The way our body responds to stress stems from how our ancestors adapted to threats. When encountering threats like predators in the wild, the body has adapted to mobilise all energy resources to either "fight" or "flight" to survive. Even though we are unlikely to have to fight or run for our lives nowadays, the way we respond to threats remains the same.
You might have experienced trembling, increased heart rate, and breathing when under stress; these are all parts of the fight-or-flight response.
Stress activates the fight-or-flight response, freepik.com
Definition of the fight-or-flight response
The flight or fight response is a way for the body to respond to threats that can be real or imaginary. Anxiety over an upcoming deadline can activate this response the same way as being attacked by a bear might.
After a threat is detected, the brain communicates with the autonomic nervous system, changing its activation from parasympathetic to sympathetic.
The fight-or-flight response can be defined as a stress response that involves activation of the sympathetic nervous system and release of stress hormones (adrenaline) to increase our chances of survival in threat situations.
The autonomic nervous system under stress
In the absence of stressors or threats, the parasympathetic division of the autonomic nervous system (ANS) is activated. When threats are detected, the activity of the autonomic nervous system switches to the sympathetic branch of the ANS.
The autonomic nervous system controls involuntary actions like breathing, heart rate, digestion or blood pressure.
The autonomic nervous system consists of two divisions: sympathetic and parasympathetic.
- The parasympathetic division acts as a "brake" it is responsible for the "rest and digest" activity. It involves digestion of food, storing energy, and slowing down the heart rate and breathing.
- The sympathetic division acts as an "accelerator". It is responsible for activating the fight-or-flight response and involves rapidly releasing energy to muscles, dilating the pupils, accelerating heart rate, increasing blood pressure and inhibiting digestion.
Examples for the fight-or-flight response
In the following text, find out more about the processes involved in the fight-or-flight response.
How the fight-or-flight response works: adrenaline fight or flight response
The fight-or-flight response starts with the activation of the amygdala, a brain region that responds to emotions like fear, which triggers the activation of the hypothalamus.
The hypothalamus communicates with the autonomic nervous system and activates the sympathetic division. The sympathetic nervous system communicates with the adrenal glands (above the kidneys) that release a hormone called adrenaline (epinephrine). The release of adrenaline causes the liver to release stored energy in the form of glucose into the bloodstream, increases heart rate, blood pressure breathing rate and stops digestion.
Hypothalamus communicates that a threat was detected to the autonomic nervous system, StudySmarter Originals
This process might seem lengthy, but it occurs instantaneously. The fight-or-flight response in our body can occur before we consciously become aware of the threat.
If someone throws an object at you, the fight-or-flight response allows you to respond quickly. Due to the instantaneous release of energy into the bloodstream, your muscles quickly react, and you can dodge the object or even catch it.
When you realise you have an important deadline the next day, the fight-or-flight response can also be triggered; you might experience increased heart-beat, increased focus and determination to finish the work before the deadline, as your body enters the state of full alertness.
The role of adrenaline in the fight-or-flight response
Adrenaline is a hormone that is secreted in times of stress or excitation. These can include bungee jumping, public speaking, taking an important test, or running away from predators. Upon secretion, adrenaline releases large amounts of energy into the bloodstream, allowing muscles to quickly use this energy to survive the threat by rapidly running away or fighting.
The effects of the adrenaline rush
After the sympathetic nervous system triggers the adrenaline release, it causes the liver to release glucose that can be quickly used for energy by the muscles. Readily available glucose allows muscles to quickly use it to fight or flight.
- Adrenaline also speeds up the heart rate and slows down digestion.
- A faster heart rate increases blood flow to important organs like the heart and important muscles, so the energy can be used to increase survival.
- Slowing down digestion means more energy can go to the muscles to run away or fight, increasing our chances of survival upon encountering a predator.
- Adrenaline also triggers sweat production, which helps regulate the body's temperature and dilation of pupils, enhancing the vision to detect or respond to a threat quicker.
Effects of adrenaline on the body, StudySmarter Originals
The HPA axis activation: Long term stress
The HPA axis response involves the hypothalamus, pituitary gland and adrenal glands. This response takes longer (minutes or hours) than the instantaneous adrenaline response and involves the sympathomedullary response.
- After the rapid effects of adrenaline release subsides, and the hypothalamus still signals the presence of a threat, the hypothalamus releases the corticotropin-releasing hormone (CRH).
- CRH travels to and stimulates the anterior pituitary gland and releases the adrenocorticotropic hormone (ACTH).
- The ACTH travels throughout the body and triggers the release of cortisol from the adrenal cortex (the outer areas of the adrenal glands)
- This causes blood sugar levels to rise, as protein is converted to glucose for energy.
Similarly to adrenaline, cortisol also raises the blood glucose level. It suppresses the immune system function and other "non-essential" processes in times of threat (e.g. digestion), which can have significant, negative long term effects on health.
Stress is normal and sometimes good for us; it helps us get things done (although we don't need to run away from bears that often anymore). However, if the stress remains and the body is put into a constant state of 'fight-or-flight', it can severely affect our health.
Effects of the fight-or-flight response
In the short term, the fight-or-flight response is beneficial for us; it allows us to focus more intensely, fight or flight in dangerous situations, and perform better when under pressure. However, in the long-term, if we chronically experience the physiological consequences of stress, it can negatively impact the body.
Doctors often have to make rapid life or death decisions under pressure. In this case, the fight-or-flight response can be beneficial; the release of adrenaline can affect doctors' motivation and focus so they can quickly make a decision.
Chronic stress and physical health
Chronic stress can cause headaches, cardiovascular problems, digestive problems, and increased chances of heart attack or stroke. Stress can even decrease the immune system function making people under stress more prone to catching colds or other infectious diseases.
Chronic stress and mental health
Chronic stress can also have negative effects on one's mental health. Prolonged exposure to adrenaline and cortisol can be associated with problems like anxiety and depression, impair the ability to focus or even be a risk factor for addictions.
The role of the parasympathetic nervous system
After the perception of the threat passes, the autonomic nervous system activates the parasympathetic division to help the body relax and restore normal functioning of the immune system and digestion.
The body returns to the "rest and digest state", and the parasympathetic nervous system slows down the breathing and heart rate. Blood vessels dilate, lowering blood pressure. Glucose is again stored, and digestion activity comes back to normal.
Following the stress response, the parasympathetic nervous system helps us return to the "rest and digest" state again, freepik.com
The fight-or-flight response - Key takeaways
- The flight or fight response is a way for the body to respond to threats. It involves activation of the sympathetic nervous system and release of stress hormones (adrenaline) to increase our chances of survival in situations of threat.
- The autonomic nervous system controls involuntary actions like breathing, heart rate, digestion or blood pressure. It consists of sympathetic (activates the fight-or-flight response) and parasympathetic (activates the rest and digest response).
- The fight-or-flight response occurs instantaneously while the HPA axis response is slower.
- The fight-or-flight response starts with the activation of the amygdala, which triggers the activation of the hypothalamus. Hypothalamus communicates with the autonomic nervous system, and the sympathetic division of the nervous system is activated.
- The sympathetic nervous system communicates with adrenal glands that release a hormone called adrenaline.
- The flight-or-fight response can be beneficial and help us perform under pressure, but chronic stress has severe negative impacts on our mental and physical health.
- After the fight-or-flight response, the parasympathetic nervous system helps the body return to the rest and digest state.
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