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Fight-or-Flight Response and The Role of Adrenaline

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Fight-or-Flight Response and The Role of Adrenaline

What do the Hulk and your voice shaking of stage fright have in common?

Fight-or-Flight Response and the Role of Adrenaline Hysterical strength StudySmarterThe Marvel character, Hulk, inspired by what is known as ‘hysterical strength’, Pixabay

Popular imagination has been long captured by rumours of regular people overturning 800kg cars when their children are in danger. They have even inspired comic book heroes like the Hulk. There have been enough anecdotal reports that researchers believe there is some truth to humans being able to increase twitch muscle strength (short-term bursts like for a sprint) when in true life-or-death situations. This is called ‘hysterical strength’, but as you can imagine, there’s no scientific consensus as to whether it’s real, as it wouldn’t be ethical to research this in a lab.

A similar physical reaction also happens when we experience stage fright. Before stepping onto a stage in front of a crowd for the first time, even the most confident people tend to experience strange physical sensations: they blush, start sweating, their body starts to shake, or their voice cracks and they feel a strong urge to run to the nearest bathroom. We have more energy and strength at that moment. Although the situation isn’t one of life-or-death, our bodies might react as if we were under threat.

These situations have in common that both phenomena can be attributed to the nervous system division (the sympathetic nervous system) reacting to perceived danger in what’s called the fight-or-flight response.

What is the fight-or-flight response of the sympathetic nervous system?

The fight-or-flight response is a specific physiological response that happens without conscious control when an individual faces perceived danger. It’s an acute stress response that mobilises the body’s resources to deal with danger quickly through hormones and physical changes. Once activated, it takes 20 minutes to an hour for the body to return to its normal state.

The fight-or-flight response, communicated via the sympathetic nervous system, affects many body parts. The body then prepares for the eventuality to either run away from the danger (flight) or fight the danger (fight.)

The ‘danger’ doesn’t have to be a real danger (like a fire breaking out on the dinner table in front of you) but can be a psychological one, influenced by a person’s worldview (like when someone with clown phobia sees a picture of a clown.) Triggered memories of childhood trauma, work stress, or even the thought that something can threaten our livelihood or emotional wellbeing can be perceived as a danger. The body doesn’t know the difference between real and perceived danger and sets off a chain reaction to ready the body for action. Although many situations today might not require this specific response, our biology is hard-wired to act as if we were still in the wild and facing predators or enemies.

In recent years, ‘freezing’ has been added to the reactions that are also caused by the sympathetic nervous system in reaction to danger, but it’s not made it into the A-Level curriculum yet. The body is immobilised with the freeze response to prepare for the next move.

What are some possible triggers for the fight-or-flight response?

Many triggers that can set off the fight-or-flight response. All of them signal to the body that it must urgently react to danger. Some possible triggers are:

  • Perceived danger.

  • Pain.

  • Physical injury.

  • Hypoglycemia (drop in blood glucose levels).

  • Sudden emotional upset.

  • Drop in blood pressure

So you see, the fight-or-flight response can also explain why you start shaking when you don’t eat enough food or when you see blood. It can even explain why some people faint after being injected.

How does the fight-or-flight system work?

When we detect a threatening stimulus such as a rustling in the bush or a loud scream, the body sets off a complex chain reaction involving the endocrine system and the nervous system. Let’s look at the sequence of events first and then at the stress hormones or messengers that cause the following reaction:

  1. Senses receive input: you hear or see something alarming, and the information is sent to the brain via the sensory neurones. The hypothalamus (part of the brain in charge of the stress response) activates.

  2. The autonomic nervous system, which includes the sympathetic nervous system, is aroused.

  3. The sympathetic nervous system sends a signal to the adrenal glands to produce adrenaline and noradrenaline.

  4. Adrenaline and noradrenaline activate the fight-or-flight response.

  5. After the threat is over, the parasympathetic nervous system brings the body back into a balanced state.

What parts of the body does the fight-or-flight response affect?

The fight-or-flight response affects many areas of the body.

  • Blood flow increases in the limbs and to the head, leading to blushing. Less blood flows to the skin, so the skin can get cold and clammy. The blood clots easier to prevent blood loss from possible injury.

  • Heart rate increases to provide the body and brain with more oxygen.

  • Lungs: bronchi dilate and breathing gets faster as the respiration rate increases. This enables the lungs to get more oxygen from the air.

  • Muscles are mobilised for action, which can cause shaking or trembling.

  • The senses: pupils dilate to let more light in. This enables us to perceive everything clearer and to react better to danger. Our ears become more selective for us to be able to focus on the danger.

  • Sweat glands: increased sweating to regulate the temperature in case of activity.

  • Bowels and bladder relax, which can lead to the need to evacuate them.

  • Pain receptors: cortisol temporarily blocks pain receptors.

  • Pancreas: glucose stores are accessible to give energy to the areas in need.

The changes in blood and oxygen flow in the body, as well as the mobilisation of glucose, lead to the body having more energy available for action in the areas that need it: the brain and the limbs. The dulling of pain and sharpening of senses enable quicker reaction times. The shaking of muscles enables us to run or fight at a given moment.

After the danger has passed, the sympathetic nervous system response is counteracted with the parasympathetic nervous system response. It acts in exact opposition to the sympathetic nervous system: slowing the heart rate, narrowing the pupils, and slowing the breathing.

What is the role of adrenaline in the fight-or-flight response?

Stress hormones

The stress hormones adrenaline, noradrenaline, and cortisol are the messengers that trigger the physiological fight-or-flight response in the various organs.

Adrenaline is one of the catecholamines produced in the medulla (core) of the adrenal glands, which sit atop the kidneys. Adrenaline helps relax the smooth muscles of the lungs to help in respiration, is responsible for the heart rate rising, the constriction of blood vessels, and the constriction of skin cells (causing goosebumps.) It also helps the liver break down glycogen to glucose and access sugar stores for immediate access to energy.

Cortisol is the stress hormone produced in our body’s reaction to chronic stress. Our bodies’ response to chronic stress is called the hypothalamic-pituitary-adrenal system (HPA axis). If stress is prolonged this system kicks in after the initial fight-or-flight response.

What are some examples of the fight-or-flight response?

The fight-or-flight shows itself in many ways.

  • You’re alone in the house and hear a loud noise from inside and your heart starts beating faster.

  • You’re in a movie theatre and someone shouts ‘fire!’ Suddenly you can see better in the dark.

  • You’re on your bike and a door of a parked car suddenly opens and you’re able to steer away quickly.

  • You’re about to go on a date and have the sudden urge to go to the toilet.

  • It’s your first time singing in public and your breathing is quick. You feel anxious.

  • A dog runs towards you, so without thinking you turn to run away and scale a hedge that you never thought you could.

What happens if there’s an underactive fight-or-flight response?

Although it seems inconvenient to have a fight-or-flight reaction to public speaking or the sight of a spider, it’s useful to the organism’s survival. Just imagine for a moment if someone were to have an underactive fight-or-flight response at the sight of a tiger or a fire. It would take them more time to perceive the danger and their reaction would be slow and sluggish.

Fight-or-Flight Response and The Role of Adrenaline This is fine meme StudySmarterA meme of a dog sitting in a burning house. This is what it might look like if someone had an underactive fight-or-flight response

The seconds that the body saves in being ready for action might mean the difference between life or death. In this case, being able to dodge a tiger in time or run away from a fire. Only our ancestors that had these quick reactions survived long enough to pass on their genes.

Fight-or-Flight Response and The Role of Adrenaline - Key takeaways

  • The fight-or-flight response is a specific physiological response that happens when an individual faces perceived danger, that mobilises resources to either flee (flight) or fight the danger.
  • The activation of the fight-or-flight response begins within milliseconds of a danger being perceived and can last up to an hour, allowing for fast reaction times.
  • The fight-or-flight response is carried out using the sympathetic nervous system to stimulate hormone production. It involves both the nervous and endocrine systems.

  • Adrenaline is the main hormone associated with the fight-or-flight response.

  • Physiological changes initiated by the fight-or-flight response include increased heart- and respiratory rate, an increase of blood flow to the brain and limbs, increased sweating, and trembling muscles.

Frequently Asked Questions about Fight-or-Flight Response and The Role of Adrenaline

Adrenaline is responsible for the immediate response to perceived danger by increasing the heart rate, respiratory rate, increasing blood flow to the brain and limbs as well as accessing energy stores via glucose within seconds. Cortisol acts similarly, but with a delayed reaction and it continues for longer.

The fight-or-flight response is a chain of physiological events in the body that happens in response to a stimulus that is perceived as dangerous. Your senses send a signal to the brain, which stimulates the sympathetic nervous system to tell your adrenal glands to produce hormones which in turn affect multiple organs throughout the body.


Adrenaline is the hormone best associated with the fight-or-flight response.

Stage fright is an example of the fight-or-flight response.

The first stage is alarm (fight or flight response), the second is resistance (body starts to recover) and the last is exhaustion (this is chronic stress). These are part of the theory of general adaption which describes the effects of chronic stress.


Final Fight-or-Flight Response and The Role of Adrenaline Quiz

Question

The fight-or-flight response is a psychological response that happens when an individual is faced with perceived danger.

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Answer

False

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Question

The fight-or-flight response is an acute stress response.

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Answer

True

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Question

To initiate the fight-or-flight response, there has to be a real danger.


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Answer

True


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Question

Which of the following is not an example of the fight-or-flight response?

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Answer

Blushing when someone shouts at you.

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Question

‘Flight’ refers to the act of removing yourself from danger.

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Answer

True

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Question

Freeze is another response the body can have to danger.


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Answer

True


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Question

The fight-or-flight response uses the parasympathetic nervous system to signal the adrenal glands to produce adrenaline.

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Answer

True

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Question

Seeing blood can trigger the fight-or-flight response.

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Answer

True

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Question

The fight-or-flight response is a combination of the nervous system and the endocrine system.


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Answer

True


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Question

Which one of the following is not an area of the body known to be affected by the fight-or-flight signal?


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Answer

Testes

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Question

You are very nervous about singing for a crowd for the first time. When you try to sing, your voice wobbles, goes off-key, and is weaker than usual. Explain this reaction using what you learned in this explanation.


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Answer

The fight-or-flight reaction has affected your muscles, particularly the muscles that control your voice box as well as your breathing. Also, the faster breathing triggered by adrenaline can mean that your breathing isn't as deep as usual, which can make your singing seem weaker.

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Question

 How do the physiological changes triggered by adrenaline help survival?


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Answer

They enable the body to quickly react to dangerous stimuli and focus on the danger.

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Question

What is the main hormone that is associated with the fight-or-flight response?


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Answer

Adrenaline

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Question

What evolutionary advantage does the fight-or-flight response give us?


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Answer

It enables our body to respond quickly and appropriately to danger.

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Question

It's possible to not feel pain during life-or-death situations even if seriously injured.


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Answer

True

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