"Immediately after an injury occurs, there is a very brief constriction of surrounding blood vessels that lasts but a moment. Then, almost immediately, blood vessels dilate, or open, and blood flow increases.
Injured tissues release a number of chemicals that affect blood vessels. These chemicals include histamine, sereotonin, and a group of chemically related compounds called kinis. All of these substances result in both vasodilation an an increase in the permeability of blood vessels so that components that would normally be retained in the blood are permitted to leak out into the tissue spaces.
In the absence of injury, blood flow through a small vessel is such that the cells tend to pass in large measure within the central two thirds of the lumen with a thin layer of plasma flowing closest to the outer walls. This is called axial flow. After an injury, blood cells no longer pass in a central stream. Microscopic examination of vessels near an injured site shows that white cels begin to accumulate in the vessel near the point of injury and then stick, or marginate, to the wall. This margination of leukocytes continues until the endothelial surface of vessel is covered with adherent white cells. Within minutes these cells begin to pass through the endothelial lining and out of the vessels into the interstitial spaces near the inury. One of the important functions of many white blood cells is phagocytosis - the process of engulfing and destroying bacteria.
Movement of white cells into the area of injury or infection is called diapedesis. The term chemotaxis describes the attraction of leukocytes, especially neutrophils, into the interstitial spaces. The attractive force is produced by the release of kinins and other chemicals by injured tissue. Leukocytosis means an increase in the number leukocytes in the blood. A substance called leukocytosis promoting(LP)factor is also released by injured tissue. It stimulates the release of white cells from storage areas and increaces the number of circulating white blood cells.
The accumulation of dead leukocytes and tissue debris may lead to the formation of pus at the focal point of infection. Should this occur, an abscess, or cavity formed by the disintegration of tissues, may fill with pus and require surgical drainage.
Increased permeability of blood vessels, increased blood flow, and the migration and accumulation of white blood cells all contribute to the formation of inflammatory exudate, which accumulates in teh interstitial spaces in the area of injury. The result is often swelling, or edema, and pain. In addition to white blood cells and tissue debris, inflammatory exudate contains the 'leaked' substances normally retained in the blood but allowed to escape into the interstitial spaces because of increased capillary permeability. One such substance is a soluble protein that is converted into fibrin inthe interstitial spaces. Fibrin formation results in development of a clot, which helps to seal off the infected area and decrease the spread of bacteria or other infectious material.
The cardinal signs of inflammation 'make sense' when examined in the light of our understanding of the process.
1.The heat (calor) is largely the result of increased blood flow to the area of injury.
2.The redness (rubor) is also caused by increased blood flow and pooling of blod folowing injury.
3.Swelling (tumor) results because of edema and accumulation of inflammatory exudate and clot formation in the affected tissue spaces.
4.Pain (dolor)is caused by chemicals such as the kinins (especially bradykinins) and other chemical mediators that are released following tissue injury and cellular death."1