The mechanisms by which poisons work are widely variable. Some act directly upon a feature of metabolism in the target species. Metabolism refers to all of the processes by which the living organism maintains itself. The most important part of metabolism is that of extracting enough energy from the surroundings in order to remain living. Metabolism depends upon the use of enzymes to enable the necessary chemical reactions to occur at body temperatures.
Enzymes are substances that facilitate chemical reactions.
Cyanide, for instance, disables the crucial enzyme cytochrome oxidase, which is essential for respiration to occur in cells. It acts very quickly in mammals, causing death in just a few minutes. Heavy metals such as lead, mercury and cadmium damage internal organs like the liver and kidneys, and initiate a chain of disastrous system failures leading to brain damage, anaemia and death. Some of these metals mimic calcium, which is pumped into and out of bone tissues as required. The toxic substitutes get in with little trouble, but can no longer be easily removed.