Fight or flight response: what is it, and how does it work?

If you’ve ever stepped too close to an oncoming car, you’ve likely experienced the fight or flight response: a sudden surge of fright, heavier breathing, and an immediate racing heart. 

Humans evolved a fight or flight reaction to survive. However, it’s often triggered for reasons that aren’t life-threatening, like freezing in place after hearing some shocking news or everyday stress at work. 

How does your body trigger it? Let’s dive into the response and how stress hormones change the body so that it can fight or flee. 

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The fight or flight response—also known as the acute stress response—is your body’s automatic reaction to anything it perceives as a danger. The threat can be real or imagined. 

The response describes the immediate physiological reaction that prepares the body to either fend off the threat or flee. In some cases, it causes you to freeze (Chu, 2021). 

Your ability to quickly jump out of the way of a moving car is thanks to your body’s sympathetic nervous system. 

When you’re faced with acute stress, a surge of hormones, including adrenaline and noradrenaline, activates your sympathetic nervous system, leading to reactions like a rapid heart rate and an increase in blood pressure (Chu, 2021).

And new research shows that your body is able to release these hormones thanks to how your amygdala—a part of your brain that helps regulate your emotions—responds when it senses danger (Berger, 2019). 

When the amygdala senses danger, it tells cells in your bones to release a hormone called osteocalcin. This hormone helps to override your parasympathetic nervous system (which controls your body’s ability to relax), making it easier for your adrenal glands to release adrenaline (epinephrine) and noradrenaline (norepinephrine) (Berger, 2019).

Once the threat—like that moving car—passes, your parasympathetic nervous system takes the wheel and puts the brakes on the stress response so your body can go back to what’s known as “rest and digest” mode (Chu, 2021).

When your body gears up for a fight-or-flight response, many things have to happen quickly and all at once. 

For example, your heart needs to rev up and be on high alert while your digestive system needs to slow down. This chart shows what happens when stress hormones flood your body (Chu, 2021; Yaribeygi, 2017):

You may recall stories of superhuman strength during times of crisis. 

For example, if someone lifted a car to help free a passenger, they experienced a fight response, and adrenaline played a role. If you are able to move from the path of an on-coming car without thinking about it, that’s also the fight or flight response at work (Kozlowska, 2015).

Other not-so-obvious examples of the fight or flight response include (Chu, 2021; Kozlowska, 2015):

• Feeling butterflies in your stomach when anxious: This feeling of butterflies is partly due to blood leaving your digestive tract, preparing your body for a threat.

• Having a phobia or psychological stressor: While mental and not physical, these stressors still trigger a reaction that prepares the body for a physical response.

• Freezing when confronted with a threat: The ability to freeze is common in mammals who need to avoid being seen to escape predators. If you freeze in response to shocking news or a public speaking event, it’s an effect of the stress response.

The fight or flight response can allow you to escape a life-threatening situation. It can also focus your attention on solving a problem. 

For example, eustress, also known as ‘good stress,’ can give you an edge when trying to perform under pressure. When stress levels are manageable, some stress can help you achieve goals and doesn’t affect your well-being (Dhabhar, 2014).

If the acute stress response is triggered too often, however, it can cause the stress hormone cortisol to remain at high levels in the body. 

Chronically elevated cortisol and chronic stress have been linked to various illnesses, including heart disease, immune system problems, and memory loss (Russell, 2019; Riaz, 2022). 

Chronic stress also increases the chances of having a panic attack or developing mental health issues like anxiety disorders and eating disorders (Toussaint, 2016; Bremner, 2020).

Depending on the level of stress you’re experiencing, some approaches can help you reverse the fight or flight response. 

Deep breathing exercises can have an immediate effect in reversing the physiological changes linked to the stress response. 

An exercise called 2:1 breathing involves exhaling and inhaling through your nose, exhaling for twice the amount of time it takes to inhale. For instance, you would count to three while inhaling and six while exhaling (Adhana, 2013). 

Studies show meditation and the practice of mindfulness are also effective in tamping down the stress response (Pascoe, 2017; Creswell, 2019).

There are many other evidence-based ways to reduce your stress levels. If you’re unable to bring down stress levels on your own, it’s important to contact a healthcare provider who can offer medical advice. Approaches like cognitive behavioral therapy are effective (Nakao, 2021). 

Getting help from a trained professional—who can guide treatment—is the best approach if you are working to manage a stress-related condition. 

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Current version

March 31, 2022

Written by

Kristin DeJohn

Fact checked by

Steve Silvestro, MD

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About the medical reviewer

Steve Silvestro, MD, Ro

Dr. Steve Silvestro is a board-certified pediatrician and Associate Director, Clinical Content & Education at Ro.