When someone's heart stops pumping blood around their body, the tissues and cells are deprived of oxygen and rapidly begin to die.
But different cells die at different rates. So, for example, brain cells die within three to seven minutes, while skin cells can be taken from a dead body for up to 24 hours after death and still grow normally in a laboratory culture.
But contrary to folklore, this doesn't mean that hair and nails continue to grow after death, although shrinkage of the skin can make it seem this way.
Rate of decomposition
From this point on, nature is very efficient at breaking down human corpses. Decomposition is well under way by the time burial or cremation occurs. However, the exact rate of decomposition depends to some extent on environmental conditions.
Decomposition in the air is twice as fast as when the body is under water and four times as fast as underground. Corpses are preserved longer when buried deeper, as long as the ground isn't waterlogged.
The intestines are packed with millions of micro-organisms that don't die with the person. These organisms start to break down the dead cells of the intestines, while some, especially bacteria called clostridia and coliforms, start to invade other parts of the body.
At the same time the body undergoes its own intrinsic breakdown under the action of enzymes and other chemicals which have been released by the dead cells. The pancreas, for example, is usually packed with digestive enzymes, and so rapidly digests itself.
Green substances and gas
The decomposing tissues release green substances and gas, which make the skin green/blue and blistered, starting on the abdomen. The front of the body swells, the tongue may protrude, and fluid from the lungs oozes out of the mouth and nostrils.
This unpleasant sight is added to by a terrible smell as gases such as hydrogen sulphide (rotten egg smell), methane and traces of mercaptans are released. This stage is reached in temperate countries after about four to six days, much faster in the tropics and slower in cold or dry conditions.
A corpse left above ground is then rapidly broken down by insects and animals, including bluebottles and carrion fly maggots, followed by beetles, ants and wasps.
In the tropics, a corpse can become a moving mass of maggots within 24 hours.
If there are no animals to destroy the body, hair, nails and teeth become detached within a few weeks, and after a month or so the tissues become liquefied and the main body cavities burst open.
Burial in a coffin slows the process
The whole process is generally slower in a coffin, and the body may remain identifiable for many months. Some tissues, such as tendons and ligaments, are more resistant to decomposition, while the uterus and prostate glands may last several months.
But within a year all that is usually left is the skeleton and teeth, with traces of the tissues on them - it takes 40 to 50 years for the bones to become dry and brittle in a coffin. In soil of neutral acidity, bones may last for hundreds of years, while acid peaty soil gradually dissolves the bones.
This article was last medically reviewed by Dr Trisha Macnair in January 2007.