SNAB topic 6: spec point 2

SNAB spec point 2 notes- determining time of death

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  • Created on: 24-02-16 18:23
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2 Describe how to determine the time of death of a mammal by examining the extent of
decomposition, stage of succession, forensic entomology, body temperature and degree of
muscle contraction.
Decomposition
Putrefaction is the first sign of decomposition in humans a greenish discolouration of the skin of the
lower abdomen
This is due to the formation of sulphaemoglobin in the blood which will spread across the rest of the
body
o Green > darken to reddish green > and then turn purple black in colour
Gases e.g. methane and carbon dioxide form in the intestine and tissues due to the action of bacteria
This causes the body to smell and become bloated
As the tissues decompose further the gas is released = and the body deflates
Decay rate of dry body is reduced when fluid associated with putrefaction drains away
In average conditions in a temperature climate:
o Discolouration of the abdominal wall 36 and 72 hours of death
o Gas formation occurs after about a week
The temperature of the body will determine the rate of decomposition
In hotter temperatures, if the body remains above 26oC gas formation may occur within 3 days
Low temperatures slow down decomposition
Warm temperatures speed up decomposition
The rate of decomposition is highest between 21 38oC
Intense heat will denature enzymes involved in autolysis= delaying the start of decay
If the body is injured decomposition may be quicker as bacteria aiding in decomposition can enter
through wounds
Stage of succession
A corpse will attract insects that feed directly off of the corpse and others that feed off of each other
As each organism feeds on a body it changes the conditions of the body which makes the body attractive
to another group of organisms
This will continue until the body is reduced to a skeleton this is a predictable process with different
groups of organisms occupying the decomposing body at different times
Normally eggs are laid in wounds or at openings to the body (nose and mouth)
The season, weather conditions, size and location of the body will influence the type and number of
species present
The length of each stage of succession depends on the condition of the body which in turn depends on
the environmental conditions
Most of the early insects remain on the body until advanced stages of decay
Insects can be used to determine if the body has been moved e.g. there may be a species of insect found
on a body that would not naturally occur in that location by determining life stage/ larval stage: egg,
larva (maggot),pupa and adult
Other decomposers
Bacteria from the gut invade the tissues after death
Other bacteria and fungi from the surroundings colonise the corpse= contributing to decay and changing
conditions on the decomposing body

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Forensic entomology
The presence of insects allows an estimation of how much time has elapsed since death
Information on the location and condition of the body are recorded
Samples of insects are taken from the body and their exact location and where they were found are
recorded
The temperature of the air, ground, body and `maggot mass' are also recorded so that the rate of
maggot development can be determined
The temperature history of where the body was found can be obtained from local meteorological…read more

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After a further period of time, rigor mortis passes and muscles are again relaxed
o After death muscle cells are starved of oxygen = oxygen dependent reactions stop
o Respiration in the cells becomes anaerobic and produces lactic acid
o The pH of the cells falls= inhibiting enzymes= inhibiting anaerobic respiration
o ATP needed for muscle contraction is no longer produced = bonds between the proteins
become fixed
o Proteins can no longer move over one another to shorten the muscle= fixing the muscle and…read more

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