Arpad Vass, a forensic anthropologist at the Oak Ridge National Laboratory, takes a stab at this morbid mystery.
As best as anyone can gauge, cell metabolism likely continues for roughly four to 10 minutes after death, depending on the ambient temperature around the body.
During this time period, oxygenated blood, which normally exchanges carbon dioxide with oxygen, is not circulating. Thus, cell respiration—which uses oxygen to make cellular energy while creating carbon dioxide as a by-product—creates carbon dioxide that is not transported out of the cell. This lowers the pH of the cell, resulting in an acidic intracellular environment. This acidic environment causes intracellular membranes to rupture—including those around the cell's lysosome, which contains enzymes for digesting everything from proteins to fats and nucleic acids. Once the membranes have burst, these enzymes are released and begin to digest the cell from the inside out. This process is known as autolysis (or self-digestion).
The rate of autolytic spread throughout the body is dependent on the quantity of enzymes present—the dispersion in liver tissue, which is rich in these proteins, would take place much more rapidly than it would in lung tissue, which has a smaller reserve. This progression also depends on the amount of water present in a tissue. (The brain, being very high in water content, would degrade faster than, say, muscle tissue.) Autolytic spread, however, is most intimately tied to environmental temperature. In cold surroundings, the autolytic process slows down, while warm conditions speed up the progression. This is why people who have drowned in very cold water and are not recovered for an hour or so can, in some circumstances, be completely revived. The cold temperatures have slowed down the autolytic process to the point that no permanent damage has occurred in the tissues.
Autolysis will eventually affect all the cells of the body, although those on the surface of organs will show self-digestion's visual effects first—in the form of very small, fluid-filled blisters. Once these rupture, the nutrient-rich fluids present within the blisters then fuel the onset of the second major phase of decomposition, called putrefaction, which is the process where microbiological organisms (in the body, on the body and in the environment) feed on the nutrient-rich fluids produced during autolysis.