Death or, as it is also colloquially referred to, passing away, is defined as ‘the cessation of all vital functions of the body, including the heartbeat.’  Whilst many are aware that this is the permanent end of someone’s life, there is little general knowledge about what occurs in the body after death, and the most common stages a corpse experiences. This knowledge is especially key for forensic pathologists, as it can provide more information to discover the time of death, which can be used as evidence in court.
We’ve all seen it in TV shows; someone dies, the ambulance arrives, and the last we see of them is a white sheet over their drained face, to protect our eyes from observing a horror that we will experience ourselves. Ironically, although dead, what goes on in humans after death is full of life.
There are 4 main stages that can be observed 1-2 days after death; pallor mortis, algor mortis, rigor mortis and livor mortis. These stages occur due to the cessation of normal bodily functions, with most rooting from the absence of a heartbeat and circulation. The rate at which these occur is heavily influenced by factors such as items of clothing, muscle mass and environmental factors.
Pallor mortis is the first sign of death, presented by a sudden white/paling complexion in those with originally pale skin tones. This is caused by the lack of blood circulating through capillaries, caused by the halted heartbeat, leading to little haemoglobin (a red respiratory pigment) penetrating through the skin. 
This is closely followed by the next stage, algor mortis or ‘the death chill’, which involves the body radiating heat and decreasing in temperature until the environmental conditions are matched.  Whilst this occurs soon after death, it is possible that the body’s temperature may increase again hours after, due to the heat produced by respiring microorganisms during decomposition. This process heavily relies on the clothing and environment they are in.
Again, this is caused by halted circulation, so thermal conduction around the body does not occur. It also involves the lack of hypothalamus function, and thermoregulatory centres not being able to respond to the environment, so homeostasis does not occur, and the body’s temperature does not remain constant.
Next, rigor mortis occurs which involves the stiffening of muscles, leaving the body tense and rigid. The speed of this step varies from body to body as it can be affected by what the deceased was doing before death, the environment they are in, and the muscle mass present on the body.
Ca2+ ions are used in the skeletal muscles to attach the actin and myosin fibres together, causing muscular contraction. When aerobic respiration occurs ATP is formed in the mitochondria, which is used by the muscles to pump Ca2+ ions out of the muscle cells by active transport, allowing the actin and myosin fibres to detach. However, when deprived of oxygen, no ATP is produced so Ca2+ pumping is halted, causing accumulation and prolonged muscular contraction. Eventually, the muscles will return to their original relaxed state. 
Livor mortis is a result of gravity taking its course, causing blood to pool in the regions of the body which navigate towards the ground (e.g the ear lobes, fingertips and soles of the feet). This is characteristically presented by a purple/blue tinge of the skin. 
When alive, the force of the residual heartbeat forces blood through the capillaries, leaving no room for gravity to intervene. This is shown when you press your hand, as your skin will turn a lighter shade due to blood moving in the capillaries but, when removed, the colour will return. Livor mortis causes blood to be fixed in a certain position, so the skin will not pale even when compressed.
There are 2 stages which also occur on a more molecular level, compared to the previously mentioned; autolysis and putrefaction. These contribute to the rotting, and bacterial breakdown of flesh.
Autolysis occurs soon after death as the cells are deprived of oxygen, and only insignificant volumes of carbon dioxide are able to be removed from the previously respiring cells. This results in lysosomes secreting hydrolytic enzymes. 
The function of the usually compartmentalised hydrolytic enzymes, present in the lysosomes, is to digest any cellular waste or unwanted tissue, and these are particularly prominent in the pancreas; this is due to the use of hydrolytic enzymes in breaking the peptide bonds between adjacent amino acids for later digestion. During autolysis the lysosomes secret these enzymes, causing cells to be digested, and the flesh begins to break down.
This is similar to apoptosis as the cell self destructs, but apoptosis is a programmed, regulated death whilst autolysis occurs in response to injured/dead tissue, and does not usually occur in healthy cells. 
The putrefaction stage, after the 2nd day, is the cause of the well known rotting flesh or ‘dead’ odour that is commonly assosciated with death. Bacteria escape from the unused intestinal tract, releasing gases during respiration which cause intense bloating and bulged eyes. They also break down the endothelial lining of blood vessels, releasing haemoglobin, causing red marbling in the skin.  Soon after, the red will begin to turn green due to the breakdown of the prosthetic heam group in Hb to a protein called biliverdin; also the cause of the green tinge in bruises.
The remaining bacteria are not, of course, broken down by the body, so cause the organs to decay. This releases chemicals such as cadaverine and putrescine  which produce the unpleasant scent of rotting flesh. Eventually maggots aid the digestion of the corpse, leaving bones.
- Medical Dictionary (2007) “Death” https://medical-dictionary.thefreedictionary.com/death
- ThoughtCo. (2018) “What Causes Rigor Mortis?” https://www.thoughtco.com/what-causes-rigor-mortis-601995
- verywellhealth (2019) “What Physically Happens to Your Body Right After Death” https://www.verywellhealth.com/what-happens-to-my-body-right-after-i-die-1132498
- ScienceABC (2019) “What Is Pallor Mortis?” https://www.scienceabc.com/pure-sciences/what-is-pallor-mortis.html
- ScienceABC (2019) “What Are The 4 Postmortem Stages Of Death?” https://www.scienceabc.com/humans/post-mortemstages-of-death-different-stages-the-body-goes-through-after-death.html
- Wikipedia (2019) “Autolysis” https://en.wikipedia.org/wiki/Autolysis_(biology)
- Explore Forensics (2017) “The Rate of Decay in a Corpse” http://www.exploreforensics.co.uk/the-rate-of-decay-in-a-corpse.html
- Semantic Scholar (2016) “Biologically inspired simulation of livor Morris” https://www.semanticscholar.org/paper/Biologically-inspired-simulation-of-livor-mortis-Frerichs-Vidler/0d1690c6a3515ffcc97c7706c5cfa46554e66dd1