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 (not a coma), 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.