Forensic Science: Postmortem changes & time of death

After death, heat production stops and the body cools towards ambient temperature. Measured rectal or deep core temperature is used to estimate the time since death.

When circulation stops, blood settles under gravity into the lowest capillaries and venules, producing reddish-purple discoloration in dependent areas, except where skin is compressed against a surface.

Muscle relaxation requires ATP to detach myosin heads from actin. After death, ATP is not regenerated, so cross-bridges remain bound and muscles stiffen until proteolysis during decomposition releases them.

Gut bacteria spread from the caecum, which lies close to the right iliac fossa. Hydrogen sulphide combines with haemoglobin forming sulfhaemoglobin, producing green discoloration there first.

Rapid loss of water in hot or dry, ventilated conditions inhibits bacterial growth. Skin and tissues dry and harden, preserving the body's general shape.

Autolysis is aseptic tissue breakdown by the cell's own enzymes, mainly lysosomal, and is prominent in enzyme-rich organs such as the pancreas. Putrefaction is the separate, bacterially driven process.

Insects colonise a body soon after death in favourable conditions. The age of the oldest immature insects, derived from developmental data and temperature, indicates the minimum time since colonisation.

Blowflies are attracted within minutes to hours and lay eggs in natural openings and wounds. Beetles, cheese skippers and mites typically arrive later, in succession, as decomposition advances.

When the eyelids remain open after death, the exposed sclera dries, producing a horizontal brownish-black discoloration. It is an artefact of drying and should not be mistaken for injury.

Gastric emptying depends on meal size and composition, fluids, drugs, emotional state and illness. Contents may suggest when a last meal was eaten relative to death, but precise timing is unreliable.

At death muscles relax as nerve control stops while ATP persists. Primary flaccidity is followed by rigor mortis and then secondary flaccidity when decomposition resolves rigor.

Henssge's nomogram models body cooling using deep rectal temperature, environmental temperature and body mass, with corrective factors for clothing, covering and air movement, giving an estimate with a confidence interval.

Water conducts heat far better than air, and flowing water continually removes warmed water from the body surface, so cooling is much faster. Insulation and body fat slow cooling.

Early lividity blanches on pressure and can shift if the body is turned. After around 8-12 hours, haemolysis and leakage of blood into tissues fix the pattern, though timing varies widely.

Lividity forms in the parts lowest at the time it develops. Fixed lividity inconsistent with the discovered position indicates the body lay in another position for several hours before being moved.

Rigor is usually first noticed in small muscles such as the jaw within 2-6 hours, becomes fully established by about 12 hours and resolves as decomposition begins, with wide variation.

Cadaveric spasm is a rare, instantaneous stiffening, usually of a group of muscles such as a hand grip, associated with intense activity at death. Ordinary rigor follows a period of primary flaccidity.

Bacteria spread through blood vessels and haemolysis releases haemoglobin that is converted to coloured derivatives. Superficial veins become visible as a branching greenish-brown network, typically after a few days.

Anaerobic bacteria, many from the gut such as clostridia, produce gases including hydrogen sulphide, methane and carbon dioxide, which distend the abdomen, face and tissues.

Potassium diffuses from retinal and other cells into the vitreous after death, increasing roughly linearly with time. Ambient temperature and method affect the rate, so estimates have wide confidence intervals.