MadSci Network: General Biology Query:

Hello Robert. As a microbiologist I find your question fascinating, mainly because my favorite little microbes play a large part in what happens to a human body after death. Some people might find it disgusting, but if you look at it from the level of the microbes, insects, and other animals involved it is just a normal part of life. Be prepared anyway because this isn't going to be pretty. First of all let's look at what happens to the human body at the time of death and soon after. At the very moment of death the heart stops beating and the lungs stop breathing. This means that the cells in the body will no longer receive blood and oxygen. Since the blood is no longer being pumped through the body it will drain from the blood vessels at the top of the body and collect in the blood vessels on the lower part of the body. The upper part of the body will become pale and the lower part of the body will become dark. If the person is lying on their back, the front of their body and face will be very pale or even grey while their back will be much darker and look almost like it is bruised. This is called lividity or liver mortis and is one of the first things that a scientist will look at to try to determine when someone died and if they were moved after death. At this point most of the cells in the body are still not dead. While the brain cells die in the first few minutes after the heart stops, muscle cells can live for several hours and skin and bone cells can stay alive for days! How is this possible? Well, the cells use a different type of respiration than when the heart and lungs were working. While the person was alive the cells used aerobic respiration (with oxygen), but after death the cells continue to survive using what is called anaerobic respiration (without oxygen). However, one of the by-products of anaerobic respiration is lactic acid. Lactic acid eventually builds up and causes the muscles to stiffen. This is the same thing that happens to a person's legs when they run a long distance. The heart and lungs can't keep up with the demand so the leg muscles start to use anaerobic respiration. In a living person this lactic acid will eventually be cleared out by the circulatory system, but in a dead body this isn't possible so the entire body stiffens. This is called rigor mortis. Rigor mortis usually starts about 3 hours after death and lasts 36 hours. Eventually all of the cells die and the body can no longer fight of bacteria. The cells' own enzymes and the enzymes of bacteria begin to cause the body to decompose and the muscles lose their stiffness. Like liver mortis, rigor mortis is another tool that scientists can use to determine the time of death. OK. Now comes the actual process of decomposition, or breakdown and decay of the body. Decomposition can be broken down into 5 steps: 1. Initial decay 2. Putrefaction 3. Black putrefaction 4. Butyric putrefaction 5. Dry decay Let's look at each of these steps in more detail. Step 1: Initial decay Initial decay occurs from 0 to 3 days after death. Although the body appears fresh from the outside, many things are going on inside the body to contribute to the process of decomposition. The bacteria that are normally inside the intestines of a living person begin to feed on the contents of the intestine and the intestine itself. Eventually these bacteria break out into the body cavity and start to digest other organs. Since the intestine is no longer intact, the body's digestive enzymes, which were kept safely inside the intestine and stomach, leak out and spread through the body helping to break down more organs and tissues. At the same time, enzymes inside individual cells leak out and digest the cell and its connections with other cells. Let's not forget about the insects! From the moment of death flies are attracted to the smell of the decomposing body. Without the normal defenses of a living body, these flies are able to lay their eggs around wounds and other body openings (mouth, nose, eyes. etc.). Within 24 hours most of these eggs hatch and the larvae, or maggots, move into the body to feed on the dead tissue. Step 2: Putrefaction Putrefaction occurs from 4 to 10 days after death. As the bacteria are breaking down the tissues and cells they are also producing a lot of gas. These gases include hydrogen sulfide, methane, cadaverine, and putrescine. All of these gases really stink, but insects love the smell. More and more flies start to show up along with beetles and mites. The gases also cause the body to inflate forcing more fluids out of the cells and blood vessels and into the body cavity. This provides even more food for the bacteria and a nice warm living space for the maggots. Step 3: Black putrefaction This stage occurs from 10 to 20 days after death. The bloated body eventually collapses and the flesh has gotten creamy (like cottage cheese). The exposed parts of the body have turned black and the body really begins to stink. A lot of the fluids have now leaked out of the body into the soil attracting more insects and mites. These insects and mites will eventually consume most of the flesh on the body. Bacteria are still at work also, and will consume the flesh if there are no insects around. The temperature of the body also increases due to all of the insect activity. Step 4: Butyric fermentation Butyric fermentation occurs from 20 to 50 days after death. All of the remaining flesh on the body is removed during this time and the body starts to dry out. It has a "cheesy" smell caused by butyric acid. This smell attracts a bunch of new organisms to the body. Mold starts to grow on the part of the body that is touching the ground and a lot of beetles show up. Since the body is beginning to dry out maggots are no longer able to eat the tough flesh. Beetles, however, are able to chew through this tough material such as skin and ligaments. Step 5: Dry decay This stage occurs from 50 to 365 days after death. The body is now dry and decays very slowly. Tineid moths and bacteria eventually eat the person's hair, leaving nothing but bones. As long as there are no large animals around to carry them away, the bones can remain almost indefinitely. That brings us to the end of the decomposition process. Most of these steps depend a lot on the climate. Temperature and moisture and the presence of insects will affect how long this whole process takes. Decomposition will occur much faster in the summer than in the winter and also will take longer in a body that is buried than a body that is left exposed on the ground. References: � Anderson, G.S. 2000. Minimum and maximum development rates of some forensically important Calliphoridae (Diptera). Journal of Forensic Sciences. 45: 824-832. � Bornemissza, G.F. 1957. An analysis of arthropod succession in carrion and the effect of its decomposition on the soil fauna. Australian Journal of Zoology. 5: 1-12. � Fuller, M.E. 1934. The insect inhabitants of carrion: a study in animal ecology. Council for Scientific and Industrial Research. Bulletin No. 82. 63 pp. � Kamal, A.S. 1958. Comparative study of thirteen species of sarcosaprophagous Calliphoridae and Sarcophagidae (Diptera) I. Bionomics. Annals of the Entomological Society of America. 51: 261-270. � Morovic-Budak, A. 1965. Experiences in the process of putrefaction in corpses buried in earth. Medicine, Science and the Law 5: 40-43. � Rodriguez, WC. and Bass, WM. (1985). Decomposition of buried bodies and methods that may aid in their location. Journal of Forensic Sciences 30: 836-852. � Spennemann, D.H.R and Franke, B. 1995. Decomposition of buried human bodies and associated death scene materials on coral atolls in the tropical Pacific. Journal of Forensic Science. 40: 356-367.

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