed by the union of the largest collecting vessels and drain large areas of the body.
• Lymph is delivered from trunks into one of two large lymphatic ducts.
• The right lymphatic duct drains the right upper arm and right side of the head and thorax.
• The thoracic duct drains the rest of the body.
• It starts as an enlarged sac called the cisterna chyli in about half of individuals.
• Each duct empties the lymph into venous circulation at the junction of the internal jugular and subclavian veins on its respective side of the body.

Lymph Transport

• The lymphatic system is a low-pressure system, similar to the venous system.
• Lymph is propelled by:
• The milking action of skeletal muscle.
• Pressure changes in the thorax during breathing.
• Valves that prevent backflow.
• Pulsations of nearby arteries.
• Contractions of smooth muscle in the walls of lymphatics.
• Physical activity increases lymph flow.
• Immobilization of an area retains inflammatory material for faster healing.

20.2 Lymphoid Cells, Tissues, and Organs

• Lymphoid cells consist of immune system cells
• Lymphocytes are cells of the adaptive immune system that mature into either T cells (T lymphocytes) or B cells (B lymphocytes).

Lymphoid Organs

• Primary lymphoid organs:
• Thymus
• Red bone marrow
• Secondary lymphoid organs:
• Lymph nodes
• Tonsils
• Spleen
• Peyer's patches
• Appendix

20.3 Lymph Nodes

• Lymph nodes act as lymph filters and facilitate immune system actication.
• Macrophages in the nodes remove and destroy microorganisms and debris from the lymph, preventing unwanted substances from reaching the blood.
• They offer a place for lymphocytes to become activated and mount an attack against antigens.

Structure of a Lymph Node

• Lymph nodes vary in shape and size, but most are bean-shaped.
• They are small, less than 2.5 cm (~1 inch).
• They are surrounded by an external fibrous capsule.
• Capsule fibers extend inward as trabeculae, dividing the node into compartments.
• Lymph nodes have two histologically distinct regions: the cortex and the medulla.

Cortex

• The superficial area of the cortex contains follicles with germinal centers that are heavy with dividing B cells.
• The deep cortex houses T cells in transit, which circulate continuously among blood, lymph nodes, and lymph.
• Abundant numbers of dendritic cells are closely associated with both T and B cells.
  • These play a role in activating lymphocytes.

Medulla

• Medullary cords extend inward from the cortex and contain B cells, T cells, and plasma cells.
• Lymph sinuses are found throughout the node.
• They consist of large lymphatic capillaries spanned by crisscrossing reticular fibers.
• Macrophages reside on these fibers and phagocytize any foreign matter.

Circulation in the Lymph Nodes

• Lymph enters the convex side of the node via afferent lymphatic vessels.
• It then travels through the large subcapsular sinus into smaller sinuses throughout the cortex and medulla.
• Lymph flows into the medullary sinuses and then exits the concave side at the hilum via efferent lymphatic vessels.
• The presence of fewer efferent vessels causes the flow to somewhat stagnate.
• This allows lymphocytes and macrophages time to function and allowing lymph to travel through several nodes.

20.4 Spleen

• The spleen is a blood-rich organ about the size of a fist, located in the left side of the abdominal cavity, just below the stomach.
• It is the largest lymphoid organ.
• The splenic artery and vein serve it, entering and exiting at the hilum.
• Functions:
• It serves as the site of lymphocyte proliferation, immune surveillance, and response.
• It cleanses the blood of aged blood cells and platelets.
• Macrophages remove debris.
• It stores breakdown products of RBCs (e.g., iron) for later reuse.
• Spleen also stores blood platelets and monocytes for release into the blood when needed.
• It may be a site of fetal erythrocyte production.
• The spleen is encased by a fibrous capsule and also has trabeculae.
• Histologically Consists of :
• White pulp: This is the site where immune function occurs, containing mostly lymphocytes on reticular fibers.
• White pulp clusters are found around central arteries and appear as islands of white in a sea of red pulp.
• Red pulp: This is the site where old blood cells and bloodborne pathogens are destroyed.
• It is rich in RBCs and macrophages that engulf them.
• Red pulp is composed of splenic cords (reticular tissue) that separate blood-filled splenic sinusoids (venous sinuses).

20.5 MALT

• Mucosa-associated lymphoid tissue (MALT) refers to lymphoid tissues in mucous membranes.
• It protects from pathogens trying to enter the body.
• It is found in the mucosa of the respiratory tract, genitourinary organs, and digestive tract.
• Largest collections are found in:
• Tonsils
• Peyer's patches
• Appendix

Tonsils

• Tonsils are the simplest lymphoid organs that form a ring of lymphatic tissue around the pharynx and appear as swellings of mucosa.
• Types of Tonsils:
• Palatine tonsils: located at the posterior end of the oral cavity; they are the largest and most often infected.
• Lingual tonsil: a lumpy collection of follicles at the base of the tongue.
• Pharyngeal tonsil: also called adenoids, located in the posterior wall of the nasopharynx.
• Tubal tonsils: surround the openings of the auditory tubes into the pharynx.
• Tonsils function to gather and remove pathogens in food or air.
• They contain follicles with germinal centers and scattered lymphocytes.
• Not fully encapsulated and has overlying epithelium that invaginates, forming tonsillar crypts.
• Bacteria or particulate matter enters these crypts, where they are trapped and destroyed.
• Although this is risky, it allows immune cells to become activated and build memory cells against these potential pathogens.

Peyer's Patches

• Peyer's patches are clusters of lymphoid follicles in the wall of the distal portion of the small intestine, also called aggregated lymphoid nodules.
• They are structurally similar to tonsils.
• Location Aids Functions:
• Destroying bacteria and preventing them from breaching the intestinal wall.
• Generating "memory" lymphocytes.

Appendix

• The appendix is an offshoot of the first part of the large intestine.
• Location Aids Functions:
• Destroying bactera
• Generate "memory" lymphocytes.

20.6 Thymus

• Thymus description:
• A bilobed lymphoid organ found in the inferior neck.
• Extends into the mediastinum and partially overlies the heart.
• The thymus functions as the lymphoid organ where T cells mature.
• THe thymus is most active and largest in size during childhood.
• It stops growing during adolescence, then gradually atrophies.
• The thymus continues to produce immunocompetent cells, though more slowly, with age.
• The thymus differs from other lymphoid organs as it functions strictly in T lymphocyte maturation.
• The blood thymus barrier keeps immature T lymphocytes isolated from any antigens to prevent premature activation.
• The thymus provides an environment in which T lymphocytes become immunocompetent.

Explore the primary functions and components of the lymphatic system. Questions cover lymph flow, vessel structure, fluid dynamics, and lymphoid organ functions. Test your understanding of how this system maintains fluid balance and supports the immune response.