The basic internal protective mechanism is called "the fight-flight-freeze" response. This is not a planned, deliberately thought-out reaction, but a rapid-fire, automatic, total body response that we share with other animals. Whenever we perceive that we are in danger our bodies make a heroic and rapid response. Numerous neurotransmitters and hormones produce massive changes in every organ system. The brain sends instantaneous signals to the adrenal glands to secrete epinephrine or, as it is also called, adrenaline. At the same time the brain releases a kindred substance, norephinephrine, which affects only the brain itself. Likewise, increased amounts of steroids flood into the bloodstream, as well as opioid substances that are pain relievers.
In the fight or flight part of the response, heart rate, blood pressure, and respiratory rate increase along with alertness and vigilance. Simultaneously a decrease occurs in feeding, reproductive activity, and immune response. This radical adjustment is in the service of survival, preparing us to make an immediate response to the dangerous situation. When this reaction is a response to a real danger, is time-limited, and is effective, it is life-saving and highly adaptive.
Problems arise only when this reaction is evoked in the absence of any threat, when the threat is prolonged, or when the organism can do nothing to protect itself from the threat. Then the chemicals related to this response keep pumping out, affecting our bodies and brains in very negative ways.
If there is no chance for survival if we try to run or fight, we may automatically freeze. The freeze response activates a very different sequence of autonomic nervous system arousal, slowing the heart rate, causing us to fall over and thus preserve blood flow, and even simulate death so that a predator loses interest.