Death is the end of life. Every living thing eventually dies, but human beings are probably the only creatures that can imagine their own deaths. Most people fear death and try to avoid thinking about it. But the awareness of death has been one of the chief forces in the development of civilization. Throughout history, people have continually sought new medical knowledge with which to delay death. Philosophers and religious leaders have tried to understand the meaning of death. Some scholars believe that much human progress results from peoples efforts to overcome death and gain immortality through lasting achievements.

Medical aspects of death. Scientists recognize three types of death that occur during the life of all organisms except those consisting of only one cell. These types are necrobiosis, necrosis, and somatic death.

Necrobiosis is the continual death and replacement of individual cells through life. Except for nerve cells, all the cells of an organism are constantly being replaced. For example, new skin cells form under the surface as the old ones die and flake off.

Necrosis is the death of tissues or even entire organs. During a heart attack, for example, a blood clot cuts off the circulation of the blood to part of the heart. The affected part dies, but the organism continues to live unless the damage has been severe.

Somatic death is the end of all life processes in an organism. A person whose heart and lungs stop working may be considered clinically dead, but somatic death may not yet have occurred. The individual cells of the body continue to live for several minutes. The person may be revived if the heart and lungs start working again and give the cells the oxygen they need. After about three minutes, the brain cells--which are most sensitive to a lack of oxygen--begin to die. The person is soon dead beyond any possibility of revival. Gradually, other cells of the body also die. The last ones to perish are the bone, hair, and skin cells, which may continue to grow for several hours.

Many changes take place after death. The temperature of the body slowly drops to that of its surroundings. The muscles develop a stiffening called rigor mortis. The blood, which no longer circulates, settles and produces reddish-purple discolorations in the lowest areas of the body. Eventually, bacteria and other tiny organisms grow on the corpse and cause it to decay.