The mechanisms that cause death are complex. Death results from the failure of a single body system or several of them, which then brings about the catas- trophic failure of the whole organism. The more complex the creature, the greater the chance of this occurring. The simplest of organisms, such as amoebae or sea-anemones, reproduce asexually, so it is hardly true to say that these animals die: each descendant is an exact genetic replica of its ancestor. Human beings and other complex animals, on the other hand, are composed of many different kinds of specialized. The vast majority of these are unable to replicate themselves and therefore the lifetime of the individual is finite. We tend to look at death at the macroscopic level -that is, in terms of how the death or destruction of an organ or tissue system causes the death of the being as a whole. But it is at the cellular, microscopic level that the critical events occur: the disruption of these microscopic systems causes death. All depend on complex membrane systems to separate their various functions. The membranes retain their integrity partly by their particular chemical structure and partly through electrochemical systems, which require energy and thus oxygen to sustain them. Nervous impulses are transmitted electrically by what is known as the sodium-potassium pump. Heart muscle contraction depends on the mobilization of calcium through another oxygen-dependent system to make it work.
If these systems are denied oxygen because of asphyxia, heart attack or loss of, the membranes stop functioning. Substances, including enzymes, that are normally involved in metabolic processes within the cell are released, and the enzymes destroy the cells through autodigestion. In cardiac and brain tissue this is a very rapid process. Certain kinds of surgery make it necessary to interrupt the supply to an organ – for example, open heart surgery. Then it is possible to delay cell destruction by cooling the organ down to low temperature and by infusing it with special salt solutions that help to prevent membrane breakdown. The cooling operation reduces the rate of cell metabolism and thus the demand for oxygen. This cannot be continued indefinitely: hearts removed from bodies for transplantation can survive for just a few hours, kidneys a little longer. Toxins of one kind or another can also bring about cellular death and eventual whole body death within a short period of time. Chemicals such as cyanide interrupt vital enzymatic pathways in intracellular respiration, once again depriving cells of their energy requirement. Certain toxins, such as those associated with cholera, damage the membranes of cells of absorption in the gut, effectively turning them into secretory cells. If this happens, the body loses massive amounts of fluids, and death ensues very quickly. Overwhelming infections and the final stage of some kinds of cancer result in death by disturbing the blood-clotting mechanism. This causes either haemorrhages throughout the body or widespread clotting.