The autonomic nervous system controls physiological responses to stress. Its chemical mediators on the sympathetic side are the catecholamines, dopamine, noradrenalin and adrenalin - particularly the last two. Dr. Cannon's original description of the flight or fight mechanism is still valid. In response to an emergency or a perceived emergency there is a major shock to the autonomic nervous system, releasing a large amount of adrenalin.

However, adrenalin is very toxic and it elevates blood pressure. The body must remove it as quickly as possible. To do so it has developed two main pathways for converting adrenaline to other substances. One pathway leads to adrenochrome, which does not elevate blood pressure. The second pathway leads to non indolic derivatives and is controlled by several enzymes called amine oxidases. Thus, a highly reactive compound which elevates pressure is replaced by other compounds that do not have this property.