Ch21_lymphatic_system

Chapter 21: The Lymphatic System Overview

• Overview of lymphatic system functions, importance, and impact of dysfunction.

• Key components: Capillary dynamics, lymphatic circulation, lymphoid cells and tissues, lymph nodes, and other lymphoid organs.

The Lymphatic System

• Components:

  • Two semi-independent parts:

    • Lymphoid tissues and organs

      • House macrophages and lymphocytes.

    • Lymphatic Circulation

      • Returns fluids that have leaked from the cardiovascular (CV) system back to the blood.

Capillary Dynamics

• Fluid movement in/out of capillaries:

  • Occurs through intercellular clefts.

  • Continuous mixing of plasma and interstitial fluid compartments essential for maintaining interstitial environment.

  • Fluid Movement Statistics:

    • 20L/day filtered out at arterial ends.

    • 17L/day reabsorbed into capillaries at venous end.

    • Approximately 3L/day of fluid, along with leaked proteins, is removed by the lymphatic system.

Pressures Affecting Fluid Movement

• Hydrostatic Pressure: The force exerted by a fluid against the walls of its container.

  • Determines the amount of fluid flowing in/out of capillaries.

• Colloid Osmotic Pressure (COP): Refers to the pressure exerted by proteins in the blood plasma.

  • Key Points:

    • At arterial end: Hydrostatic pressure > Osmotic pressure, resulting in fluid being forced out.

    • At venous end: Osmotic pressure > Hydrostatic pressure, resulting in fluid being drawn back in.

Lymphatic Vessels

• Structure:

  • Lymphatic capillaries are interwoven with blood capillaries.

  • Pick up fluid leaked from blood vessels; termed lymph once inside the lymphatic capillaries.

• Extremely permeable, allowing large proteins, viruses, and bacteria entry due to:

  • Overlapping endothelial cells (one-way flaps).

  • Anchoring collagen filaments.

Lymphatic Circulation

• Flow pathway:

  • Lymphatic capillaries → Lymphatic collecting vessels → Lymphatic trunks → Lymphatic ducts → Venous system.

• Major trunks and ducts include:

  • Right lymphatic duct, thoracic duct, and various trunks associated with the jugular and subclavian veins.

Transport of Lymph

• Mechanisms maintaining lymph flow towards the cardiovascular system:

  • Skeletal muscle pump: Movement during muscle contraction.

  • Respiratory pump: Changes in thoracic pressure during breathing.

  • One-way valves: Prevent backflow.

  • Pulsation of arteries: Assists lymph movement.

  • Smooth muscle contraction in lymphatic trunks & ducts.

Common Lymphatic Issues

• Lymphedema: Swelling due to lymph accumulation.

  • Primary: Caused by vessel malformation.

  • Secondary: Due to underlying conditions, e.g. post-mastectomy swelling.

• Elephantiasis: Caused by blockage of lymphatic vessels by filarial worms transmitted by mosquitoes.

  • Affects >120 million globally.

  • Treatment available.

Lymphoid Cells

• Types and functions:

  • Lymphocytes:

    • T cells (T lymphocytes).

    • B cells (B lymphocytes).

  • Plasma cells: Produce antibodies.

  • Macrophages: Engulf foreign substances.

  • Dendritic cells: Capture and present antigens to T cells.

  • Reticular cells: Produce reticular fibers, supporting lymphocyte architecture.

Lymphoid Tissue

• Composed of reticular connective tissue:

  • Found in small amounts in nearly every organ and large amounts in lymphoid organs.

  • Functions include housing lymphocytes and providing optimal surveillance locations for immune response.

Primary and Secondary Lymphatic Structures

• Primary structures: Sites of lymphocyte formation and maturation:

  • Red Bone Marrow: Produces B and T lymphocytes.

  • Thymus: Maturation of T lymphocytes; most active in early life and atrophies later.

• Secondary structures: Include lymph nodes, spleen, and MALT, where immune responses are initiated.

Lymph Nodes

• Clustered along lymphatic vessels:

  • Function to filter lymph through macrophages and activate immune responses via lymphocytes.

• Structure:

  • More afferent than efferent vessels, slowing down lymph flow.

  • Cortex contains lymphatic nodules with T & B cells, dendritic cells, and macrophages.

  • Medulla contains B & T cells; macrophages are located within sinuses spanned by reticular fibers.

Swollen Lymph Nodes (Buboes)

• Caused by trapped bacteria leading to swelling and tenderness.

• Historically significant as a symptom of the Bubonic Plague.

Spleen

• Divided into red and white pulp:

  • Red pulp: Removes old blood cells, stores breakdown products, platelets, and monocytes.

  • White pulp: Site for lymphocyte proliferation and immune response surveillance.

Mucosa-Associated Lymphoid Tissue (MALT)

• Composed of sets of lymphoid tissue within mucous membranes of various tracts:

  • Respiratory, urogenital organs, digestive tract.

  • Acts as a first line of defense against entering pathogens.

  • Key structures: Tonsils, Peyer's patches, and appendix.

MALT: Tonsils

• Form a ring around the entrance to the pharynx, capturing bacteria through tonsillar crypts.

MALT: Peyer’s Patches

• Located in the small intestine, function similarly to tonsils by sampling intestinal pathogens.

MALT: Appendix

• Offshoot of the large intestine with lymphoid follicles for pathogen destruction and as a reservoir for symbiotic bacteria.

Review Questions

1. The primary mechanism driving filtration out of capillary beds is hydrostatic pressure.

2. Capillary colloid osmotic pressure created by nondiffusible plasma proteins; draw fluids into the capillary.

3. Lymphatic vessels collect excess fluid from interstitial spaces.

4. All substances such as bacteria, large proteins, and interstitial fluid can enter lymphatic capillaries.

5. The red pulp of the spleen is concerned with blood filtering, while the white pulp is concerned with immunity.