While it may be morbid, it's normal to wonder what happens to our bodies post-mortem. While what happens to the human consciousness remains a mystery, we can learn the established facts of the physical process. The stages of decay occur right away, and what transpires when a body decomposes in a casket can be a long, complex process.
Decomposition begins as soon as someone passes and continues until the body becomes fossilized. Depending on the pre-burial preparations, decomposition in a casket can slow things down, essentially elongating the natural course for decades. Learning the facts of the matter may leave you feeling humbled, as it's a reminder that, however important we are in life, we all end up dust and bones in the end.
Decomposition begins immediately post-mortem, in a stage known as autolysis, or self-digestion. During this process, as the name implies, the body begins digesting itself. As cells are deprived of oxygen, they begin to breakdown and release membranes that are then digested by enzymes.
Typically, this begins in the liver and brain due to the high water and enzyme content in these areas.
As the process of self-digestion occurs, the body's internal temperature drops - paralleling the cessation of flowing blood. As a result, the blood begins to thicken. The coagulation causes the arteries, veins, and capillaries to essentially harden. This leads to the well-known phenomenon of rigor mortis, in which corpses become stiff and immobile.
This process of stiffening usually occurs two to six hours post-mortem.
Rigor mortis lasts about 24 hours. After it passes, blood settles in the area of the body that was closest to the ground at the time of death. The area where blood settles develops a red-brown color, also called the "post-mortem stain." For example, if someone passed after falling down headfirst, then there would be a blotchy stain in the facial region.
Other body parts begin to take on a bluish tinge within 8 to 10 hours post-mortem.
A bad odor begins two to three days post-mortem, in a process known as putrefaction. It's caused by micro-organisms in the intestines. These organisms do not expire at the same time as the body. Instead, they begin eating through the intestines. After a few days, micro-organisms spread across the thighs and stomach. The process of putrefaction eventually causes a foul odor, similar to the scent of rotten eggs and methane.
Other side effects include a protruding tongue, a greenish patch on the belly, and fluid oozing from the mouth and nostrils.