Inflammation Mechanisms Overview

Inflammation-1

• Inflammation is a protective mechanism, reacting to irritants in local tissues, blood vessels and lymph vessels
• Inflammation aims to eliminate the offending agent, prevent its spread and initiate tissue repair
• Inflammation involves local tissue and vascular reactions:
  • Local tissue reactions occur at the site with maximum irritant concentration, leading to cell death (necrosis)
  • Chemical mediators (e.g., histamine, serotonin, prostaglandins) are released to trigger other reactions in the inflamed area
  • Local vascular reactions involve a series of steps:
    • Transient vasoconstriction of small arterioles (a protective mechanism lasting seconds to minutes only)
    • Vasodilation of arterioles, venules and capillaries (increasing blood flow to the inflamed area; leading to redness and heat)
    • Slowing of blood flow (stasis) due to increased blood viscosity and sticky endothelium
    • Formation of inflammatory exudates (plasma and cells escape into interstitial spaces, including inflammatory fluid and cellular exudates, such as leucocytes)
    • Dilation of lymphatic vessels to drain fluid exudates

Intended Learning Objectives

• Recall definitions, nomenclature, causes and types of inflammation
• Describe the pathogenesis of acute inflammation
• Describe the mechanism, characteristics and function of inflammatory exudates

Nomenclature of Inflammation

• Inflammation is named by adding the suffix "-itis" to the affected organ's name (e.g., appendicitis, hepatitis, arthritis)

Causes of Inflammation

• Living irritants: bacteria, their toxins, viruses, parasites, and fungi
• Non-living irritants:
  • Physical: excess heat, cold, radiation
  • Chemical: concentrated acids, alkalis, poisons
  • Mechanical: trauma, friction, foreign bodies
  • Immunological: allergic reactions

Types of Inflammation

• Acute inflammation: sudden onset, short duration (days to weeks)
• Chronic inflammation: gradual onset, long duration (months to years)

Acute Inflammation Pathogenesis

• The acute inflammatory reaction consists of local tissue reactions and local vascular reactions.

Inflammatory Fluid Exudate

• Mechanism of formation:

  • Increased capillary permeability (primarily due to histamine)
  • Increased capillary hydrostatic pressure (due to vasodilation and increased blood flow)
  • Increased interstitial fluid osmotic pressure resulting from tissue necrosis

• Characteristics:

  • High protein content (4-8 g/dL) compared to normal interstitial fluid (1 g/dL)
  • High fibrinogen content (leading to turbidity and clotting)
  • High specific gravity (above 1018)
  • High cellular content (polymorphs and macrophages)

• Functions:

  • Dilutes toxins
  • Delivers antibodies to the site of inflammation
  • Delivers nutrients and removes waste products
  • Forms a fibrin network that serves as a mechanical barrier to infection spread and facilitates cell (leucocytes) migration to infected area