A bruise is a discoloration of the skin caused by a localized injury to the body. A bruise, often known as a black-and-blue mark or a contusion, differs from most other wounds in that there is no breakage of the skin. A typical example of a bruise is a "black eye."
The discoloration, known medically as an ecchymosis, is caused by the escape of blood from broken blood vessels. The blood, which seeps out into the deep layer (dermis) of the skin, appears black or bluish rather than red. This is due to the phenomenon of light scattering (Tyndall effect), in which light passing through a turbid, or cloudy, medium such as the skin has its shorter wavelength components (the blue waves) scattered backward and to the sides more than the longer red wavelengths. As a result, a bluish reflection is seen. If the blood is located higher in the dermis, the overlying tissue is not thick enough to cause a bluish reflection and the bruise may appear purplish or reddish.
Over a period of several weeks the red blood cells degenerate, and their red pigment, hemoglobin, is converted into a yellowish brown pigment, hemosiderin. The presence of these two pigments and their differing depths below the skin surface are responsible for the color variations that later occur in a bruise. While the red blood cells are degenerating, special white blood cells called macrophages enter the area and begin ingesting the cell fragments and hemosiderin. Usually within several weeks, the bruise disappears.