How Does the Fight-or-flight Response Work?


By Catherine A. SandersonAmherst College

The fight-or-flight reaction in response to threat was first described by Walter Cannon, a physiologist at Harvard Medical School in the early 1900s. When we first notice any kind of threat, such as a snake, our sympathetic nervous system and our endocrine system are stimulated, leading to increases in two hormones: adrenaline, or the more technical name of epinephrine; and norepinephrine, also called noradrenaline.

Illustration of fight-or-flight with two sharks and a school of fish
The fight-or-flight reaction can be simply viewed as our natural response to any threat. (Image: desdemona72/Shutterstock)

Different Stages of Fight-or-flight

Cannon demonstrated that increases in these hormones in the bloodstream activate the cardiovascular system, so that blood is directed to the brain and muscles. This is what prepares someone to either fight off a threat or run away.

This original fight-or-flight model was extended in 1936 by Hungarian endocrinologist Hans Selye to describe not just our immediate response to a threat, but also the stages the body goes through over time. This model, known as the general adaptation syndrome, describes three stages, of what Selye was the first to call stress

Portrait of Hans Seylel
Hans Seyle created the word stressor. (Image: Jean-Paul Rioux/Public domain)

Given how much we all use the word stress, it’s interesting to note that Selye himself later said that if his English had been better, he would have used the word strain. That’s partly because, in physics, the term stress refers only to the force applied, like saying, “I’m under a lot of stress”, whereas strain in physics refers to what happens in the response to the stress.

And it’s a good point: in psychology, we are interested not only in the stressors (another word Selye created) but also in the stress response. However, unlike in physics, where strain typically refers to an impairment from which there is no recovery, in psychology, we study the response to stressors because recovery is an option.

When the Alarms Go Off

The first stage in the stress response is basically fight-or-flight, also known as the alarm stage. In this stage, the body is mobilizing all of its resources to do whatever is necessary to fight off (or escape from) the immediate threat. 

During this phase, both epinephrine and norepinephrine are released into the bloodstream, triggering increases in heart rate, blood pressure, and breathing rate. If the stressor lasts more than a very short time, a second system, often called the HPA axis, releases an additional hormone called cortisol.

Illustration of the release of epinephrine and cortisol into the bloodstream
In the alarm stage, both epinephrine and cortisol are released if the stressor lasts for more than a short time. (Image: Pikovit/Shutterstock)

Cortisol releases extra glucose into the bloodstream for immediate energy. But, it’s also involved in stopping or slowing down processes that do not help fight off an immediate threat, such as digestion, reproduction, and even the immune system. After all, although illness and disease may also kill you, those threats act slowly. If you don’t survive the attack from the lion, it doesn’t really matter if your immune system is strong enough to protect you from the flu.

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Going Back to Normal, Gradually

Next is the resistance stage. During this stage, the body releases less of the alarm-stage hormones in response to the threat. Heart rate, blood pressure, and breathing are all still elevated to help deliver oxygen and energy quickly throughout the body, but they are subsiding. 

Nonessential functions, such as digestion, growth, and reproduction, may resume, at least partially, which can help the body return to its regular level of functioning, but at a slower pace than normal. This is the stress response to more typical daily life stressors like interviewing for a new job, having a conflict with a friend, or getting stuck in traffic.

However, in other cases, the stressors, and the stress-response, are not short-lived, but rather can continue over long periods of time. This could include the stress of living in poverty, constant interpersonal conflict, or intense work pressure.

Stress and Exhausting

When the threat, real or perceived, continues for a long time, the exhaustion stage sets in. This stage occurs when the body’s state of arousal (meaning a state of physiological and/or psychological activation) persists for a long time. 

In the exhaustion stage, maintenance operations have been delayed so long in response to stress that the body experiences extra wear and tear on multiple systems, including cardiovascular, neuroendocrine, and immune. Prolonged stress can also lead to nutritional imbalances, such as low magnesium, which reduces the body’s ability to produce and use energy. 

And, as the body’s resources are depleted, this further increases the risk of developing an infection or disease that the body would otherwise typically be able to fight off. This explains why people who are under ongoing stress are more likely to develop cardiovascular disease, high blood pressure, arthritis, colds, and flu.

Common Questions about How the Fight-or-flight Response Works

Q: How did Walter Cannon describe the fight-or-flight response?

According to Walter Cannon, fight-or-flight response starts from the moment one sees a threat. This leads to the stimulation of both the sympathetic nervous system and the endocrine system. After which, two hormones are released, called epinephrine and norepinephrine. These hormones activate the cardiovascular system and ready the body to either fight or run away.

Q: Why did Hans Selye later regret the name he used for his model for stress?

Hans Selye expanded on the fight-or-flight response with a model that explained what happened not only in the first few moments but also during a long amount of time, and he used the word stress. However, in physics, stress refers to the force applied, while strain refers to the reaction. Considering this fact, he later stated that were his English better at the time, he would have used the term strain.

Q: What are the three stages of stress according to Hans Selye’s model?

Hans Selye’s model describes three stages for stress: First is the alarm stage, or the fight-or-flight stage, where the body mobilizes every resource possible to defend itself. Second is the resistance stage, where processes such as digestion slowly work again. And finally, there is the exhaustion stage, where the body’s maintenance systems have been on hold for too long and multiple systems are worn out.

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