Death is often perceived as an abrupt switch, but biologically, it is a complex, continuous process of physiological shutdown and decay. The body’s transition is marked by a series of cascading failures, beginning the moment systems that maintain homeostasis cease to function. This progression involves charting the immediate systemic failure, the observable changes in the hours that follow, and the eventual internal and microbial breakdown of tissue.
The Cessation of Vital Functions
The moment death is defined clinically is when the body’s primary life-support systems—cardiovascular, respiratory, and nervous systems—undergo irreversible failure. The heart stops beating, known as cardiac arrest, halting the circulation of oxygenated blood. This cessation of blood flow and breathing is termed clinical death, a state that can sometimes be reversed with prompt intervention like cardiopulmonary resuscitation.
If oxygen and blood flow are not restored, the process progresses rapidly to irreversible biological death due to the death of brain cells. Without oxygen, brain cells accumulate ischemic injury faster than any other organ, with permanent damage likely after only six to ten minutes. The loss of measurable brain activity, confirmed by a “flatline” on an electroencephalogram (EEG), marks the final loss of organized function.
Initial Physical Changes After Death
Following the cessation of vital functions, the body undergoes externally observable changes used by forensic science to estimate the time since death.
Algor Mortis
Algor Mortis is the cooling of the body. Since the body no longer generates internal heat, its temperature steadily declines, generally at a rate of approximately 1.5 degrees Fahrenheit (0.83 degrees Celsius) per hour, until it matches the ambient temperature.
Livor Mortis
Livor Mortis, or post-mortem lividity, results from the passive settling of blood due to gravity. Blood pools in the lowest dependent areas, creating a reddish-purple discoloration visible about two hours after death. Initially, this discoloration blanches when pressed, but after approximately eight to twelve hours, the pooled blood becomes fixed.
Rigor Mortis
Rigor Mortis is the stiffening of the muscles, preceded by primary flaccidity where muscles relax completely. Rigor mortis begins as the body’s energy molecule, Adenosine Triphosphate (ATP), is depleted, preventing muscle fibers from detaching after contraction. Stiffness typically starts in smaller muscles within two to six hours, becoming fully pronounced throughout the body within twelve hours.
The Internal Timeline of Cellular Breakdown
While external signs are setting in, a profound internal process known as autolysis, or self-digestion, begins at the cellular level within minutes of death. Without oxygen and waste removal, the body’s cells quickly become acidic. This acidic environment causes the membranes surrounding the cells and their internal compartments to rupture.
The rupture releases powerful digestive enzymes, which begin to break down the cell’s own proteins and carbohydrates, effectively digesting the tissue from the inside out. Different tissues break down at varying rates; internal organs, for example, begin to decompose within twenty-four to seventy-two hours post-mortem.
The rate of autolysis is influenced by conditions like the body’s temperature at the time of death. This chemical process compromises the structural integrity of tissues and occurs concurrently with the microbial degradation that marks later decomposition stages.
The Later Stages of Decomposition
Autolysis quickly merges with putrefaction, the primary cause of tissue degradation involving bacterial activity. The most significant contributors are anaerobic bacteria residing naturally within the digestive tract. After death, these bacteria spread throughout the body, consuming surrounding tissues.
Putrefaction produces various gases, including hydrogen sulfide, methane, and foul-smelling compounds like cadaverine and putrescine. This gas buildup causes the body to swell, entering the bloat stage, which can begin three to five days post-mortem. The gases also lead to discoloration, often turning the abdomen greenish to black, indicating bacterial activity is underway.
As the process continues into the active decay stage, gas pressure may cause skin to blister and rupture, allowing fluids to drain. During this period, soft tissues liquefy as they are broken down by microbial action. Environmental factors like high temperature and humidity accelerate decomposition, while cold conditions slow the entire process.