It is not easy to give a definition of dying because, with increasing biological knowledge, it seems to be practically impossible to draw a sharp dividing line between living and non-living matter. For mammals and stone this classification is not much of a problem but micro-organisms such as viruses are on the borderline. Yet it might be said that one of the essential characteristics of life is continuous metabolism. In this process there is a continuous breaking down of the organism with the release of energy while at the same time new substances are formed. To balance this process energy needs to be added from the outside. Ultimately this energy, needed by all living creatures on earth, originates from the sun.. (Nearly all animal organisms eat plants or eat animals that feed on plants, and practically all plants derive the energy for photosynthesis – their tissue-building process – from sunlight.) Consequently, there is a continuous process of building up and breaking down that keeps approximately the same pace. During growth, building up has the upper hand. In mammals, including the human race, nutrients and oxygen are absorbed, transported by theand used by every part of the body in order to supply energy and to rebuild .
As an organism ages this very complicated process runs less efficiently and finally life may end because mutually dependant functions one by one cease functioning. When aging is the sole cause of death, meaning that no specific organ or system in the body is upset or ill, one speaks of death through natural causes.
Usually death is closely preceded by a disorder or total arrest of one of the life-supporting functions such as thecirculation or respiration. This causes other body functions to become disturbed or to stop which leads finally to death. It is often difficult to determine the cause of death. Whenever it is important to know the cause, for instance for or legal reasons, a pathologist may perform an autopsy (post mortem). It is equally difficult to determine the exact moment of death. One of the problems is that doctors have to distinguish between clinical and biological death. When the heart stops beating or when respiration ceases someone is pronounced dead. Seconds after the moment that the brain ceases to receive oxygenated the patient becomes comatose. When immediate action is taken recovery is possible. By means of resuscitation it may be possible to restore the blood supply to the brain, after which the patient regains consciousness. However, if the brain has been deprived of oxygenated blood for more than five minutes, brain damage is irrevocable. Although the most important control centre has then been incapacitated and the organism as a whole is not able to function any more, other in the body are able to continue living. At such moments doctors speak of clinical death.
The expression biological death is used when all body functions necessary for life have stopped, such as the blood circulation, respiration, brain and nerve functions and metabolism of the.