Definition: The lymphatic system is an elaborate network of drainage vessels, known as lymphatic vessels, that return fluid leakages, called lymph, back to the bloodstream.
Function: Lymph is cleansed in lymph nodes as it passes through the body, and lymphoid organs/tissues play a crucial role in the body’s immunity and resistance to disease. Key components include:
Spleen
Tonsils
Thymus, etc.
Anatomy of the Lymphatic System
Meningeal Lymphatic Vessels
Location: Situated around the brain and spinal cord.
Key Structures
Internal Jugular Vein: The main vein for draining blood from the head and neck.
Right Lymphatic Duct: Drains lymph from the right upper limb and right side of the head and thorax.
Thoracic Duct: Drains lymph from the rest of the body, including lower limbs.
Cisterna Chyli: An enlarged sac that acts as a reservoir for lymph before it flows into the thoracic duct.
Aorta: Major artery supplying blood to the body.
Collecting Lymphatic Vessels
Structure: Collecting vessels possess three tunics, have thinner walls, a larger diameter, and contain more valves and anastomoses than veins.
Lymphatic Capillaries: Begin as microscopic blind-ended structures that are highly permeable, equipped with flap-like mini-valves to ensure one-way flow of lymph.
Specialized Lymphatic Capillaries (Lacteals): Located in the villi of the small intestine, play an essential role in absorbing and transporting digested fats, forming a milky fluid known as chyle.
Types of Lymphatic Vessels
Superficial Lymphatic Vessels: Travel along superficial veins in the skin.
Deep Lymphatic Vessels: Located with deep arteries.
Buttons and Zippers: Describes junctions between lymphatic capillary endothelial cells that contribute to their structural integrity.
Anchoring Filaments: Help to keep lymphatic capillaries open during periods of interstitial edema.
Lymphatic Trunks
Definition: Larger vessels formed from collecting vessels.
Categories: There are nine trunks in total, each named for the regions they drain:
Paired trunks:
Lumbar
Bronchomediastinal
Subclavian
Jugular
A single intestinal trunk.
Drainage Functions
Right Lymphatic Duct: Drains the right upper limb and right head/thorax into the venous circulation at the junction of the right internal jugular and right subclavian veins.
Thoracic Duct: Drains the rest of the body, including the lower limbs, into the venous circulation at the junction of the left internal jugular and left subclavian veins.
Lymph Nodes
Structure and Function: Discrete enclosed collections of diffuse tissue and follicles that filter lymph and activate the immune system. Their organization supports a one-way traffic flow through lymph nodes:
Afferent Vessels: Bring lymph into the node.
Efferent Vessels: Carry lymph away from the node.
Regions in Nodes: Present two main regions:
Cortex (Outer Region): Contains follicles with germinal centers rich in dividing B cells, T cells, and dendritic cells.
Medulla (Inner Region): Contains medullary cords rich in B cells, T cells, and macrophages. The medulla has sinuses for lymph collection, divided by trabeculae.
Lymphoid Cells
Types of Immune Cells:
T Lymphocytes (T Cells): Manage immune responses and can attack/destroy infected cells when activated.
B Lymphocytes (B Cells): Produce plasma cells that mark antigens with antibodies to trigger their destruction.
Macrophages: Activate T cells and perform phagocytosis of foreign substances.
Dendritic Cells: Capture and deliver antigens to lymph nodes to activate T cells.
Types of Lymphoid Tissue
Diffuse Lymphoid Tissue:
Description: Loose arrangement of lymphoid cells and reticular connective tissue (CT) fibers.
Function: A proliferation site for lymphocytes; present in virtually every body organ with larger collections in mucous membranes.
Lymphoid Follicles/Nodules:
Description: Solid, spherical structures made of tightly packed lymphoid cells and reticular fibers, proliferating B cells found in germinal centers.
Location: Found in Peyer's patches and appendix.
Lymphoid Organs
Division: Grouped into two categories:
Primary Lymphoid Organs: Sites of T/B cell maturation (Thymus and Red Bone Marrow).
Thymus: Site of T cell activation, most prominent in childhood, not directly involved in targeting antigens.
Spleen: Largest lymphoid organ, involved in lymphocyte proliferation, immune response, blood cleansing, storage/release of RBC products, and fetal RBC production.
Tonsils: Ring of lymphatic tissue located at the entrance to the pharynx to remove pathogens.
Types of tonsils:
Palatine Tonsils: Largest, most often infected.
Lingual Tonsils: Located at the base of the tongue.
Pharyngeal Tonsils (Adenoids): Located at the posterior wall of the nasopharynx.
Tubal Tonsils: Located near the auditory tubes.
Other Lymphoid Organs
Peyer's Patches: Clusters of lymph nodules in the ileum of the small intestine; serve in destroying bacteria and generating memory lymphocytes.
Appendix: Lymph nodule formation in the lower part of the cecum; also destroys bacteria and generates memory lymphocytes.
MALT (Mucosa-Associated Lymphoid Tissue): A collective term for tonsils, Peyer's patches, appendix, and nodules in walls of bronchi and GI tract.
Lymph Formation and Transport
Origin of Lymph: Lymph originates as an excess fluid in blood capillaries, traveling to lymph capillaries, collecting vessels, trunks, and eventually enters particular ducts (right lymphatic duct or thoracic duct).
Transport Mechanisms: Lymph travels through lymphatic vessels using mechanisms similar to veins:
Muscle contractions
Respiratory pressure changes
Unidirectional valves allow only upward flow to the circulatory system.