The Stress Response

Erin Moloney
Jan 24, 2021
8 min read

Updated: Feb 1, 2021

Its Evolution, Neurobiology, and Lasting Impact on the Body

Stress - the seemingly one constant of the human condition. From the moment we wake up, there are countless tasks and thoughts and people and situations that call for our attention, steadily depleting our energy until our head hits the pillow at night, and we finally get a break - that is, if we are actually able to release that built-up tension, which is a constant struggle for many. If left unchecked, stress can accumulate into a relentless presence that grinds away at our physical, mental, and spiritual wellbeing.

But what is stress - biologically speaking?

What are the physiological processes that produce this response in our bodies, and why do we react this way? What lasting symptoms appear when stressful situations overwhelm our ability to cope?

Types of Stress

Stress can generally be thought of in two different ways:

Acute stress is the most frequently experienced and is often the result of negative thinking in response to recent, current, or future situations.

Maybe you have a big exam coming up, or your roommate made a mess in the kitchen, or the roads are so icy that you feel like you’re bound to careen off the highway at any minute. Acute stress is normal and actually beneficial if managed in healthy ways. It forces us to deal with these stressors - so we actually study for that test, communicate with our roommate, and focus on the road instead of getting distracted by that podcast you were listening to. After the stressor is dealt with, the accompanying stress usually goes away.¹

When stress levels are not regulated, this can lead to chronic stress, a state of constant low-level stress that sustains even after the main stressor isn’t in the picture anymore.

Basically, our brains adapt to the constant stress and rarely are we able to relax, causing our bodies to kick into overdrive just to regulate its basic functions. This may cause those who suffer with it to react to harmless scenarios as if there was a real threat of danger. In addition, they are constantly enduring the hazardous effects of stress on mind and body. Left unaddressed, chronic stress can cause a significant negative impact on a person’s mental and physical health.

Many mental health professionals help those struggling with stress to find coping mechanisms in order to manage their negative symptoms. But why do we get so stressed out in the first place? To answer that question, we’re going to have to look to our past.

Evolution of the Stress Response

Let’s go back 200,000 years around the time when modern humans (homo sapiens) first evolved and observe a day in the life of one of our ancestors. So here’s Bob, a very accurately named hunter-gatherer just chilling by the water hole and snacking on some of the berries he found earlier.

An image of an early modern human, known as homo sapien, or in this case, Bob.

Suddenly, a rustling sound alerts him to the unmistakable presence of a menacing cat of some kind in the bushes, looking at Bob like he’s next on the menu. Almost instantly, he jumps into action, high-tailing it for the nearest tree to climb.

Bob is demonstrating one outcome of the well-known fight-or-flight response, our automatic physiological response to a perceived threat.

There are actually two more components to the stress response, including Freeze and Fawn, but we’ll get into those in another article. What’s important to know is that this response evolved in a context in which death-by-cougar-mauling was an everyday possibility. Those who could most efficiently transition from the relaxation response (a state commonly known as “rest and digest” ) and the stress response would live to see another day, and more importantly, pass on their genes to the next generation. Reinforced by natural selection, the Autonomic Nervous System evolved.²

Biology of the Autonomic Nervous System

So what is happening in Bob’s brain when he first catches wind of the threat? Well, first his sensory organs relay sights and sounds by sending electrical signals to the

amygdala, the emotional processing area of the brain.

If the sensory information is interpreted as a threat, the amygdala sends a distress signal to the hypothalamus.

The hypothalamus functions as a command center, communicating with the rest of the body through the autonomic nervous system.

The Autonomic Nervous System controls involuntary body functions and is made up of 2 components that serve contrasting functions.

If we can picture our bodies functioning like a car, the sympathetic nervous system is like the gas pedal, bringing energy to the system. On the other hand, and the parasympathetic nervous system acts as the brake, inhibiting activity.

These systems work in a close-knit partnership in order to maintain homeostasis, or an internal state of balance. We’ll talk about the parasympathetic division, or the relaxation response, in another article.

But right now, Bob has seen and heard that cougar, and the next step his brain takes in response to this threat is his hypothalamus signaling the adrenal glands to release the hormone epinephrine, more commonly known as adrenaline, into the bloodstream.

This triggers an array of physiological changes, and in our car metaphor, essentially “starts up his engines”:

He starts to breathe more rapidly, and small airways in the lungs open up to maximize the amount of oxygen entering the body. The extra oxygen is sent to his brain to increase his alertness, his senses quickly sharpening.
At the same time, Bob’s heart rate increases, sending blood rushing to his muscles, which are tensing in preparation to run.
Stored glucose and fats are released into the bloodstream, supplying Bob with the energy he’ll need to escape his feline foe.

As the initial dose of adrenaline subsides, the second component of the stress response system is activated:

the HPA axis, a network consisting of the hypothalamus, the pituitary gland, and the adrenal glands.

A diagram of the HPA axis. CRH and ACTH are hormonal messengers.

Through a series of hormonal signals, the adrenal glands release cortisol, the stress hormone, keeping the body in a state of high alert, essentially holding down the gas pedal as long as Bob is still perceiving a threat. When he has made it safely to the top of a tree out of the cougar’s reach, Bob’s cortisol levels finally begin to fall, and the parasympathetic nervous system kicks, acting as the brake to dampen the stress response.³

Nature has come up with an amazingly intricate system to balance the need for both arousal and relaxation. Unfortunately, this system evolved in a much different context than we find ourselves today.

Instead of cougars, we have deadlines, traffic jams, and relationship difficulties. Our culture has evolved much faster than our biological mechanisms ever could.

With the rapid development of science and technology, and a transition from a hunter-gatherer to a modern way of life, the nature of stress has changed dramatically.⁴ Instead of the brain reacting to one big stressor, efficiently transitioning between states of stress and relaxation, we are constantly bombarded with low-level stressors, resulting in many people today existing in a constant state of hyperarousal.³

Just like a car, the constant pressure of our foot on the gas pedal causes wear-and-tear on the system over time. How does this constant stress affect our bodies in the long term?

Lasting Impact of Chronic Stress

Increasing amounts of research is beginning to show a strong link between chronic stress and an increased risk for physical and psychiatric disorders.

Surprisingly, an estimated 75%–90% of human diseases are related to the activation of the stress system.

Just some of these include cardiovascular and metabolic diseases, psychotic and neurodegenerative disorders, and cancer.

Cardiovascular diseases are the number 1 cause of death worldwide, with 17.9 million lives lost each year. Both stress in early life, such as child abuse and family conflict, and adult life, including low-income and high job demands, have been linked to increased risk of coronary heart disease.⁴

What is the mechanism underlying this extensive link between stress and human disease?

While much research still needs to be done, one promising contender is inflammation.

Inflammation is the immune system’s response to a foreign invader, be it a bacteria, virus, cancer, or a transplanted organ. It can also act in response to psychological stressors and tissue injury.

Just like the autonomic nervous system, inflammation strives to maintain homeostasis of the bodily functions. It too is regulated by hormonal messengers, one of them being cortisol. The increased release of cortisol from the stress response causes the body to habituate to these higher levels, effectively shifting the body’s natural balance.

Because of these increased levels, tissues become less sensitive to cortisol and both the inflammatory and immune response become less regulated. This results in tissue breakdown and decreased immune system functioning.⁵

Many researching this link are coming to the conclusion that stress-induced chronic low-grade inflammation might be the “common soil” of stress-related diseases. Of course, there are many factors that contribute to this, including genetic predisposition, aging and lifestyle.⁴

There are many negative affects of stress on the mind and body as we have seen. But can our fight-or-flight system be used for good?

Good Stress?

While the effects of chronic stress are nothing to ignore, there is a positive impact that our stress response has on our lives. If we were just relaxed all the time, we would never get anything done! The secret is to channel the fight-or-flight response and use that excess energy towards an actionable goal, and in moderate amounts. This kind of stress is referred to as

eustress - where the gap that exists between a stressful situation and the ability one has to deal with it is seen as a challenge to be overcome, as opposed to a debilitating hindrance.

Eustress can supply us with motivation for an unpleasant task, like the stress of a deadline for your English paper powering you through a late-night writing session. Stress can also push us out of our comfort zones, forcing us to confront and adapt to uncomfortable situations. Though giving a presentation in front of your entire class may be far from your cup of tea, you will come away from the experience with improved public speaking skills, leaving you more prepared for your next speech.

Stress originating from a shared experience can also promote bonding, as there’s a sense that everyone is trudging through the mud together. From my own experience, I know for a fact I wouldn’t have gotten through my earlier coursework in computer engineering if it weren’t for the relationships with my peers that were strengthened through our collective struggle.

There are many ways in which stress can be utilized for good. And it would be a bleak life if all stress was removed, one characterized by stagnation and insignificance. For what is life without goals, dreams, aspirations, adventures into the unknown, and the hard work it takes to pursue them? Many of the most rewarding, memorable and meaningful aspects in life are only made possible through our ability to harness our stress for good.

It’s clear that stress can have a profound impact on our physical and mental wellbeing. I want to underscore the importance of regularly doing a personal inventory of the effect of stress on your daily life and taking steps to better manage it for the benefit of your overall health. In my next post, I will explore some of these coping mechanisms and explain how to activate the natural relaxation response that we all have access to at any time. Additional mental health resources can be found on my "Resources" tab.

If you feel overwhelmed by stress, get immediate help by calling the SAMHSA’s National Helpline – 1-800-662-HELP (4357).

Love and Light, Erin.