Determining Time of Death

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  • Determining Time of Death
    • Rigor mortis
      • This is where muscles usually totally relax and then stiffen.
        • Joints become fixed and their position, either flexed or extended, will depend on the body position at the time of death.
      • First,shortly after death, muscle cells become starved of oxygen and oxygen-dependent reactions stop.
        • Then, respiration in the cells becomes anaerobic and produces lactic acid.
          • The pH of the cells falls, inhibiting enzymes and thus inhibiting anaerobic respiration.
            • The ATP needed for muscle contraction is no longer produced. As a result, bonds between the muscle proteins become fixed.
              • The proteins can no longer move over one another to shorten the muscle, fixing the muscle and joints.
      • Smaller muscles stiffen before larger ones.
        • The rigor mortis passes off as muscle tissue starts to break down, in the same order in which it developed.
      • Most human bodies will have complete rigor mortis six to nine hours after death.
        • However, rigor mortis will set in more quickly and last for a shorter period if the environmental temperature is high, or if the person has been physically active before death
    • Decomposition (putrefaction)
      • After death, tissues start to break down due to the action of enzymes.
        • Autolysis occurs first. This is when the body's own enzymes, from the digestive tract and lysosomes, break down cells.
        • Bacteria from the gut and gaseous exchange system invade the tissues after death, releasing enzymes that result in decomposition.
      • The first sign of decomposition in humans is a greenish discolouration of the skin of the lower abdomen.
        • This discolouration is due to the formation of sulfhaemoglobin in the blood
          • It will spread across the rest of the body, darken to reddish-green and then turn a purple-black colour. Gas or liquid blisters may appear on the skin.
      • Due to the action of bacteria, gases such as hydrogen sulphide, methane, carbon dioxide ammonia and hydrogen form in the intestines and tissues.
        • This causes the body to smell and become bloated.
          • As the tissues further decompose, the gas is released and the body deflates.
            • When the fluid associated with putrefaction drains away, the soft tissues shrink and the decay rate of the dry body is reduced.
      • There is variation in the time taken for decomposition. In average conditions in a temperature climate, discolourationof the abdomen wall will occur 36 to 72 hours after death.
        • Gas formation occurs after about a week.
        • The temperature of the body will determine the rate of decomposition.
          • If the body remains above 26°C, gas formation may occur within 3 days.
      • Environmentaltemperatures have a major influence on the rate of decomposition.
        • Low temperatures slow down decomposition. Warm temperatures speed it up.
        • The rate of decomposition is highest between 21 and 38°C.
          • Intense heat denatures the enzymes involved in autolysis, delaying the start of decay. Injuries to the body allow the entry of bacteria that aid decomposition.
    • Forensic Entomology
      • The presence of insects allows a forensic entomologist to make an estimate of how much time has elapsed since death.
        • They record information such as location and condition of the body. They take samples of any insects on or near the body - noting exactly when/where they were found. They take measurements of the temperature of the air, ground, body and a measurement of 'maggot mass'. They also use meteorological records and weather stations to establish temperature history.
          • Some of the maggots will be killed at the time of collection (to determine age later) and some will be taken back to the lab alive.
            • The live maggots are fed on meat, allowing them to complete their development; this aids with species identification and establishing when they pupate.
      • By determining the time when eggs were laid, a minimum time since death can be estimated.
        • The age of the maggot can be read off a graph (linking age and temperature) if the temperature has remained fairly constant.
        • If the maggot is not C. vicina and its stage of development is unclear, it will be allowed to develop into a pupa. This gives a date of pupation which makes it possible to calculate a date when the eggs were laid.
      • The time of egg laying may give an underestimateof time of death as it is hard to know how long it took the flies to find the body.
        • Generally, flies will lay eggs on a body within one day of death in the summer. Factors such as cocaine in the body would accelerate development.
      • Succession occurs on corpses. Some insects feed off the decaying corpse, whereas others feed on the insects around it.
        • One group of organisms feed on a decomposing body, conditions change in a way which makes it more attractive to another group of organisms.
          • As the corpse continues to change, it attracts different organisms that feed on it until only the skeleton remains.
            • Forensic scientists use the predictable sequence of different organisms that feed on a decomposing body to determine the time of death.
              • Insects can also help determine if a body has been moved. For example, insects normally found in the woods on a body found indoors would indicate it has been move some time after death.
                • Other organisms, such as bacteria and fungi also colonise the body, contributing to decay and changing conditions. These are referred to as decomposers
                  • But there does not seem to be a set sequence of succession for bacteria and fungi.
                    • Corpses are a good source of organic carbohydrates, proteins, fats and nucleic acids and this energy allows the bacteria and fungi to grow and multiply rapidly, ensuring more decomposition.

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