Lymphatic System Overview and Functions

Lymphatic System: Overview

  • Components:
    • Two semi-independent parts:
    • A network of lymphatic vessels.
    • Lymphoid tissues and organs scattered throughout the body.
  • Functions:
    • Returns interstitial fluid and leaked plasma proteins back to the blood.
    • Lymph: Interstitial fluid once it has entered lymphatic vessels.

Lymphatic Vessels

  • Flow Direction:
    • One-way system; lymph flows toward the heart.
  • Types of Vessels:
    • Microscopic, permeable, blind-ended capillaries.
    • Lymphatic collecting vessels.
    • Larger trunks and ducts.

Lymphatic Capillaries

  • Structure:
    • Very permeable, similar to blood capillaries but with modifications.
    • Loosely joined endothelial minivalves.
    • Can withstand interstitial pressure and remain open.
  • Function of Minivalves:
    • Allow interstitial fluid to enter lymph capillaries (one-way gates).
    • Prevent lymph from escaping capillaries.
  • Role During Inflammation:
    • Absorb cell debris, pathogens, and cancer cells.
    • Cell cleansing occurs in lymph nodes.
  • Lacteals:
    • Specialized lymph capillaries in intestinal mucosa; absorb digested fat and deliver chyle to blood.

Lymphatic Collecting Vessels

  • Similarities to Veins:
    • Have three tunics (layers) like veins.
    • Thinner walls, more internal valves.
  • Anastomosis:
    • Comprise a network that frequently recombines.
  • Traveling Pathways:
    • Superficial vessels travel with veins; deep vessels travel with arteries.
    • Nutrients supplied via the vasa vasorum.

Lymphatic Trunks and Ducts

  • Formation of Trunks:
    • Formed by the convergence of the largest collecting ducts.
  • Major Trunks Include:
    • Paired lumbar, bronchomediastinal, subclavian, jugular trunks, and a single intestinal trunk.
  • Lymphatic Duct Functions:
    • Right Lymphatic Duct: Drains the right upper arm and right side of head/thorax.
    • Thoracic Duct: Arises from cisterna chyli, drains the rest of the body.

Lymph Transport

  • Pumping Mechanism:
    • Lacks a dedicated pumping organ; relies on:
    • Pulsations of nearby arteries.
    • Smooth muscle contractions in lymphatic vessel walls.

Lymphoid Cells

  • Main Cells Involved:
    • Lymphocytes:
    • Two main types: T cells and B cells.
  • Functions:
    • Protect against antigens (foreign substances):
    • Bacteria, viruses, mismatched RBCs, cancer cells.

T Cells and B Cells

  • T Cells:
    • Manage immune response and kill foreign cells.
  • B Cells:
    • Produce plasma cells that secrete antibodies to immobilize antigens.

Other Lymphoid Cells

  • Macrophages:
    • Phagocytize foreign substances and activate T cells.
  • Dendritic Cells:
    • Similar functions to macrophages; spiny appearance.
  • Reticular Cells:
    • Produce stroma (support network) for lymphoid organs.

Lymphoid Tissue

  • Types of Tissue:
    • Diffuse Lymphatic Tissue: Scattered reticular elements in organs.
    • Lymphatic Follicles (Nodules): Solid, spherical bodies, densely packed reticular elements.
    • Comprise germinal centers with B cells.

Lymph Nodes

  • General Characteristics:
    • Principal lymphoid organs embedded in connective tissue.
    • Clusters occur near the body surface (inguinal, axillary, cervical).
  • Functions:
    • Filtration of lymph (removal of microorganisms and debris).
    • Activation of immune system by monitoring for antigens.

Structure of Lymph Nodes

  • Anatomy:
    • Bean-shaped, surrounded by fibrous capsule.
    • Divided into compartments by trabeculae.
  • Distinct Regions:
    • Cortex: Contains follicles (B cells) and dendritic cells.
    • Medulla: Contains B cells, T cells, plasma cells, and lymph sinuses.

Circulation in Lymph Nodes

  • Pathway:
    • Enter via afferent lymphatic vessels, travel through sinuses, exit via efferent vessels.
    • Fewer efferent vessels cause lymph stasis, allowing time for immune function.

Major Lymphoid Organs

  • Tonsils:
    • Form a ring around the pharynx; trap and destroy bacteria (types: palatine, lingual, pharyngeal, tubal).
  • Thymus:
    • Secretes hormones for T lymphocyte maturation; most active during youth.
  • Spleen:
    • Largest lymphoid organ, serves in lymphocyte proliferation, blood cleansing, and immune response.

Structure and Functions of Spleen

  • Composition:
    • Surrounded by fibrous capsule; contains white pulp (immune function) and red pulp (disposal of worn-out RBCs).
  • Other Functions:
    • Stores breakdown products of RBCs, iron salvage, fetal erythrocyte production, and platelet storage.

Thymus Characteristics

  • Size Variation with Age:
    • Increases in size and activity in children, atrophies in adolescence.
  • Internal Anatomy:
    • Cortex with densely packed lymphocytes, medulla with fewer lymphocytes and thymic corpuscles.

Aggregates of Lymphoid Follicles

  • Peyer’s Patches:
    • Clusters of lymphoid tissue in the intestine; prevent bacteria from breaching the intestinal wall, generate memory lymphocytes.
  • MALT (Mucosa-associated Lymphatic Tissue):
    • Includes Peyer’s patches, tonsils, and appendix; protect digestive and respiratory systems from foreign matter.