Lymphatic System

1. Three Main Functions of the Lymphatic System

  • Fluid Balance: The lymphatic system returns excess interstitial fluid (about 3 liters daily) to the bloodstream, preventing tissue swelling (edema).

  • Immune Function: Lymphoid organs produce and house lymphocytes (T cells and B cells), which help defend against infections and foreign substances.

  • Lipid Absorption: The lymphatic system, specifically lacteals in the small intestine, absorbs dietary lipids and fat-soluble vitamins (A, D, E, and K), transporting them to the bloodstream.

2. Role of the Lymphatic System in Fluid Movement vs. the Cardiovascular System

  • The cardiovascular system is a closed circuit where the heart pumps blood through arteries, capillaries, and veins.

  • The lymphatic system is an open-ended network that collects excess interstitial fluid and returns it to the circulatory system.

  • Unlike the cardiovascular system, the lymphatic system has no central pump; instead, fluid moves via skeletal muscle contraction, respiratory pressure changes, and one-way valves.

3. Forces Moving Fluid into Lymphatic Capillaries and Through Lymphatic Vessels

  • Forces Moving Fluid into Lymphatic Capillaries:

    • Hydrostatic Pressure: Interstitial fluid pressure forces excess fluid into lymphatic capillaries.

    • Overlapping Endothelial Cells: Lymphatic capillaries have flap-like openings that allow fluid to enter but not exit.

  • Forces Moving Lymph Through Vessels:

    • Skeletal Muscle Pump: Contraction of muscles squeezes lymph vessels.

    • Respiratory Pump: Changes in thoracic pressure during breathing help move lymph.

    • Smooth Muscle Contraction: Larger lymphatic vessels contain smooth muscle that contracts rhythmically.

4. Flow of Lymph from Capillary to Brachiocephalic Vein

  • Lymphatic Capillaries → Larger Lymphatic Vessels → Lymph Nodes (filtration) → Lymphatic Trunks → Lymphatic Ducts → Subclavian Veins → Brachiocephalic Veins

  • Lymphatic Trunks & Regions Drained:

    • Jugular Trunks (head & neck)

    • Subclavian Trunks (upper limbs, chest wall)

    • Bronchomediastinal Trunks (thoracic cavity)

    • Intestinal Trunk (digestive organs)

    • Lumbar Trunks (lower limbs, pelvic organs)

  • Lymphatic Ducts:

    • Right Lymphatic Duct: Drains the right upper body (right arm, right head & neck, right thorax) into the right subclavian vein.

    • Thoracic Duct: Drains the rest of the body into the left subclavian vein.

5. Primary Lymphatic Tissues & Their Roles

  • Bone Marrow: Produces lymphocytes; site of B cell maturation.

  • Thymus: Site of T cell maturation; shrinks after puberty.

6. Secondary Lymphatic Tissues & Their Roles

  • Lymph Nodes: Filter lymph, trap pathogens, and activate immune cells.

  • Spleen: Filters blood, removes old red blood cells, and houses immune cells.

  • Mucosa-Associated Lymphoid Tissue (MALT): Found in mucosal linings (e.g., tonsils, Peyer’s patches) to trap pathogens.

  • Tonsils: Trap inhaled/ingested pathogens.

  • Peyer’s Patches (small intestine): Monitor gut bacteria.

  • Appendix: Contains lymphoid tissue that may help regulate gut bacteria.