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How does stress work?

6 min read

How does stress work?

Stress is the system of changes that is activated in your brain and body in response to stressors. Stressors are things that activate your stress response, such as a job interview, worries about finances, relationship problems, or big changes.

What is stress?

Stress is mostly illustrated as something negative, but it’s actually a healthy response of the body to changing environments or threats. Stress also helps you feel alert and concentrated which can be beneficial for work or productivity. 

When you are exposed to a stressor, your stress system releases adrenaline and cortisol into the bloodstream leading to several physiological events. Examples of these events are increased heart rate, respiration rate, blood pressure, glucose release, higher alertness, muscle tension, and decreased digestive function. Also, small blood vessels of the skin and the intestines will contract, while blood vessels in the muscles dilate. 

All these changes prepare you for the action to come, either fight or flight. However, these days, we are not chased by lions or claws anymore. Most stress nowadays is more of a chronic and low-intensity nature.

The role of the autonomic nervous system in stress

Think about what you can and can't control. You can decide to move your limbs, but you can't motivate your stomach to function or stop functioning. The autonomic nervous system (ANS) regulates vital functions in your body that you can't control, like your intestines, kidneys, heart and lungs. 

The ANS is subdivided into two systems (or actually three but let’s not focus on the third one): the parasympathetic nervous system and the sympathetic nervous system. The sympathetic nervous system reacts to stress responses and primes you for fight or flight, for example when you are anxious about your presentation the next day. Your adrenaline and cortisol levels rise, sweating increases, your heart rate goes up, your respiratory rate increases and muscle tension gets higher. Conversely, the parasympathetic nervous system helps the body to relax after stressful events to maintain a natural balance. Your blood pressure, heart rate and feelings of anxiety fall, and your body is ready for digestion, also called the rest and digest response. The two systems work together to establish homeostasis.


McCorry LK. Physiology of the autonomic nervous system. Am J Pharm Educ. 2007;71(4):78. doi:10.5688/aj710478

The quick and slow stress response

Whenever a stressful event occurs, the brain processes this within the amygdala and sends a signal to the hypothalamus which activates the pituitary gland. This is a pea-sized gland that plays a big role in many vital functions of our bodies. In stressful situations, the pituitary gland produces ACTH, a hormone that acts on the sympathetic nervous system and activates the adrenal glands. The adrenal glands will produce catecholamines including adrenaline and noradrenaline and release cortisol to initiate processes that lead to stress responses within the body. These stress responses prepare the body for fight or flight actions to promote survival in dangerous situations and help to maintain homeostasis.

Healthy humans have two stress responses: a quick (nor)adrenaline response that acts within seconds, the sympathetic-adrenomedullary system (SAM), and a slow cortisol response within around 25 minutes, the hypo-thalamic-pituitary-adrenocortical system (HPA).


The SAM system has quick metabolic effects and stress responses within seconds. The brain detects a threat or stressor and initiates rapid physiological and behavioral reactions. Chromaffin cells in the adrenal glands are activated by the sympathetic nervous system and produce adrenaline and noradrenaline, also called epinephrine and norepinephrine. Adrenaline travels via the bloodstream and binds to receptors in various organs throughout the body to play a role in fight or flight reactions and enter survival mode.


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Fight or flight reactions induced by adrenaline:

  • Increased heart rate and blood pressure.

  • Increased breathing rate.

  • The rapid increase of sweat gland activity.

  • Vasodilatation in muscles and vasoconstriction in blood vessels of the skin and gut to ensure blood flow to the brain and muscles. The skin may be pale, and feet and hands may be cold. 

  • Inhibition of the stomach and intestines.

  • Increased blood glucose through glycolysis in the liver for quick energy supply.

  • Increased mental alertness, arousal and attention.

  • Inhibition of salivation causes a dry mouth.

  • Dilation of pupils.

  • Relaxation of the bladder.

  • Tunnel vision and hearing loss.

The duration of the effects of adrenaline is determined by the trigger of stress. After the stressor is removed, it may take 20-30 minutes to calm down, but the effects on the body can take longer to subside.


Cortisol is the body’s stress hormone and helps the body deal with stressors. Cortisol has basal levels that follow a 24-h circadian rhythm and has an impact on the brain, the autonomic nervous system, the heart and blood vessels to establish optimal cardiovascular function throughout the day. These basal levels are lowest in the evening at the beginning of sleep, rise towards the end of sleep and are highest at wakening.


Mohd Azmi NASJuliana NAzmani Set al. Cortisol on Circadian Rhythm and Its Effect on Cardiovascular System. Int J Environ Res Public Health. 2021;18(2):676. Published 2021 Jan 14. doi:10.3390/ijerph18020676

On top of these basal levels, cortisol levels increase when stressors occur. The effects of cortisol become present in a slower time frame than adrenaline effects. When the brain processes a stressful situation, the brain sends a signal to the adrenal cortex to produce cortisol. Then, cortisol binds to cell receptors throughout the whole body, activating the transcription of genes in the cells. Depending on the genes that are transcripted, different physiological stress responses take place. The time frame between stress detection and stress response is around 25 minutes. The effects of cortisol are more complex than the effects of adrenaline because they can differ between cell types and gene transcriptions. The main goal of cortisol in stressful situations is to suppress the impact of fight or flight reactions and calm you down after stressful events. The effects include:

  • Release of glucose into the bloodstream from the liver

  • Increase of blood pressure

  • Inflammatory and immune effects

NOWATCH continuously measures your stress levels so you can manage your stress on time.

Sources and References

Cacioppo JT, Berntson GG, Malarkey WB, Kiecolt-Glaser JK, Sheridan JF, Poehlmann, KM, Burleson, MH, Ernst JM, Hawkley LC, Glaser R: Autonomic, neuroendocrine, and immune responses to psychological stress: the reactivity hypothesis. Ann NY Acad Sci 1998; 840:664–673. 37 Herman JP, Ulrich-Lai YM: Neural regulation of endocrine and autonomic stress re- sponses. Nat Rev Neurosci 2009;10:397–409.

Stratakis CA, Chrousos GP. Neuroendocrinology and pathophysiology of the stress system. Ann N Y Acad Sci. 1995 Dec 29;771:1-18. doi: 10.1111/j.1749-6632.1995.tb44666.x.  Frankenhaeuser, M. (1986). A psychobiological framework for research on human stress and coping. In M. H. Appley & R. Trumbull (Eds.), Dynamics of stress: Physiological, psychological, and social perspectives (pp. 101–116). Plenum Press.

Gunnar, M., and Quevedo, K. (2007). The neurobiology of stress and development. Annu. Rev. Psychol. 58, 145–173 McCorry LK. Physiology of the autonomic nervous system. Am J Pharm Educ. 2007;71(4):78. doi:10.5688/aj710478 Mohd Azmi NAS, Juliana N, Azmani S, et al. Cortisol on Circadian Rhythm and Its Effect on Cardiovascular System. Int J Environ Res Public Health. 2021;18(2):676. Published 2021 Jan 14. doi:10.3390/ijerph18020676 Gunnar, M., and Quevedo, K. (2007). The neurobiology of stress and development. Annu. Rev. Psychol. 58, 145–173


M. van Oers

Scientific writer @ Nowatch

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