The skin serves to protect the body from the external environment. Since this layer is relatively thin, physical trauma can penetrate the skin and cause wounds. Blunt injuries that do not puncture the skin are also possible, yet they can cause serious physical damage to the internal organs. The body's typical response to injury is inflammation, with the classic symptoms of heat, redness, pain, and edema in the affected area. Other causes of injury include chemical, radiation, electrical, thermal, infectious, and metabolic factors.