Stress | Fight or Flight Triggers with Sympathetic Nervous System

Mar 11 , 2022


Fluid Fitness

Stress | Fight or Flight Triggers with Sympathetic Nervous System

Fight or flight triggers with sympathetic nervous system/adrenal

Breaking down the play-by-play of the ‘fight or flight’ response and how it relates to our central nervous system can give us a better understanding of what we can do to better regulate stress and its effects on the body.

Flight or Fight Response:

The ‘flight or fight’ response is a physiological reaction that happens in the body in response to stress or a perceived harmful event. The sympathetic nervous system (SNS) is activated to prepare the body for either fighting or fleeing, and it does this by stimulating blood glucose, dilating the pupils, slowing digestion/peristalsis and increasing heart rate. Therefore, it is important to remember that chronic stress in our daily lives can affect very real bodily events and sympathetic triggers. When the sympathetic nervous system is activated due to stress, a cascade of reactions and hormones cause the production or norepinephrine and epinephrine, as well as the hormones estrogen, testosterone and cortisol.


Hypothalamic-Pituitary-Adrenal Axis:

The hypothalamic-pituitary-adrenal axis (HPA axis) is a complex set of interactions between the 3 endocrine glands, hypothalamus, pituitary gland and adrenal glands. It is what controls the body’s reaction to stress, as well as regulates several body processes, for example digestion, immune system, mood and emotions, and energy storage and expenditure. In the response to stress, the hypothalamus releases CRH (corticotrophin-releasing hormone), which travels to the pituitary causing it to release ACTH (adrenocorticopropic hormone). ACTH enters the circulatory system and travels to the adrenal glands causing them to produce a number of glucocorticoids, including cortisol. Glucocorticoids work as a feedback mechanism to the HPA axis and reduces certain aspects of immune function, including inflammation. However, excessive activation of the HPA axis can contribute to the development of certain diseases.


Vagus Nerve Response:

The vagus nerve or “wandering nerve” starts in the brainstem and travels down through the body, creating a network between the brain and the organs, including stomach, digestive tract, lungs, heart, spleen, intestines, liver and kidneys. The vagus nerve also makes up a significant part of the autonomic nervous system and it is an essential part of the parasympathetic nervous system, which is responsible for calming organs after the stress induced ‘flight or fight’ response. The strength of the vagus response is called vagal tone, and this can vary between people. Having a high vagal tone, means that the body is better at regulating blood glucose levels, which can reduce the risk of developing associated diseases. But in the same respect, having a low vagal tone, is associated with chronic inflammation, which can lead to the damage of organs and blood vessels. 

Stress and Response:

Daily stressors cause repeated and/or constant activation of the stress “flight or fight” response system within the body, for which the system was not developed. Over-activation from any number of sources can cause the breakdown of many bodily systems. A healthy individual will have certain cycling levels of cortisol in their body throughout the day, however disruption of the is natural cycle due to chronic stress has been linked to chronic fatigue syndrome, insomnia and burnout. Also, increased production of cortisol causes an increase in the availability of glucose, while down-regulating the immune systems drain on available glucose. Chronic elevated blood glucose can lead to insulin resistance and diabetes. It is also important to note that, the action of the vagus nerve can be disrupted or altered by emotional stress or pain. This can cause dramatic change in blood pressure, including a drop that can cause fainting (vasovagal syncope).



  1. Continue stress journal:
    1. Keep a simple journal, paying extra special attention to any symptoms of chronic stress that you might be experiencing.
  1. Remember that there are physical, emotional and behavioral symptoms
  2. Note if any of the symptoms occur at specific times during the day or after specific events
  1. Start to reduce the amount of physical stress cause by your diet:
  • . Go at least one day this week without any processed foods.
  1. Plan for the day and focus on preparing all your meals for the day with whole foods, such as: fresh produce and protein, whole grains and mostly unsaturated forms of fat.
  1. Knowledge is power – Know your options for gaining a better understanding of your body’s response to stress and vagal tone.
  • . If you are participating in the Heart Rate Variability project through Fluid: Make sure you are up to date on the protocols and are following through.
      1. If you have questions about getting and interpreting data, talk to Ryan or your trainer.
  1. Metabolic testing is also available through Fluid and can give you another avenue for understanding how your body is processing energy.
      1. This can give you additional information on how your body might be adapting to stressors.


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