Ironically, despite spending a lifetime walking around in the same body and doing our best to care for it (or wishing we would), few seem to wonder what happens to their physical remains right after death occurs. Here is a timeline of the processes involved, assuming the deceased remains undisturbed.
At the moment of death, all of the muscles in the body relax, a state called primary flaccidity. Eyelids lose their tension, the pupils dilate, the jaw might fall open, and the body's joints and limbs are flexible. With the loss of tension in the muscles, the skin will sag, which can cause prominent joints and bones in the body, such as the jaw or hips, to become pronounced.
The human heart beats more than 2.5 billion times during the average human lifespan, circulating about 5.6 liters (6 quarts) of blood through the circulatory system. Within minutes of the heart stopping, a process called pallor mortis causes the usually pinkish tone of a Caucasian person to grow pale as blood drains from the smaller veins in the skin.
At the same time, the body begins to cool from its normal temperature of 37° Celsius (98.6° Fahrenheit) until reaching the ambient temperature around it. Known as algor mortis, the decrease in body temperature follows a somewhat linear progression. (Two degrees Celsius in the first hour; One degree each hour thereafter.) This enables forensic scientists to approximate the time of death if necessary, assuming the body hasn't completely cooled and depending upon other external factors, such as indoors vs. outside, humidity, etc.
Hours 2 to 6
Because the heart no longer pumps blood, gravity begins to pull it to the areas of the body closest to the ground, a process called livor mortis. If the body remains undisturbed long enough (several hours), the parts of the body nearest the ground can develop a reddish-purple discoloration from the accumulating blood. Embalmers sometimes refer to this as the "postmortem stain."
Beginning approximately in the third hour after death, again depending upon numerous factors, chemical changes within the body's cells cause all of the muscles to begin stiffening. Known as rigor mortis, the first muscles affected include the eyelids, jaw and neck. Over the next several hours, rigor mortis spreads upward into the face and down through the chest, abdomen, arms and legs until it reaches the fingers and toes.
Interestingly, the old custom of placing coins on the eyelids of the deceased might have originated from the desire to keep the eyes shut, since rigor mortis affects them soonest. Also, it is not unusual for infants and young children who die not to display rigor mortis, possibly due to their smaller muscle mass.
Hours 7 to 12
Maximum muscle stiffness throughout the body occurs after roughly 12 hours due to rigor mortis, although this will be affected by the decedent's age, physical condition, sex, the air temperature, etc. At this point, the limbs of the deceased are difficult to move or manipulate. The knees and elbows will be slightly flexed, and fingers or toes can appear unusually crooked.
Hour 12 and Beyond
After reaching a state of maximum rigor mortis, the muscles will begin to loosen due to continued chemical changes within the cells and internal tissue decay. This process occurs gradually, over a period of one to three days, and will be influenced by external conditions such as temperature (cold slows the process down). Rigor mortis dissipates in the reverse order in which it occurred, i.e., from the fingers and toes, through the arms and legs, and then up through the chest to the neck and face. Eventually, all of the muscles will again relax, reaching a state known as secondary flaccidity.
"Introduction: Death." www.newscientist.com. 2007. Jo Marchant and Lucy Middleton, New Scientist. Retrieved July 17, 2012. http://www.newscientist.com/article/dn12759-introduction-death.html?full=true
"What Happens to Your Body After You Die." www.fountia.com. Fountia. Retrieved July 17, 2012. http://www.fountia.com/body-after-death
"Rigor Mortis and Other Postmortem Changes." www.deathreference.com. Encyclopedia of Death and Dying. Retrieved July 17, 2012. http://www.deathreference.com/Py-Se/Rigor-Mortis-and-Other-Postmortem-Changes.html
"Amazing Heart Facts." www.pbs.org. 1997. NOVA/WGBH. Retrieved July 17, 2012. http://www.pbs.org/wgbh/nova/heart/heartfacts.html