Chapter 20 (Part I) - Lymphatic System

Chapter 20: The Lymphatic System

Overview

• The Lymphatic System can be compared to the sewer system of the body.

  • Drains excess fluids and discards unwanted substances.

  • Serves as a navigation route for defense forces (immune cells) aside from the bloodstream.

• Main components:

  • Lymphatic Vessels

  • Lymphatic Organs: Includes lymph nodes, spleen, and thymus.

  • Lymphatic Tissue: Includes tonsils, appendix, and Peyer’s patches.

Functions of the Lymphatic System

• Regulation of Interstitial Fluid Volume

  • Interstitial fluid accumulates due to unequal hydrostatic and osmotic pressure leading to about 2-4 liters lost daily.

  • Lymphatic vessels collect excess interstitial fluid, which becomes lymph.

  • Lymph and interstitial fluid share a similar composition.

  • Lymphatic vessels transport excess fluid back to circulation, thereby preventing hypotonic blood.

• Absorption of Dietary Fats

  • Fats from the small intestine, broken down during digestion, are unable to pass through capillaries.

  • They enter lymphatic vessels called lacteals and eventually get returned to the bloodstream.

• Immune Functions

  • Lymphoid organs filter pathogens and house leukocytes for immune response.

  • Lymphatic vessels aid in mobilizing the immune system through pathways that complement the bloodstream.

  • Lymph nodes serve as sites where white blood cells (especially lymphocytes) reside before activation in response to infections.

Lymphatic Vessels

• Structures are similar to blood vessels serving a network for lymph circulation.

  • Lymph is formed from excess interstitial fluid and has various components including water, proteins, fats, leukocytes, waste, and pathogens.

• Composition of Lymph:

  • Made from water, proteins, fats, leukocytes, waste products, and damaged cells gathered from tissues.

• Vessel Structure:

  • Lined by endothelium (simple squamous).

  • Walls allow flaps for larger fluid and cell entry, therefore functioning as a one-way system toward lymph nodes.

Direction of Lymph Flow

• Afferent vs. Efferent Lymphatic Vessels:

  • Afferent: Carry lymph from tissues to lymph nodes.

  • Efferent: Carry lymph away from lymph nodes to major vein junctions returning lymph to the bloodstream.

Lymph Nodes

• Function as checkpoint stations in the lymphatic system.

  • Key clusters found in axillae, neck, groin, and abdominal cavity.

  • Rich in lymphocytes (B-cells in cortex, T-cells in the middle, and macrophages in the medulla).

  • Serve as filtering points for pathogens and sources of lymphocyte activation.

Main Lymphatic Trunks and Ducts

• Lymph-collecting vessels form larger trunks:

  • Nine Major Trunks include lumbar, jugular, and broncho-mediastinal trunks.

  • Two Primary Ducts: Thoracic Duct (drains left upper and lower body) and Right Lymphatic Duct (drains right upper body).

Lymphatic Vasculature

• Movement of Lymph:

  • Circulation relies on the contraction of surrounding muscles and the pressure of nearby arteries, lacking direct pumping force.

  • Skeletal muscle movements aid lymph propulsion, underscoring the importance of physical activity in maintaining flow.

Normal vs. Obstructed Lymphatic Flow

• Normal Flow: Ensures tissue hydration and nutrient delivery through lymphatic vessels.

• Obstructed Flow: Results in conditions like lymphedema characterized by fluid retention in tissues.

Swollen Lymph Nodes

• Commonly signify infection; lymphocyte proliferation causes node swelling indicating immune activation.

Lymphoid Tissues

• Primarily composed of reticular tissue, lymphocytes (B-cells, T-cells), phagocytes, and reticular cells.

Mucosa-Associated Lymphatic Tissue (MALT)

• Found in mucous membranes of the body, helping trap pathogens.

  • Key Locations include:

    • Gastrointestinal Tract: Peyer’s patches, tonsils, and appendix.

    • Genitourinary Tract: Limited presence in kidney and urethra areas.

    • Respiratory Passages.

  • Tonsils and Peyer's Patches are significant components of MALT and respond to oral and nasal pathogens through lymphocyte production.

The Spleen

• Largest lymphoid organ located in abdominal cavity.

  • Functions in filtering blood, removing old erythrocytes, and pathogen filtering.

  • Contains:

    • Red Pulp: Destroys erythrocytes.

    • White Pulp: Contains leukocytes for pathogen clearance.

The Thymus

• Located in the superior mediastinum, crucial for T-cell maturation.

  • Undergoes positive and negative selection processes ensuring proper immune function.

  • Maximal size around ages 12-14; reduces and replaces with fat over time, though it does not compromise overall immunity significantly.

Conclusion

• The Lymphatic System plays essential roles in fluid regulation, fat absorption, and immune function, essential for general health.