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The Fight-or-Flight Response

The fight-or-flight response—also known as the acute stress response—refers to the body’s automatic reaction to something mentally or physically terrifying. When the stress response is triggered, the body gets ready either to stay and deal with the threat or to run away to safety. 

Fight or flight was first described in the 1920s by American physiologist Walter Cannon, who noted how the chain of rapidly occurring reactions helped the body mobilize resources to deal with threat. This chain of physiological changes are as follows: 

1 – The body’s sympathetic nervous system, which promotes the fight-or-flight response, is activated by a sudden release of hormones, causing:

    • A quickening pulse and breathing rate
    • A burst of adrenaline
    • Redirection of blood away from extremities and toward major organs
    • Release of cortisol and other hormones, bringing other short- and long-term changes

2 – The body’s parasympathetic nervous system helps calm the body once the threat is gone, usually taking 20-60 minutes.

This burst of energy was an evolutionary benefit that allowed ancient ancestors to face extreme physical and short-lived threats from predators—and stay safe. Today, whether threats are physical and/or psychological, the stress response prepares the body to react to danger or deal with stressful situations—or stressors—where actual or perceived demands outweigh one’s resources to successfully cope. 

Three fight-or-flight stages

Alarm stage: Central nervous system is ramped up, preparing to fight or flee

Resistance stage: Body attempts to normalize and recover from initial elevated response

Exhaustion stage: If first two stages occur repeatedly over time, as when under chronic stress, the body can become exhausted and begin to break down.

Physical signs of fight-or-flight response

  • Dilated pupils: Allow more light into the eyes, resulting in better vision of dangerous surroundings
  • Pale or flushed skin: Commonly occurs, as blood rushes to the head and brain. Reduced blood flow to the body’s surface areas allows increased blood flow not only to the brain but to muscles, arms and legs. An increased blood-clotting ability prevents excess blood loss in case of injury.
  • Rapid heart rate and breathing: An increase that provides the body with energy and oxygen to fuel a rapid response to danger
  • Trembling or shaking: From muscles tensing and priming for action

Benefits and drawbacks of fight or flight


Essential to how one deals with the environment. A body primed for action is better prepared:

      • To cope with actual danger or threat 
      • Perform more effectively under pressure, such as at school or work
      • Survive life-threatening situations

Research suggests, those with an urge to fight can shift from intending to harm others toward protecting them instead, especially when triggered by negative emotions, such as anger and fear.


While automatic, fight or flight is not always accurate and can be triggered by both real and imaginary threats, as in phobias. Constantly being in a state of fight or flight—when facing repeated stressors—can harm one’s health. Chronic stress can increase the risk of:

      • Chronic fatigue
      • Depression
      • Gastrointestinal issues
      • Headaches, migraines
      • Heart attack, stroke, high blood pressure, high cholesterol
      • Metabolic disorders, such as diabetes and obesity
      • Poor immune function
      • Reproductive and sexual dysfunction
      • Worsened asthma-related breathing problems 

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