There are differing definitions of death, but it is clear that if a person is unconscious, their heart has stopped and their brain ceased to function, that person has died. They are beyond resuscitation, and their body, now just an object that resembles a person, meets none of the criteria for life. The body is no longer a series of self-sustaining chemical reactions; through the same miraculous processes that allowed it to live, it will begin an inexorable return to the ecosystem from whence it came.
Following death, pallor mortis is the first stage in the process of post-mortem disembodiment. As the heart has stopped, blood no longer circulates, and the red blood cells (being denser than the plasma) drift downwards, away from the upper surface of the body. This is the cause of the paleness of dead skin, and happens within the first half-hour of death.
With the body no longer generating heat, algor mortis is the next noticeable stage. The human cadaver, with no internal heat source, will lose about 1°C of heat every hour following death (until the body reaches ambient temperature). This is one of the ways that forensics officers can use to determine time of death, though in this case, the body must be found within 24 hours.
Rigor mortis is the most well-known stage of death, begins after about 4 hours, peaks at 12 hours, and subsides gradually over the next two to three days. It is difficult to explain why dead muscles can stiffen without describing how muscles work when they are alive. One of the ways that energy is stored in living tissue is stockpiling the high-energy molecule ‘ATP’. ATP is used by muscle cells to pump calcium into tiny compartments, where it in turn is stockpiled, ready for massive energetic release during muscle contraction. When a person dies, their muscle cells naturally contain plenty of ATP, which continues to drive the calcium pumps, despite the death of their owner. Once it runs out, the calcium leaks from its compartment, and ‘glues’ the muscle fibres together.
Livor mortis could easily be described as the continuation and conclusion of pallor mortis. The blood continues to drift downwards by the action of gravity, but as the body cools and blood vessels begin to leak, the blood can drift further and deeper, and begins to pool near the lowest parts of the body. Interestingly, where there is pressure acting on the tissue (such as where the person’s hand is draped, or where the weight of their body is compressing the tissue in direct contact with the ground) the blood will not pool. As the blood decomposes, it irreversibly stains the tissue it is in contact with. This allows forensics experts to tell if a body has been moved after death.
Over the next few weeks, putrefaction and then decomposition set in. This is where our own digestive enzymes, along with the bacteria found naturally within the gut, turn on us and begin to break down our tissues. This is highly temperature dependant (as it would never happen in the arctic, but could be complete in a week in the tropics), but is generally seen to progress in the following rate: in the two week, the body bloats with gas and the skin discolours; in the third week, internal organs and skin rupture, producing the foul stench of death; by the fourth week the tissues have begun to liquefy, removing all resemblance to the living person.
Skeletonisation is the last stage in the process of death (unless conditions allow fossilisation to occur, but this is unlikely). The skeleton is the last part of the body to survive, because it is constructed of the stable mineral calcium phosphate, which is, in effect, a rock. Our soft tissues have been consumed and digested, in order to make new living organisms. It is said that we only borrow our bodies from this world – and we must return them after we have died.