How the Fight or Flight Response Works

fight or flight response illustration

Illustration by Joshua Seong. © Verywell, 2018.

The fight-or-flight response, also known as the acute stress response, refers to a physiological reaction that occurs in the presence of something that is terrifying, either mentally or physically. The response is triggered by the release of hormones that prepare your body to either stay and deal with a threat or to run away to safety.1

The term ‘fight-or-flight’ represents the choices that our ancient ancestors had when faced with danger in their environment. They could either fight or flee. In either case, the physiological and psychological response to stress prepares the body to react to the danger.

The fight-or-flight response was first described in the 1920s by American physiologist Walter Cannon. Cannon realized that a chain of rapidly occurring reactions inside the body helped to mobilize the body’s resources to deal with threatening circumstances. Today the fight-or-flight response is recognized as part of the first stage of Hans Selye’s general adaptation syndrome, a theory describing the stress response.1

What Happens During the Fight-or-Flight Response

In response to acute stress, the body’s sympathetic nervous system is activated due to the sudden release of hormones. The sympathetic nervous systems stimulate the adrenal glands triggering the release of catecholamines, which include adrenaline and noradrenaline. This results in an increase in heart rate, blood pressure, and breathing rate.2 After the threat is gone, it takes between 20 to 60 minutes for the body to return to its pre-arousal levels.

You can probably think of a time when you experienced the fight-or-flight response. In the face of something frightening, your heartbeat quickened, you begin breathing faster, and your entire body becomes tense and ready to take action.
The fight-or-flight response can happen in the face of an imminent physical danger (such as encountering a growling dog during your morning jog) or as a result of a more psychological threat (such as preparing to give a big presentation at school or work).

Some of the physical signs that may indicate that the fight-or-flight response has kicked in include:

  • Rapid Heart Beat and Breathing: The body increases heartbeat and respiration rate in order to provide the energy and oxygen to the body that will be needed to fuel a rapid response to the danger.2
  • Pale or Flushed Skin: As the stress response starts to take hold, blood flow to the surface areas of the body is reduced and flow to the muscles, brain, legs, and arms are increased. You might become pale as a result, or your face may alternate between pale and flushed as blood rushes to your head and brain.3 The body’s blood clotting ability also increases in order to prevent excess blood loss in the event of injury.
  • Dilated Pupils: The body also prepares itself to be more aware and observant of the surroundings during times of danger. Another common symptom of the fight-or-flight response is the dilation of the pupils, which allows more light into the eyes and results in a better vision of the surroundings.4
  • Trembling: In the face of stress or danger, your muscles become tense and primed for action. This tension can result in trembling or shaking.3

Why It’s Important

The fight-or-flight response plays a critical role in how we deal with stress and danger in our environment. Essentially, the response prepares the body to either fight or flee the threat. It is also important to note that the response can be triggered due to both real and imaginary threats.

By priming your body for action, you are better prepared to perform under pressure. The stress created by the situation can actually be helpful, making it more likely that you will cope effectively with the threat. This type of stress can help you perform better in situations where you are under pressure to do well, such as at work or school. In cases where the threat is life-threatening, the fight-or-flight response can actually play a critical role in your survival. By gearing you up to fight or flee, the fight-or-flight response makes it more likely that you will survive the danger.

While the fight-or-flight response happens automatically, that does not mean that it is always accurate. Sometimes we respond in this way even when there is no real threat. Phobias are good examples of how the fight-or-flight response might be triggered in the face of a perceived threat.

A person who is terrified of heights might begin to experience the acute stress response when he has to go the top floor of a skyscraper to attend a meeting. His body might go on high alert as his heartbeat and respiration rate increase. When this response becomes severe, it may even lead to a panic attack.

Understanding the body’s natural fight-or-flight response is one way to help cope with such situations. When you notice that you are becoming tense, you can start looking for ways to calm down and relax your body.
The stress response is one of the major topics studied in the rapidly-growing field of health psychology. Health psychologists are interested in helping people find ways to combat stress and live healthier, more productive lives. By learning more about the fight-or-flight response, psychologists can help people explore new ways to deal with their natural reaction to stress.