Lymphatic System

Lymphatic System

Overview of the Lymphatic System

  • 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:
    1. Cortex (Outer Region): Contains follicles with germinal centers rich in dividing B cells, T cells, and dendritic cells.
    2. 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

  1. 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.
  2. 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:
    1. Primary Lymphoid Organs: Sites of T/B cell maturation (Thymus and Red Bone Marrow).
    2. Secondary Lymphoid Organs: Sites for mature lymphocyte activation (Lymph Nodes, Spleen, MALT).

Specific Organs

  • 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:
    1. Palatine Tonsils: Largest, most often infected.
    2. Lingual Tonsils: Located at the base of the tongue.
    3. Pharyngeal Tonsils (Adenoids): Located at the posterior wall of the nasopharynx.
    4. 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.