The human body lives on average 79 years. Inevitably death is caused by pathology or disease or multiple organ failure. There are a number of changes that take place both sequentially and simultaneously shortly after death which precedes decomposition.
So what happens the human body following clinical death?
Diagnosing death used to be simple. A subject was diagnosed dead if the heart was no longer functioning and the person was unable to breath. The lack of oxygen would quickly halt brain function, starved of oxygen the neutrons within the brainstem would die. This has been rethought as it is possible to keep somebody alive using ventilators. If this is the case the subject will never regain consciousness.
Clinical death is now described as loss of function of the brainstem, this can be done by testing reactivity to light. Failure for the autonomic nervous system to react indicates the persons brain stem is not functioning.
Following clinical death...
15-20 mins | Pallor Mortis
Pallor Mortis is the paling of the skin caused by the loss of blood pressure. As the heart stops beating the blood pressure falls meaning the blood in the skins capillary beds sinks more visceral. This causes the person to loose the fleshy appearance we associate with somebody healthy and living.
20 - 30 mins | Livor Mortis
Again due to the lack of cardiac pressure, the blood quickly comes to a stand still inside the blood vessels. As this occurs the plasma sinks into the lowest tissues not in contact with a surface under gravity. This causes purple/red blemishes on the skin in the lowest down parts of the body.
4 hours | Rigor Mortis
Rigor Mortis is caused by the shut down of cellular respiration in the muscles. Cellular respiration produces ATP which all cells need to carry out basic life functions. Muscles require huge amounts of ATP to break bridging proteins between actin & myosin protein filaments. As the bridging proteins break, actin and myosin can slide past each other giving muscles the ability to contract and relax.
As clinical death results in the loss of breathing (gaseous exchange) and circulation, then the delivery of oxygen and glucose the muscles stops, this causes the muscles cellular respiration to shut down. No ATP is therefore produced and the bridging proteins stay fixed between the actin and myosin filaments. This means the muscles loose the ability to contract and relax. Muscles work in antagonistic pairs around joints. As the muscles become stiff the joints become difficult to manipulate and see rigidity in the extremities.
From clinical death | Algor Mortis
As respiration stops, the body fails to produce metabolic heat. The body cools away from the optimum internal temperature to the ambient temperature.
2 - 3 days | Endogenous Autolysis
Endogenous autolysis is the 'self-digestion' of the human body by its own internal secretions. Acid in the GI tract along with digestive juices i the pancreas begin to digest and damage the internal structures. Often the contents of the closed cavities begin to liquify, liquified structures may begin to leak from various orifices, particularly the brain from the nostrils or ears.
2-3 days | Putrefaction
Putrefaction occurs when bacteria being to decompose the soft tissues. This may cause bloating or ballooning of the closed cavities as gas from bacterial decomposition builds up within the human body.
Halting the above processes allows pathologists to study the internal organs to determine the cause of death. This can be done by freezing the cadaver. Alternatively the subject can be treated with formaldehyde and preserved for several years and used in medical education.