Fight/Flight Reaction

The fight/flight reaction, also called the fight-orflight response, is the body's emergency response to danger. It is triggered by fear and prepares the body to defend itself or avoid the danger—quick action or quick escape. The fight/flight reaction is both a physiological and an emotional adaptation for the survival of humans and other animals.

The fight/flight reaction is an evolutionary adaptation for protecting against bodily harm and promoting survival. As soon as danger is perceived by the senses, the body redirects its energy from conscious thought and routine bodily functions to automatic functioning that invigorates physical performance so that the muscles of the body can strike, lift, or run with abnormally strong force. The fight/flight reaction triggers major changes throughout the body:

In his 1872 treatise, The Expression of the Emotions in Man and Animals, Charles Darwin (1809–82) hypothesized that the fight/flight reaction is a survival adaptation in humans and other animals and that the similarities of emotional expressions, such as fight/flight, among all groups of people suggests that humans all evolved from a common ancestor. In the 1880s, the American psychologist William James (1842–1910) also described the fight/ flight reaction as an emotion. Since the 1950s, the fight/ flight reaction has been recognized as a physiological response.

Sometimes stimuli other than danger can trigger the fight/flight reaction. Possible triggers include being startled, memory of a frightening or traumatic experience, or even an intensely pleasurable stimulus.


The fight/flight reaction is under the control of the sympathetic nervous system, which mobilizes the body for fight or flight. The sympathetic nervous system is one of the two branches of the autonomic nervous system that controls bodily functions—such as respiration and digestion—that are outside conscious control. The other branch of the autonomic nervous system, the parasympathetic system, slows down bodily functions to conserve energy—the rest-and-digest mode.

Based on sensory input and processing, as well as memories of previous experiences, the brain decides whether a threat exists. If it perceives a threat, the brain circuitry shifts from the prefrontal cortex—the seat of conscious thought—to the more primitive limbic system, since conscious thought would slow down the response. The fight/flight reaction is activated by the amygdalae— basal ganglia in each cerebral hemisphere—and the hypothalamus at the base of the brain. The thalamus signals the amygdalae that there is a threat, and the amygdalae and the hypothalamus responds. The amygdalae also appear to be involved in both learning fear and overcoming fear. The hypothalamus signals two other hormone-releasing endocrine glands—the pituitary in the brain and the adrenal glands. The adrenals sit on top of each kidney and are part of the autonomic nervous system. The adrenal glands secrete adrenaline (epinephrine), which is the primary fight/flight hormone. Adrenaline activates the sympathetic nervous system and damps down the activity of the parasympathetic nervous system. The adrenals also secrete norepinephrine (noradrenaline), a stress hormone and neurotransmitter in the sympathetic system. The hypothalamus signals the pituitary gland to release adrenocorticotropic hormone (ACTH),which causes theadrenal glands to secrete the stress hormone cortisol. Cortisol supplies the body with bursts of energy, improves memory, and suppresses inflammation for the short term.

This activation of the sympathetic nervous system in response to the three stress hormones (adrenaline, norepinephrine, and cortisol) results in immediate and dramatic physiological changes:

If the brain determines that the fight/flight reaction is inappropriate, the freeze response is initiated. Heart rate and breathing slow, and the body shuts down in a final effort to survive. Stage fright is an example of the freeze response.

Once the danger has passed, the fight/flight stress response is turned off, and the body recovers and returns to equilibrium. The parasympathetic nervous system takes over, releasing acetylcholine, a neurotransmitter that activates nicotinic and muscarinic receptors in cells and muscles throughout the body to shift it to a rest-and-digest mode. This shift lowers metabolism, heart rate, and blood pressure; boosts the immune response; and activates cellular repair.

Adverse effects

The fight/flight reaction evolved to protect humans and other animals from predators and promote survival, and its short-term activation has no ill effects. However, most people in modern society have little to fear from wild animals. Instead, the fight/flight reaction can be initiated by stresses of everyday life, such as the following:

Some researchers believe that fast-moving modern society, with its constant barrage of information and frantic stress, puts the sympathetic nervous system into a constant state of overdrive, in which fight/flight reactions are continually activated. There are a myriad of stresses that can trigger and maintain the fight/flight reaction. Prolonged excessive stress may damage the system, causing symptoms of hyperarousal, such as insomnia, irritability, anxiety, and jumpiness. Without abrupt actions that release the flight/flight response, hyperactivation of the sympathetic nervous system can turn into a panic attack. Post-traumatic stress disorder (PTSD) is a condition in which the fight/flight reaction has been altered or damaged, so that it is activated when there is no danger.

Over the long term, hyperactivation of the fight/ flight reaction can cause or worsen health problems and contribute to the following:

See also Post-traumaticstressdisorder(PTSD); Stress .


Adrenal glands—
The endocrine glands located on top of the kidneys that secrete several stress hormones, including adrenaline, norepinephrine, and cortisol.
Adrenaline (epinephrine)—
A neurotransmitter and the primary fight/flight hormone that is secreted by the adrenal medulla to stimulate the heart and raise blood pressure.
Adrenocorticotropic hormone (ACTH)—
A protein produced by the pituitary gland that stimulates the adrenal glands during the fight/flight reaction.
Amygdala; pl. amygdalae—
The almond-shaped basal ganglia in each cerebral hemisphere; part of the limbic system that is activated in stressful situations, such as the fight/flight reaction, and that triggers fear.
A corticosteroid hormone produced by the adrenal gland in the fight/flight reaction.
Part of the diencephalon beneath the thalamus on each side of the brain that controls the autonomic nervous system and regulates hormone production by the adjacent pituitary gland.
Limbic system—
Brain structures, including the hypothalamus, hippocampus, and amygdalae, that are involved in mood, emotion, and motivation and are responsible for the fight/flight reaction.
A neurotransmitter, adrenal hormone, and the precursor of epinephrine.
Parasympathetic nervous system—
The rest-anddigest division of the autonomic nervous system that lowers heart rate and blood pressure and stimulates digestion, among many other functions.
Pituitary gland—
A small endocrine organ in the brain that is associated with various hormones that control and regulate other endocrine organs and affect most basic bodily functions, including growth and development and the fight/flight reaction.
Post-traumatic stress disorder (PTSD)—
Ongoing fight/flight responses in the absence of a threat.
Sympathetic nervous system—
The fight-or-flight division of the autonomic nervous system that increases heart rate, blood pressure, and respiration, among other functions, in active or stressful situations.
The largest portion of the diencephalon, which relays impulses, especially sensory impulses, to and from the cerebral cortex.



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American Psychological Association, 750 First St. NE, Washington, DC, 20002-4242, (202) 336-5500, (800) 374-2721, .

Mental Health America, 2000 N. Beauregard St., 6th Fl., Alexandria, VA, 22311, (703) 684-7722, (800) 9696642, Fax: 1(703) 684-5968, .