Lymphatic System and Lymphoid Organs and Tissues Notes

Overview of the Lymphatic System

  • The lymphatic system has three main components:
    • Network of lymphatic vessels (lymphatics)
    • Lymph: fluid contained within lymphatic vessels
    • Lymph nodes: structures that cleanse lymph and filter pathogens

20.1 Lymphatic System Functions

  • The lymphatic system returns interstitial fluid and leaked plasma proteins back to blood through:
    • Lymphatic vessels (lymphatics): An elaborate network offering a one-way system that ensures lymph flows only toward the heart.
    • Flow Rate: Circulates approximately 3L of interstitial fluid daily. Once this fluid enters the lymphatic system, it is referred to as lymph.

Lymphatic Vessels Structure

Lymphatic Capillaries
  • Characteristics:
    • Blind-ended vessels located between tissue cells and blood capillaries.
    • More permeable than blood capillaries; can absorb larger molecules such as:
    • Proteins
    • Cell debris
    • Pathogens
    • Cancer cells
  • Can facilitate the spread of pathogens throughout the body.
Permeability of Lymphatic Capillaries
  • Increased permeability due to:
    • Overlapping endothelial cells forming minivalves, allowing one-way flow.
    • Collagen filaments anchor the minivalves to the matrix, which opens them during increases in extracellular fluid (ECF) volume.
Collecting Lymphatic Vessels
  • Structure:
    • Drain lymph capillaries; consist of:
    • Collecting vessels
    • Lymphatic trunks
    • Ducts
    • Thinner walls and more internal valves than veins; anastomose frequently.
  • Major ducts:
    • Right lymphatic duct: Drains the right upper arm and right side of the head and thorax.
    • Thoracic duct: Drains the rest of the body; may start as an enlarged cisterna chyli.
    • Both ducts empty lymph into venous circulation at the junction of internal jugular and subclavian veins.

Lymph Transport Mechanisms

  • The lymphatic system is a low-pressure system similar to the venous system and requires the following mechanisms to facilitate flow:
    1. Skeletal muscle contraction: Effective "milking" action.
    2. Pressure changes in thorax: Help pump lymph as one breathes.
    3. One-way valves: Prevent backflow of lymph.
    4. Pulsations of nearby arteries: Encourage lymph movement.
    5. Contractions of smooth muscle in vessel walls: Aid in propelling the lymph.
  • Physical Activity: Increases lymph flow; immobilization may trap inflammatory materials to promote healing.

Clinical Considerations: Lymphedema

  • Definition: Severe localized edema due to disrupted lymph flow.
  • Causes: Tumors blocking lymph vessels or removal of vessels during surgery.
  • Recovery: Can improve if alternative lymphatic pathways remain open.

20.2 Lymphoid Cells, Tissues, and Organs

Lymphoid Tissue Functions
  • Functions to house and provide proliferation sites for lymphocytes; offers surveillance vantage points for lymphocytes and macrophages.
  • Composed mainly of reticular connective tissue: Macrophages live on reticular fibers, with spaces for lymphocytes.
Lymphoid Cells
  • Comprised of immune system cells and supporting cells:
    • Lymphocytes: Adaptive immune cells that mature into T cells and B cells.
    • Macrophages: Phagocytize foreign substances and activate T cells.
    • Dendritic cells: Capture antigens and activate T cells in lymph nodes.
Types of Lymphoid Tissue
  • Diffuse Lymphoid Tissue: Found in every organ, with dense collections in mucous membranes.
  • Primary Lymphoid Organs: Red bone marrow and thymus, where T and B cells mature.
    • B cells: Mature in bone marrow.
    • T cells: Mature in the thymus.
  • Secondary Lymphoid Organs: Where mature lymphocytes first encounter antigens, e.g., nodes, spleen.

20.3 Lymph Nodes

  • Function: Principal secondary lymphoid organs that filter lymph - cleansing and immune system activation.
  • Nodes contain:
    • Cortex: Follicles with B cells, deep areas with T cells.
    • Medulla: Hosts B cells, T cells, and plasma cells.
  • Structure:
    • Surrounded by fibrous capsule; bean-shaped with compartments created by trabeculae.
Homeostatic Imbalance: Buboes
  • Inflamed swollen lymph nodes overwhelmed by pathogens. Can be pus-filled.
  • Swollen "glands" noted in conditions such as the bubonic plague.

20.4 The Spleen

  • Function: Largest lymphoid organ with multiple roles:
    • Lymphocyte proliferation, immune surveillance, blood cleansing.
    • Stores breakdown products of RBCs and platelets; may generate erythrocytes in fetuses.
  • Structure:
    • Dense with white pulp (lymphocyte concentration) and red pulp (removal of aged blood cells).
Clinical Considerations: Splenectomy
  • Ruptured spleen removal due to trauma or infection; though spleen can self-repair in some cases.

20.5 Mucosa-Associated Lymphoid Tissue (MALT)

  • Definition: Lymphoid tissues in mucous membranes; protects against pathogens.
  • Primary locations include:
    • Tonsils
    • Peyer’s patches (in the small intestine)
    • Appendix (contains many lymphoid follicles).

20.6 Thymus

  • Location: Bilobed organ found in the neck, extending into the mediastinum.
  • Function: Site of T cell maturation, active in childhood, diminishes with age.
  • Unique because:
    • Lacks follicles and B cells, does not fight antigens directly.
    • Contains blood-thymus barrier to protect immature T cells.

Summary of Lymphoid Organs and Tissues

  • Each lymphoid organ and tissue has distinct functions and structures, crucial in maintaining body immunity and homeostasis.
  • Understanding these components helps clarify their roles in both normal physiology and in response to clinical conditions.