The medical profession divides death into three periods.

1. The dying period, also known as dying state, is the last stage of life activities and the beginning stage of the death process. Also known as struggle period, near-death struggle period. The last stage of human struggle before death. At this stage, due to terminal illness or accidents, the physiological functions of the main organs of the human body tend to fail, and the functions of the nerve center above the brain stem are in a state of inhibition or loss, and death is about to occur.

2. In the clinical death stage, also known as somatic death stage or individual death stage, the inhibition process of the central nervous system spreads from the cerebral cortex to the subcortical part, and the medulla oblongata is also in a state of deep inhibition. The clinical manifestations are cardiac arrest, disappearance of various reflexes and dilated pupils, but various tissues and cells still have short-term weak metabolic activities.

3. The biological death period is the last stage of the death process. During this period, the metabolism of the whole central nervous system and various organs of the body stopped one after another and irreversible changes occurred. After the clinical death period, total body resuscitation is impossible.