Acute Inflammation - Introduction

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Introduction

  • Acute inflammation is characterised sudden onset and may last for a few hours to a few days.
  • Vascular, humoral and cellular alterations cause the 5 cardinal signs as a result of exposure of tissues to various causes.
  • Acute inflammation can:
    1. Be fatal
    2. Resolve by regeneration in association with the host defence mechanisms.
      • May be assisted by therapeutic measures.
    3. Undergo repair by fibrosis.
    4. Become chronic.
      • First goes through a subacute phase.
      • Is dependent upon the persistence of the agent and the amount of damage caused.

Sequence of Events

  • The following sequence of events is provoked by the presence of the irritant.
  1. Momentary vasoconstriction
    • Following contact with the irritant, there is momentary vasoconstriction of the blood vessels in the affected area.
    • This is reversed within minutes.
  2. Dilation of the blood vessels
    • Initially, dilation of the capillaries is caused by the release of chemical mediators.
    • Arterioles then dilate under the influence of a local axon reflex.
    • This gives rise to an initial acceleration of the blood flow to the area.
      • This later gives way to a slowing of blood flow, caused by alterations in vascular endothelial permeability and the filling of previously closed capillaries.
  3. Exudation of fluid
    • Follwing the slowing of blood flow and altered capillary permeability, a protein-rich fluid is exudated.
  4. Margination of leukocytes
    • Circulating white blood cells are attracted to the altered endothelial surfaces.
  5. Emigration of leukocytes
    • Leukocytes migrate through the altered endothelium to reach the injured area.
    • This is an active process - chemotaxis.
      • The cells are attracted to the endothelium by release of proteins, and further into the tissues by factors released from cells in the damaged area.
      • The emigrated leukocytes and components of the fluid exudate are also chemotactic.
      • More cells and fluid are attracted to the area.
  6. Emigration of red blood cells
    • Erythrocytes migrate through the gaps in the altered endothelium to the damaged tissue.
  7. Induction of an increase in temperature
    • This may occur either locally or systemically.
    • A systemic rise in temperature is known as pyrexia.
      • Occurs in generalise acute inflammation.
      • Pyrogens act on the temperature control centres in the hypothalamus, and are released from:
        • Neutrophils, eosinophils and macrophages
          • Particulary neutrophils when they begin to phagocytose.
        • The cellular coat of gram-negative organisms.
        • Necrosis of damaged tissue cells.
        • Antigen-antibody complexes.
        • Tumours.
          • Particularly those which have metastasised
          • It may be difficult to separate this from the pyrexia caused by the central necrosis in such tumours.

Cells