Bruises (contusions) occur when small blood vessels under the skin tear or rupture, most often from a twist, bump, or fall. Blood leaks into tissues under the skin and causes a black-and-blue color that often turns colors, including purplish black, reddish blue, or yellowish green, as the bruise heals.
Most bruises are not serious and will go away on their own within 2 to 4 weeks. Sometimes, gravity causes them to spread down the body. A leg bruise usually will take longer to heal than a bruise on the face or arms.
Severe bruising, swelling, and pain that begin within 30 minutes of an injury may mean a more serious injury, such as a severe sprain, dislocation, or fracture.
Sudden unexplained bruising or a sudden increase in the frequency of bruising may be a sign of an abnormal type of bruising that may be caused by medicine side effects, a bleeding or clotting disorder, or a medical condition.
WebMD Medical Reference from Healthwise