Death, Mechanism Of

Death is defined as the complete and permanent cessation of all vital functions, such as lack of blood pressure and cardiac activity, absence of reflex response to stimuli, and cessation of spontaneous breathing. However, since some clinical pathological conditions may mimic some death characteristics, the certainty of death is achieved through the observation of the following negative vital signs: loss of consciousness, loss of sensibility, absence of motility or muscular tonus, cessation of blood circulation, and ultimately, absence of cerebral activity. Once all these criteria are met, an individual is declared dead.

From the biologic perspective, however, dying is a gradual process, with different organs and tissues halting their respective functions at different moments. Cells of the central nervous system need greater amounts of oxygen to survive than other cell types. Therefore, neurons in the brain can only live for three to seven minutes in the absence of oxygen, whereas epidermal cells (skin cells) can survive much longer, up to 24 hours or, depending on environmental conditions surrounding the corpse, even longer. Bone cells may survive for several days.

When the heart stops beating, blood is no longer pumped through the veins and arteries to be oxygenated by the lungs and further transported to cells. The blood is drained from the capillary vessels into the larger veins present near the surfaces of the body, due to the collapse of arterial pressure (blood pressure). Therefore, cells no longer receive oxygen and muscle cells start the respiratory process known as anaerobic catabolism, whereby they break down complex chemical substances into simpler ones, in order to extract energy. Through anaerobic respiration, cells of some tissues and organs such as the heart, internal muscular fibers, and limb muscles are able to survive for a while after a heart failure. Anaerobic catabolism, also known as anaerobic glycolysis, causes the buildup of lactate in the muscular tissues that leads to lactic acidosis (high levels of lactic acid). High concentrations of lactic acid in the muscles cause muscular contraction and the body stiffens, losing all flexibility. This state is known in forensics as rigor mortis and occurs within three or so hours after death. The body remains in this state for approximately 36 hours. When muscle cells finally die, the rigor mortis ceases and the process of cellular decomposition begins. After death, the body tends to progressively lose heat, with the extreme parts such as feet, hands, and the face cooling first. However, the internal organs remain warm for about 24 hours. Environmental temperatures may accelerate or slow down the cooling process.

The above biological data, in combination with other information such as characteristics of the site where the corpse was found, environmental temperature, and presence or absence of insects, is used in forensic investigation to estimate the time of death in suspicious cases.

see also Crime scene investigation; Death, cause of; Decomposition; Entomology; Time of death.