aqa  The fight or flight response including the role of adrenaline.

To produce the fight-or-flight response, the hypothalamus activates two systems: the sympathetic nervous system and the HPA axis. The sympathetic nervous system uses nerve pathways to initiate reactions in the body, and the HPA axis uses the bloodstream. The combined effects of these two systems are the fight-or-flight response.

When the hypothalamus tells the sympathetic nervous system to kick into gear, the overall effect is that the body speeds up, tenses up and becomes generally very alert. If there’s a burglar at the door, you’re going to have to take action — and fast. The sympathetic nervous system sends out impulses to glands and smooth muscles and tells the adrenal medulla to release epinephrine (adrenaline) and norepinephrine (noradrenaline) into the bloodstream. These patterns of nerve cell firing and chemical release cause our body to undergo a series of very dramatic changes. Our respiratory rate increases. Blood is shunted away from our digestive tract and directed into our muscles and limbs, which require extra energy and fuel for running and fighting. Our pupils dilate. Our awareness intensifies. Our sight sharpens. Our impulses quicken. Our perception of pain diminishes. Our immune system mobilizes with increased activation. We become prepared—physically and psychologically—for fight or flight. We scan and search our environment, “looking for the enemy.”

When our fight or flight system is activated, we tend to perceive everything in our environment as a possible threat to our survival. By its very nature, the fight or flight system bypasses our rational mind—where our more well thought out beliefs exist—and moves us into “attack” mode. This state of alert causes us to perceive almost everything in our world as a possible threat to our survival. As such, we tend to see everyone and everything as a possible enemy. We may overreact to the slightest comment. Our fear is exaggerated. Our thinking is distorted. We see everything through the filter of possible danger. We narrow our focus to those things that can harm us. Fear becomes the lens through which we see the world.

All of these changes happen so quickly that people aren’t aware of them. In fact, the wiring is so efficient that the amygdala and hypothalamus start this cascade even before the brain’s visual centers have had a chance to fully process what is happening. That’s why people are able to jump out of the path of an oncoming car even before they think about what they are doing.

When the threat passes, the parasympathetic nervous system — the “brake” — then dampens the stress response.

forf2How stress affects your endocrine system

As the initial surge of epinephrine subsides, the hypothalamus activates the second component of the stress response system — known as the HPA axis. This is also known as the ‘resistance phase’. This network consists of the hypothalamus, the pituitary gland, and the adrenal glands.

The HPA axis relies on a series of hormonal signals to keep the sympathetic nervous system — the “gas pedal” — pressed down. If the brain continues to perceive something as dangerous, the hypothalamus releases corticotropin-releasing hormone (CRH), which travels to the pituitary gland, triggering the release of adrenocorticotropic hormone (ACTH). This hormone travels to the adrenal glands, prompting them to release cortisol. The body thus stays revved up and on high alert.

On the right-hand side is the HPA axis. The left is the activation of the sympathetic nervous system.


The resistance phase, where the body starts releasing cortisol, or stress hormone, which is responsible for many of the damaging effects of stress. Some of the effects of cortisol include:

  • Suppressing the immune system.
  • Reducing inflammation in the body.
  • Breaking down fat in tissues.
  • Preventing protein synthesis.

Though cortisol has all of these negative features, it should be noted that they do play a purpose in the fight or flight response. If you were faced with danger, all of these actions would help ensure you stay safe. But because there is no danger and because cortisol release is chronic, that is why cortisol ends up being so damaging to your health and the body.

Thyroid hormone is also released during the resistance phase. Thyroid hormone is used to boost your metabolism, breaking down fats and proteins and turning them into energy. Stress and anxiety are known to be responsible for weight gain when you struggle with them over time, but for a few moments when you have anxiety you’re actually burning fat, not gaining it. The problem is that long term anxiety causes fat deposits to grow.

Human growth hormone is also released during the resistance phase. Most of these processes are caused by the release of a separate hormone, known as the adrenocorticotropic hormone, which itself is released as a result of the corticotropin-releasing hormone. Yet these two hormones simply stimulate the secretion of other hormones. They themselves do not play a significant role in the physical symptoms of the fight or flight response.

Interestingly, it is during the resistance phase that the body is also trying to help itself return back to its normal levels (homeostasis). But for those with chronic stress,homeostasis tends to not occur.

Finally: The Exhaustion Phase

Those that respond to short term stresses tend to not reach the exhaustion phase, but those that have chronic anxiety do. The exhaustion phase is the point where the body starts to experience the more harmful effects of anxiety, because it has run out of energy to respond to anxiety and stress in a healthy way.

Cortisol continues to run rampant in your body, and the ability to stop the more harmful long term effects of cortisol decrease. Your immune system will remain suppressed (which may lead to a risk for illness), fat stores will start to grow in your midsection, your organs may start to experience structural damage because they won’t have the proper defenses against stress hormones, and more.

During the exhaustion phase, which can be chronic, it becomes harder and harder to deal with stress. While exhaustion doesn’t necessarily mean tiredness, this stage may also lead to fatigue, breakdown in the muscles, and much more. Many people talk about feeling drained from anxiety, and it’s true that anxiety is not only draining mentally, but also draining physically.