Category:Acute Inflammation
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:
- Be fatal
- Resolve by regeneration in association with the host defence mechanisms.
- May be assisted by therapeutic measures.
- Undergo repair by fibrosis.
- 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.
- Momentary vasoconstriction
- Following contact with the irritant, there is momentary vasoconstriction of the blood vessels in the affected area.
- This is reversed within minutes.
- 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.
- Exudation of fluid
- Follwing the slowing of blood flow and altered capillary permeability, a protein-rich fluid is exudated.
- Margination of leukocytes
- Circulating white blood cells are attracted to the altered endothelial surfaces.
- 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.
- Emigration of red blood cells
- Erythrocytes migrate through the gaps in the altered endothelium to the damaged tissue.
- 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.
- Neutrophils, eosinophils and macrophages
Fluids
Serous
- Serous fluids are formed due to mild vascular injury in an organ or in vessels underlying a
surface.
- The fluid is clear to cloudy fluid.
- Little protein is present.
- Examples:
- Vesicles of the skin.
- Many inflammations in joints.
Catarrhal
- Catarrhal inflammation is a mild form of inflammation.
- Occurs on mucous membranes where there are many mucus cells.
- The consistency of catarrhal exudate varies from water to gelatinous.
- Colour varies from cloudy to pinkish.
- This form of exudation is essentially a shedding of epithelium containing many mucus cells, neutrophils, some RBCs, and flecks of fibrin.
- Common in mild forms of rhinitis, tracheitis, bronchitis, gastritis and enteritis.
Fibrinous
- Fibrinous exudation occurs in more severe endothelial injury.
- Injury results in the escape of fibrinogen, which is converted to fibrin.
- The fibrin formed appears as a yellowish coagulation on the surface of or within a tissue.
- Common in the lungs and on serous surfaces.
- In hollow organs the fluid may coagulate to form casts of the lumen.
- Fibrin will peel off from the underlying tissue without causing damage to it.
Diptheritic
- Diphtheritic exudate a more severe form of fibrinous exudate in which there is considerable necrosis of the underlying tissues.
- Diptheritic exudate is firmly adherent to the underlying tissue.
- Attempts at removal cause tearing of this tissue>
- Commonly seen with internal surface fungal infections.
- E.g. in the nose of the dog and the guttural pouch of the horse.
- Fungal toxins penetrate the underlying tissue causing coagulation necrosis.
Haemorrhagic
- This is a severe acute to peracute inflammation in which haemorrhage is the main component.
- Seen in the lymph nodes, lungs and intestine in severe inflammation.
Purulent
- In purulent inflammation, pus is the predominant feature.
- Pus is an admixture of dead and dying neutrophils with necrotic cells and a pyogenic agent.
- Proteolytic enzymes released by the dying neutrophils lyse tissue cells to produce a fluid.
- Pus is an admixture of dead and dying neutrophils with necrotic cells and a pyogenic agent.
- Colour varies depending upon the agent.
- May be white, yellow, green or brown.
- An abscess is a circumscribed sphere of pus surrounded by a pyogenic membrane.
- The pyogenic membrane is composed of capillaries bringing neutrophils into the sphere.
- This rapidly becomes enveloped by a fibrous tissue capsule.
- A local connective tissue response attempting to wall off the purulent irritant from nearby normal tissue.
Functions of Exudate
- Dilutes the toxic agent, reducing its effect upon the tissue cells.
- The protein components may contain antibodies which attack or coat (opsonise) the irritant.
- This makes the irritant more digestible to neutrophils and macrophages.
- Exudates may contain fibrin.
- This is very sticky - immobilises the irritant.
- Provides a framework over which the leukocytes crawl to reach the irritant.
- Fibrin is also chemotactic to neutrophils, bringing more of these cells into the injured area.
- Exudate on a suface will wash away the irritant.
- E.g the skin and alimentary track.
- Brings the irritant to the lymph nodes, via the lymphatics, for further processing or antigen presentation.
- Preferably within white blood cells such as macrophages and neutrophils.
- Local lymph nodes may also be inflamed following inflammation of a tissue which drains into them.
Sequelae to Exudation
- Catarrhal and serous exudationresolve when the irritant is overcome.
- Alternatively, they may progress to a more serious reaction.
- Fibrinous exudates may also resolve if the fibrin is digested by macrophages.
- In the pleural and peritoneal cavities, the fibrin may become rganised into fibrous tissue, producing adhesions between the visceral surface.
- Diphtheritic inflammation will only repair by scar formation.
- If haemorrhagic lesions are widespread, they most commonly associated with acute deaths.
- For example in acute viral, bacterial or toxic diseases.
- If the exudation is strictly localised, for example in brusing, then repair may occur.
- The resolution of abscesses depends upon their location.
- If the abscess is near to a surface it will rupture onto it.
- This is beneficial in the skin where it discharges to the exterior, hopefully getting rid of the pyogenic organism.
- If the abscess is in deeper tissues, there is extensive fibrous capsule formation.
- The fluid becomes inspissated due to withdrawal of water content.
- Macrophages digest the necrotic remains.
- Fibrous tissue organises the interior.
- The very end result is a fibrous scar.
Cells
- The classical cells of acute inflammation are neutrophils, eosinophils, macrophages, mast cells and basophils.
- Macrophages are a common feature of acute and chronic inflammation.
Pages in category "Acute Inflammation"
The following 8 pages are in this category, out of 8 total.