Intro to the Science of Stress

This post is the first in a series I am writing to answer the question, “What is stress?”

I think answering this question is especially necessary because, in spite of us all having bodies, none of us come with an instruction manual for how our complex and interconnected systems work. With the amount of diseases out there which have a greater chance of occurring or worsening with higher stress levels, this knowledge can be crucial to our own self-care and those for whom we are responsible.

When I first came up with the idea of talking about stress, it was remarked by a colleague that it could take up several posts, and even then I’d likely only be scratching the surface. And it is true— as important as it is to understand stress, it is a colossal topic.

To deal with a topic of this size I thought it best to approach it by asking the simplest questions, assuming the least familiarity with the subject possible, and then working backwards from that to give readers as complete an understanding of the answer to the main question as I am able.

Homeostasis

To talk about stress I first need to talk about homeostasis. This is a Greek word which roughly means “staying the same.”

All living things thrive under certain conditions. This is easy to see with plants, which require adequate sunlight, water, and heat, but not too much of any. For plants the levels of light, water, and heat all have a particular “Goldilocks Zone” within which plants thrive the most. All organisms have certain specific conditions under which they live and grow best, and that includes humans also. When an organism lives within these ideal conditions, it is said to be in a state of homeostasis. The specific levels of light, heat, and water a plant likes to maintain is called the set point for those conditions.

For humans, one such condition having an adequate core temperature, the set point for which in most people is 36 to 37 degrees celsius¹. When we depart from those conditions on a hot day in July, our body perceives the excess heat raising our body’s temperature above the set point as a threat to homeostasis, and by extension our wellbeing. When something moves the body away from homeostasis it is called a stressor, which as you can imagine triggers what’s called the stress response. In this instance the body responds by producing sweat to cool off and stay within homeostasis; another way it does this is by dilating blood vessels closer to the skin so there’s greater heat exchange with the air.

Alternatively, if it’s the middle of winter and our bodies perceive cold in the environment, our body hair stands on end to try and trap heat. If conditions worsen and our core temperature drops, we begin to shiver and our metabolism fires up to produce more body heat. The body almost always has more than one mechanism to help maintain homeostasis, and if the finer controls (such as erectile tissue in our hair follicles) doesn’t stave off the cold sufficiently our bodies can resort to more extreme measures. One of the advantages that most animals have over plants is that we can move and respond with complex behaviours to avoid stressors as well (for example finding shelter from the heat or cold).

Temperature regulation is an easy example to come up with when thinking of conditions for which we have set points. Just to give an idea of the amount of complex balancing the body does, our bodies also maintain levels of oxygen, blood sugar, water, carbon dioxide, pH, electrolytes, levels of pressure in the blood and the lungs, and many other biological compounds. Because of human complexity homeostasis also has a psychoneurological dimension to it as well, which will be a subject of later posts.

Allostasis

Even though our bodies function optimally at homeostasis, it is often necessary to depart from homeostasis in some instances to stave off or prevent a greater departure from homeostasis further down the road.

Our bodies function optimally at around 36 to 37 degrees Celsius. At times when our body’s immune system perceives an infection, the body can respond by producing a fever. It does this by raising the homeostatic set point for temperature to above 37 degrees. Even though our bodies do not function as well at 38 or 39 degrees as at 36 or 37, the homeostasis of the bacteria infecting our bodies is far more disrupted than our own, which is why the immune system induces fever in addition to a whole host of other responses to destroy intruders.

What’s noteworthy is that this change in the body’s temperature set point, controlled by a part of the brain called the hypothalamus, causes the body to feel cold and chills in spite of being the same core temperature as before the fever. The actual conditions did not change; the perception of them did. Our body hair stands on end, we shiver, and we bundle up more when we experience chills, and our body temperature is raised— all physiological and behavioural changes resulting from this change in perception. Then when the fever breaks we begin to feel hot and sweat instead of shiver, and our bodies eventually cool down.

The more complex an organism is, the more mechanisms they have internally for tolerating changes in homeostatic set points in order to achieve greater stability over the long-term. In the late 20th century the term allostasis was coined to describe these complex adaptations. This is also a Greek word, which to transliterate roughly means “to vary in order to remain stable.”

To help drive the point home with an example I came up with Chart 1 below:

Chart 1

Chart 1 is an abstract diagram of the change in body temperature over time when a fever starts and then ceases. Under regular conditions the core temperature swings around a regular temperature set point (in blue), but when the fever begins the body’s temperature set point elevates to a new point (in red). The core temperature then starts to oscillate around a new target temperature. This continues until the immune system switches off the fever response at 5 hours after the start of the fever, after which the set point drops back down to the regular temperature².

In the following chart we just examine the change in temperature set point itself instead of core body temperature:

Chart 2

In this more simplified image of the change in set point, we see a distinct jump. The overall move away from the more regular set point is called allostatic load. This is a planned departure from the body’s regular homeostatic set points (i.e. a stress response) in order to meet a particular stressor which may be posed to those states. In the case of a fever, the body switches to a higher set point because, even if not ideal, it is preferable to the tissue and organ damage an infection may cause.

Googling the term allostatic load produces a lot of associations with chronic stress and its associated negative effects on the organs, immune system, mental health, et cetera. This is a large area of concern for medicine, on account of the number of diseases associated with chronic stress and excessive allostatic loads straining the organ systems of the body for prolonged periods. This is the reason I take a personal interest in studying stress and its effects on our nervous system.

In spite of this, allostatic load is no more a scary concept than allostasis itself is. The circadian rhythm (day-night cycling) of our bodies, and the changes in activity levels resulting from hormonal changes throughout the day, is another example of allostasis budgeting our body’s resources and shifting set points to meet the demands of the day. Every day and every moment your body is in dialogue with the environment and figuring out how best to proceed in such a way that its own health and wellbeing is preserved.

Key Takeaways

Living things thrive when they are in a state of homeostasis. Stressors, or environmental influences which can threaten that homeostasis, are met with a stress response from the body designed to push the body back toward homeostasis. Allostasis the process by which the body manages controlled departures from homeostasis in response to, or anticipation of, dealing with current or future stressors. A change in the body’s set points away from homeostasis to deal with a such stressor is called acquiring an allostatic load.

In this (very brief) introduction I have used body temperature to illustrate the core concepts of homeostasis, stressors, allostasis, and allostatic load. In the next instalment I will be bringing these concepts to the human nervous system and hormonal system, so we can understand scientifically how our bodies, minds, and emotions use behaviour to respond to perceived threats. Hope to see you there.

1. I state a range because even though the conventional wisdom is “37 degrees Celsius” the truth is that all of these parameters exist on a bell-curve, and each of us has a core temperature which differs slightly due to our metabolism, thermal mass, insulation, stress level, circadian rhythm, et cetera.

2. Body temperature itself varies much throughout the day as a result of the circadian rhythm, but for the purpose of illustrating temperature allostasis with a fever the chart is sufficiently accurate.