nfortunately, inflammation is uncomfortable. And inflammation can sometimes be triggered inappropriately (e.g., by allergies or autoimmune diseases). Many common medical treatments (e.g., aspirin and other "anti-inflammatory agents") are intended to relieve the discomfort and/or reduce the attendant tissue disturbance that inflammation may cause.
Any tissue or organ can become inflamed. Inflammation of a particular body part is named according to the site, with the "-itis" suffix appended (e.g., tonsillitis, appendicitis, dermatitis, arthritis, sinusitis, etc.). Thus many disease names are really just words that identify sites of inflammation. (Examples)
Tissue repair. Following inflammation, injured tissue is usually replaced by new cells and extracellular materials, with undamaged surrounding cells proliferating and migrating to fill the void. Although some tissues, especially surface epithelium, can grow back quite efficiently, complex organization is seldom matched in the regenerated site. Gaps are quickly filled by collagen fibers produced by proliferating fibroblasts. A mass of collagen which replaces tissue that has been destroyed is known as a scar. Scars in the skin appear white because collagen is colorless, and healing often creates a more compact arrangement of collagen with fewer capillaries than the surrounding tissue. Scars in other organs also exist as firm masses of collagen in which normal organ function cannot occur. For example, cirrhosis of the liver represents extensive replacement by collagen of damaged liver cells.
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CARDINAL SIGNS OF INFLAMMATION
The four cardinal signs of inflammation --the four "ORs" -- are:
(A fifth sign, loss of function, is sometimes included in this list.)
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Where do these signs come from? What is happening to your body during inflammation? As you understand inflammation, you should be able to answer the following specific questions.
- Why does tissue become red and warm around a site of injury?
- Why does inflamed tissue swell?
- What is pus, and how does it form?
- How does healing occur?
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PHYSIOLOGY OF INFLAMMATION
The four cardinal signs of inflammation are readily explained by the behavior of the underlying cells and tissues. The inflammatory response consists of several physiological processes (illustration from WebPath), all of which are triggered by the release of pharmacologically active substances such as histamine and heparin. These triggers of acute inflammation are released by mast cells, sensitive cells which are scattered throughout ordinary connective tissues and which react to tissue damage or other disturbance.
Chronic inflammation is maintained by more complex interactions among several cell types; diagram from WebPath.
The basic components of the inflammatory response are:
See illustration from WebPath.
Vasodilation (increased vascular perfusion).
- Relaxation of the smooth muscle which surrounds terminal arterioles results in increased blood flow into the connective tissue capillary bed.
- Increased tissue perfusion in turn causes redness (rubor), as more red blood cells pass through the tissue, and warmth (calor), as blood carries body heat from the body's core to cooler peripheral tissues.
Increased vascular permeability.
- The endothelial lining of capillaries (endo = inside; the endothelium is the layer of thin, flat cells which line blood vessels) becomes more leaky, allowing more fluid (blood plasma) to exude into the connective tissue spaces.
- There is normally a balance between fluid leaving vascular spaces and fluid re-entering the system. Inflammation shifts this balance, causing accumulation of interstitial fluid.
- The fluid build-up which follows this permeability change is called edema and is visible as puffiness or swelling (tumor).
Note that edema can take a variety of forms. The following links will take you to gross images at WebPath.
Emigration of leukocytes.
- Vasodilation and increased vascular perfusion are designed to prepare the way for the inflammatory infiltrate to enter the inflamed tissue.
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