When there is a perceived threat of bodily harm, the initial physical response can vary from "Frozen Fright" to "Fight or Flight”. During this automatic, involuntary response, an area of the brain stem will release increased quantity of norepinephrine, which in turn causes the adrenal glands to release more adrenaline. The immediate change that follows is a rise in heartbeat, which triggers an increased blood pressure. One or more physical senses may become more acute while others shut down. Concentration and alertness are greatly increased at this stage. When faced with possible danger, the two common reactions of fear and anger, individually or in tandem, can take control of the body. Fear causes extra blood to be diverted from the face, neck and head to those muscles that most aid in self-defense. Incidentally, this diversion of blood away from the head causes some people to turn pale and/or faint when petrified. Anger produces a similar response with the exception that blood rushes to the face, neck and head, thus giving the appearance of a threatening adversary. Failure to act when the bloodstream is pumped with adrenaline causes weakness and possible nausea. The unmetabolized hormones produce trembles and shaking in the arms and legs, loss of concentration and composure. This can often lead to exhaustion before the body regains its balance and achieves equilibrium. Therefore, physical arousal associated with fight or flight cannot be sustained indefinitely.