Our bodies do not let us down when faced with a life-endangering situation. They prepare us for the fight with the many means available to us as a result of evolution. The flight or fight response can be activated instantly when needed.

The fight or flight response is activated by the sympathetic nervous system

The sympathetic nervous system triggers the fight-or-flight response before we consciously make any decision on how to act. Many things happen very fast. First the hormone epinephrine (also known as adrenaline) and norepinephrine (also called noradrenaline) are released into our system.

We notice the effects:

  • Rapid pulse and respiration increase oxygen intake for fast action
  • Blood pressure goes up and extra oxygen is sent to the brain, increasing alertness.
  • Sight, hearing, and other senses become sharper.
  • Blood sugar (glucose) and fats from energy stores are released into the bloodstream to give us the extra power we need.
  • Skin temperature goes up and the increased sweat on the palms of our hands improves our grip– should we need to climb a tree to flee.
  • Digestion is slowed down – all our energy is now conserved for staying alive.

Skin temperature goes up and the increased sweat on the palms of our hands improves our grip– should we need to climb a tree to flee.

All these reactions are caused by some very fast chemical processes in our body. Our preparedness for the fight is automatic and we flee away from the threat without conscious cognitive processing.

The flight-or-fight response and Walter B. Cannon

Walter B. Cannon

The term fight or flight was first used by M.D. Walter B. Cannon in 1915. He studied and taught at Harvard University department of psychology and specialized in the research of physical reactions of laboratory animals under pressure.

In his research Cannon observed noticeable physical changes in the digestive systems of animals experiencing fear. He subsequently spent some 20 years studying the relationship of psychological and physical effects of stress on animals.

Cannon also redefined the biological term homeostasis to signify the internal balance of the body. According to Cannon, our bodies continuously seek to maintain a predefined state of equilibrium by regulating the complex interdependent system of organs. Changes in variables such as body temperature and fluid balance set off a series of processes aimed at returning the body to its original balance.

‘The homeostatic definition of stress: A condition where expectations, whether genetically programmed, established by prior learning, or deduced from circumstances, do not match the perception of the environment. This discrepancy between what is observed or sensed and what is expected or programmed elicits patterned responses.’

Still in danger

Let´s get back to the threat of coming face to face with a snake. If the snake we see turns out to be a plastic toy, we take a deep breath and laugh out in relief.

If, however, the threat is real and a fight is unavoidable, the hypothalamic-pituitary-adrenal axis (HPA axis or HTPA axis) is activated after the first surge of adrenaline subsides. The HPA axis keeps the sympathetic nervous system up and running as long as needed, until the fight is over.

This adrenal cortex produces hormones that contribute to the release of cortisol. Cortisol is a steroid hormone that has several functions, including the controlling of the blood sugar level during stress reaction. The hormonal effects induced by the adrenal cortex are called indirect stress responses as they work through the bloodstream. The effects of these responses take place within 20-30 seconds. In contrast, the immediate stress responses described in the beginning of this article are induced by the sympathetic nervous system and visible in a few seconds.

Recovery from a stress reaction

When the threat has been removed and the brain no longer perceives the environment as dangerous, the frontal cortex gets a message of ‘alarm cancelled’. The high levels of reaction by the sympathetic nervous system come down and the amygdala makes the parasympathetic nervous system return the body to its normal relaxed state. The fight-or-flight response is over.

The body needs about 20 minutes to physically recover from an acute stress reaction. An adrenaline surge impacts our bodies up to an hour form reaction. The release of hormones by the adrenal cortex started later and thus also last longer. The production of cortisol will cease too once the danger has passed, and consequently the balance between the sympathetic and parasympathetic nervous systems is attained.

Our bodies can uphold a stress reaction for a very long time. Humans are built to face threats and fight for their lives, normally in rapidly escalating situations that are also over quickly. Activation and preparedness to attack are normal reactions, as well as excitement and joy of victory.

Multitasking, taxes, interests, tormenting colleagues, lost phones and broken household appliances – these were non-existent in the early days of the human species. The problem is that we cannot turn off the surge of adrenaline when our attacker is a phone bill.

The fight or flight response in short

  • The brain reacts to threat
  • The sympathetic nervous system activates the fight or flight response, also called an acute stress response
  • The reaction is very rapid and happens unconsciously
  • Adrenaline adds strength momentarily
  • If the threat goes on, the hypothalamic–pituitary–adrenal axis activates
  • The HPA axis keeps the sympathetic nervous system active as long as needed by releasing ao. cortisol
  • When the threat passes, the parasympathetic nervous systems begins to return the body back to balance

The complete set of 5 articles explains the Moodmetric measurement, science behind and the applications:


  1. Part 1: Fight or flight response
  2. Part 2: Chronic stress
  3. Part 3: Tools for long term and continuous stress measurement
  4. Part 4: Measuring stress with the Moodmetric smart ring
  5. Part 5: Moodmetric measurement in preventive health care

The picture and information about Walter Cannon 


Jaa artikkeli

Niina Venho

acute stress response adrenaline cortisol fight or flight response homeostasis stress response sympathetic nervous system Walter B. Cannon