Lecture Notes: The Lymphatic System

Introduction to the Lymphatic System

• The term "lymph" means "clear water" in Latin.
• Lymph flows through lymphatic vessels and lymph nodes, comprising the lymphatic system.
• Three Major Roles of the Lymphatic System:
  1. Fluid Return: Returns fluid from tissues back to the heart.
  2. Transport of Large Molecules: Assists large molecules like hormones and lipids in entering the blood.
  3. Immune Surveillance: Helps prevent infections.

Origin of Lymph

• Arteries have high pressure to reach body extremities.
• Blood flows from arteries → arterioles → capillaries (one cell thick, slightly porous).
• Fluid Movement:
  • 20 liters of fluid seep out daily from capillaries.
  • 17 liters reabsorbed into capillaries, 3 liters remain, needing return to the blood.

Lymphatic Vessels

• Lymphatics: Collect excess interstitial fluid and return it to the blood.
• Once in vessels, interstitial fluid is called lymph.
• System Characteristics:
  • Not a closed loop; lymph enters lymphatic capillaries and is dumped into veins.
  • Capillaries have one-way minivalves formed by overlapping endothelial cells.
  • Movement aided by:
    • Smooth muscle reacting to nearby arterial pulses.
    • Skeletal muscle contractions.
    • Valves preventing backward flow.

Lymphatic Trunks and Ducts

• Trunks Named After Regions:
  • Lumbar, bronchomediastinal, subclavian, jugular, intestinal.
• Major Ducts:
  • Right Lymphatic Duct: Drains right arm, head, and chest.
  • Thoracic Duct: Drains lymph from the rest of the body.

Transport of Large Molecules and Nutrients

• Hormones and Lipids: Too large for capillaries, enter bloodstream via lymphatics.
• Chylomicrons: Fatty acids packaged by intestines enter lacteals (special lymph vessels), look milky, and reach venous blood via thoracic duct.

Immune Function

• Lymphoid Organs: Remove foreign material from lymph.
  • Diffuse Lymphoid Tissue: Loose arrangement in gastrointestinal and respiratory tracts.
  • Lymph Nodes: Packed with lymphoid cells and protein.
    • Found in neck, armpit, groin, and intestinal wall (Peyer's patches).

Pathogen Detection and Immune Response

• Lymph Nodes Role:
  • Dendritic cells sample lymph for pathogens, present antigens to B cells.
  • B Cells transform into plasma cells, produce antibodies.
  • T Cells patrol for tagged pathogens or abnormal cells.

Additional Lymphoid Organs

• Spleen:
  • Filters antibody-coated bacteria (white pulp).
  • Destroys old blood cells (red pulp).
  • Stores emergency red blood cells and platelets.
• Thymus:
  • Active in childhood, atrophies post-puberty.
  • Develops T cells, targets self-reactive cells.
• Tonsils:
  • Includes adenoid, tubal, palatine, and lingual tonsils.
  • Traps pathogens from food and air.

Summary

• The lymphatic system is a one-way network facilitating nutrient transport, waste removal, and key immune functions.

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