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The difference between clinical death and biological death is small: just a few crucial minutes, in fact. Clinical death is the point at which a person's heart stops beating. At that time, breathing and blood circulation stop. Biological death occurs some four to six minutes later, when the brain cells die from lack of oxygen.

When brain death occurs, all neurological functions irreversibly cease. The brain simply cannot survive for very long without oxygen. When oxygen is withheld beyond that six-minute threshold, brain death is the result. The reason the brain-death clock starts ticking down once the heart stops is because cardiac activity is the whole ballgame for our bodies, and brains. When the heart stops, oxygen cannot be transported to the brain and the brain dies from lack of it. The legal time of death is noted at the moment in which a doctor determines that a patient's entire brain has ceased to function.

It's still possible to resuscitate a person who has undergone clinical death and keep them alive by artificial means of life support, such as a respirator. However, when biological death has been established and the brain has been too long deprived of oxygen, resuscitation is impossible. Often, at that point, another clock starts ticking away in the background: the clock for organ donation.