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Death is the full cessation of vital functions in the biological life. This article discusses death in the biological sense of the term, and its place in various cultures.
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The process of dying
Cell death
The body of a dead walrus on Punuk Island, in the Bering Sea
Normal cellular function involves the production of free energy required for vital cellular metabolism, the production of enzymatic and structural protein, the maintenance of chemical and osmotic homeostasis of cell, and cell reproduction. During normal functions, cells require oxygen, phosphate, calcium, hydrogen, carbon, nitrogen, sulphur, nutritional substrates, ATP (which is required as a source of energy), intact cell membranes, and a steady-state activity that requires O2 consumption. Cell death may occur when any of these functions is interrupted.
Changes after death
After death, the body core temperature falls (algor mortis). Rate and degree of the cooling depends on a number of external and cadaveric factors including the environment, clothing, body temperature at time of death and size of the body.
Furthermore, in mammals, rigor mortis begins prior to decomposition. During this process, the muscles gradually stiffen due to decreasing availability of ATP and lactic acidosis within muscle fibrils. This usually begins 2-4 hours after death, though the process may begin earlier. After 9-12 hours, or in a warm enough climate, these effects may disappear. Onset and duration of rigor mortis are influenced by environmental temperature and the degree of muscular activity prior to onset of death.
Another post mortem reaction includes livor mortis. Fibrolysins (lytic enzymes) are released from serous membranes, and cause lytic degradation of fibrinogen (which is responsible for the clotting of blood). Due to this process, blood becomes permanently incoaguable (unable to clot) within 30-60min after death. Gravitational pooling of blood results in characteristic changes in skin colour, starting with those areas that support the body on whatever surface it is lying on. This is usually seen within 2 hours of death, with the process of livor mortis reaching its maximum at 8-12 hours. The colour of the post mortem lividity differs with cause of death and environmental conditions. The distribution of the lividity depends on the position of the body and the pressure at the location.
Decomposition of a body progresses in the following stages:
- Autolysis: The "self digestion" of the body driven by its own enzymes. Cell membranes lose their structural integrity, lytic enzymes are released and denature macromolecules and remaining membranes. Autolysis occurs first in the most metabolically active cells, secretory cells and macrophages.
- Putrefaction: Anaerobic bacterial digestion of the remains. At the end stage of autolysis, an aerobic environment is established within the corpse. This favours the growth of anaerobic bacteria of mostly endogenous (colon fauna) and to a lesser extent exogenous (soil bacteria) origin. These bacteria degrade carbohydrates, proteins and lipids of the corpse to products such as acids and gases, resulting in colour changes, odours, bloating, and liquefaction of the corpse. The rate of putrefaction is dependent on the presence of moisture and moderate environmental temperatures.
- Decay: Aerobic bacterial and fungal digestion of remains. At the end stage of putrefaction, putrefactive juices have drained away and soft tissue is shrunken. All remaining tissues are in a relatively dry state. Decay is characterized by a slow breakdown of proteins by aerobic microorganisms and leads to skeletonisation of the corpse.
- Diagenisis: Decomposition of hard tissues like bone and teeth. Microorganisms (bacteria, algae, fungi) invade the bone by either following physiological channels or actively penetrating the bone substance. The latter is accomplished by excretion of acidic and/or enzymatic metabolites which results in characteristic non-physiological cavities or channels, the so called "drill channels." The invading microorganisms metabolize the organic bone matrix. The resulting metabolites destroy the surrounding mineral matrix. Furthermore, the decomposition of the mineral matrix, which is composed of crystalline calcium phosphates, is influenced by chemical factors from the environment. An acidic environment results in dissolution of calcium phosphates which partly leads to demineralisation of the bone, and partly is followed by re-crystallisation to molecules that are significantly larger than the original ones and more water soluble. These processes result in micro- and macrostructural disintegration (cracking) in the first case and progressive demineralisation in the latter.
