Death, or the cessation of life, is the irreversible loss of the essential characteristic of an entity. In the case of living organisms, that characteristic has traditionally been considered cellular metabolism. In humans, however, death has always been associated with loss of characteristics above the cellular level.
The living cell is a thermodynamically unstable system. Without continuous input of energy, a cell will degrade spontaneously into a nonliving collection of molecules. This energy is supplied by synthesis of a high-energy compound called adenosine triphosphate, or ATP. More than 90% of the ATP of most mammal cells is made in the cells by reacting oxygen with chemical compounds derived from glucose or from fatty acids. Arrest of the blood supply to any body area cuts off nutrition and oxygen and interferes with waste product removal. This leads to a breakdown of a cell's internal structure and inactivation of its enzymes.
The loss of oxygen is critical. Many cells have means of maintaining minimal nutrition for some time, but there is no means of storing oxygen.
Changes following a momentary interruption of oxygen supply can be reversed, but irreversible changes result from longer periods of loss. Body cells vary widely in their survival potential. Brain neurons can last only five to eight minutes; cells of the heart, kidney, liver, and various other organs can survive for about an hour; skin and connective tissue cells may survive for several hours. These differences are related primarily to differences in metabolic rate. Death of even large numbers of cells, such as in gangrene of a leg, does not necessarily lead to the death of an organism. Death of the neurons of the brain's cerebral cortex, however, destroys intellectual capacity, and loss of neurons in the midbrain and medulla destroys the brain center that controls breathing. Unless the organism is placed on a mechanical respirator, the remaining cells of the body will die shortly.