A bruise, medically referred to as a contusion, is caused when tiny blood vessels are damaged or broken as the result of a blow to the skin. The raised area of a bump or bruise results from blood leaking from injured blood vessels into the tissues, such as muscle, as well as from the body's response to the injury. A purplish, flat bruise that occurs when blood leaks out into the top layers of skin is called an ecchymosis. Petechiae, another type of bruise, refers to very small, one to three millimeter accumulations of blood beneath the skin.
Bruises change colors over time in a predictable pattern, so that it is possible to estimate when an injury occurred by the color of the bruise. Initially, a bruise will be reddish, the color of the blood under the skin. Individuals with darker skin tones may have trouble distinguishing a bruise. After one to two days, the red blood cells begin to break down, and the bruise will darken to a blue or purplish color. This fades to green at about day six. Around the eighth or ninth day, the skin over the bruised area will have a brown or yellowish appearance, and the skin will gradually change back to its normal color.
Bruising can occur due to trauma or injury, certain medications that thin the blood (including warfarin or Coumadin® and aspirin), physical abuse, bleeding disorders (such as hemophilia), and other medical conditions, including immune disorders (such as human immunodeficiency virus (HIV)), leukemia (cancer of the blood), drug and alcohol addiction, and aplastic anemia (lack of red blood cell production).
The injury required to produce a bruise varies with age. While it may take quite a bit of force to cause a bruise in a young child, even minor bumps and scrapes may cause extensive bruising or ecchymosis in an elderly person. Blood vessels become more fragile as individuals age and bruising may even occur without prior injury in the elderly.

Related Terms

Bleeding disorders, broken blood vessels, bump, compartment syndrome, contusion, ecchymosis, injured blood vessels, myositis ossificans, pain, petechiae, skin discoloration, swelling.