Chapter 9 – Stress

General Adaptation Syndrome

Even if you know the physical effects of stress, you may be unaware of the different stages of stress, known as the general adaptation syndrome (GAS). The general adaptation syndrome is a universal and predictable response pattern to all stressors, whether good stress (called eustress) or bad stress (called distress).

Endocrinologist Hans Selye first described GAS in the 1930s and 1940s. He believed that when we are chronically exposed to stress, over time, the stress response causes aging and disease. During experiments with rats, Selye observed a series of physiological changes in the rats after they were exposed to stressful events.

After additional research, Selye concluded that these changes were not an isolated case, but rather the typical response to stress. He subsequently identified these stages as alarm, resistance, and exhaustion. Understanding these different responses and how they relate to each other may help you cope with stress. The sequence of physical responses associated with GAS is the same for eustress and distress and occurs in three stages.

Three Stages of GAS

1. Alarm reaction stage

In this stage, your body experiences the “fight or flight” response. This natural reaction prepares you to either flee or protect yourself in dangerous situations. The sympathetic branch of the autonomic nervous system is activated and the adrenal glands secrete two hormones to stimulate your reactions to stress: epinephrine (also known as adrenalin) and norepinephrine (also known as noradrenalin).

Adrenalin mobilizes glucose and fatty acid release from fatty cells. The body is able to use both as energy to respond to stress. Adrenalin and noradrenalin also have powerful effects on the heart. Both the heart rate and stroke volume are increased, thereby increasing the body’s cardiac output. They also help to shunt blood away from the other parts of the body and thereby push more blood to the heart, brain, and muscles as the body prepares to attack or flee. At the same time, the adrenal glands also release cortisol, to help meet the body’s energy needs in times of stress.

2. Resistance stage

After the initial reaction to the stressor during the alarm reaction stage, the parasympathetic branch of the autonomic nervous system counteracts the changes that the stressful stimulus has produced, and attempts to restore a state of homeostasis, the default state in which the body functions normally.

During the resistance stage, the results of the hormonal changes which occurred in the previous stage are still apparent, including increased glucose levels in the blood and higher blood pressure, but stress hormone levels begin to return to normal, enabling the body’s focus to shift from alertness to repair.

If the resistance stage continues for too long the body will stay in a state of alertness and continue to produce the stress hormones. Signs of the resistance stage include:

  • Irritability
  • Frustration
  • Poor concentration

3. Exhaustion stage

After an extended period of stress, the body enters this final stage of GAS. At this stage, the body has depleted its physical, emotional, and mental resources and is unable to maintain normal function. Once the body is no longer equipped to fight stress and may experience these symptoms:

  • Fatigue
  • Depression
  • Anxiety
  • Feeling unable to cope

The Fight or Flight Response

When an individual perceives a potential stressor as dangerous, the body enters into a stress response termed “Fight or Flight”. This is a natural physiological deviation from homeostasis designed to protect an individual from harm. When our ancestors lived among other animals out in the wild, it was important for survival that when faced with danger, an automatic “alarm” response would take over causing them to take immediate action (attack or run). This is still an important response mechanism in today’s world. Imagine a bus speeding toward you, horn blasting, and you experienced no sense of danger or alarm. You would probably be killed. Luckily, your fight-or-flight response automatically steps in and takes over.

During the initial stress response, a person’s brain sends messages to a part the nervous system called the autonomic nervous system. The autonomic nervous system has two branches: the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system is the fight-or-flight system which gets the body aroused and ready for action, and the parasympathetic nervous system returns the body to a normal, non-aroused state.

When activated, the sympathetic nervous system releases a chemical called adrenalin. Adrenalin is used as a messenger to continue sympathetic nervous system activity, so that once activity begins, it often continues and increases for some time. Adrenalin takes time to fully exit the blood stream so even after your sympathetic nervous system has stopped responding, you are likely to continue to feel “stressed”. The parasympathetic nervous system takes over when the perceived danger is over (or the fight or run response took place).

Exercise and Fight or Flight

Physical activity or regular exercise serve to act as simulated “fight or flee” scenarios that trigger the parasympathetic nervous system and return the body to homeostasis.


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Health and Fitness for Life Copyright © 2019 by Dawn Markell and Diane Peterson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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