Death occurs in two ways: 1) from cessation of circulatory (heart-lung) function; or 2) from the cessation of brain function (brain death). Brain death occurs when a person has an irreversible, catastrophic brain injury, which causes all brain activity to permanently stop. In such cases, the heart and lungs can continue to function if artificial-support machines are used. However, these functions will also cease when the machines are discontinued.
Brain death is an accepted medical, ethical and legal form of death. The standards for determining that someone is brain dead are strict. Only about one out of a hundred individuals in the United States will die through the process of brain death and have the potential for organ donation. Tissue donation can occur after either type of death, but organ donation can only occur after brain death.