The physiology of stress

Posted on July 03, 2015

With the exams having recently ended at most universities in South Africa, none of us needs to be reminded of what causes stress. But what does stress really do to our bodies? Now, we all know how it makes us feel. Whether your hands tremble as you flip through a difficult exam paper, or you have trouble sleeping because you are worried about approaching deadlines – we have all experienced it at some time or another. While we all understand the symptoms of the stress response, few of us know what the underlying physiological mechanisms are.  

In a talk delivered at the University of Pretoria’s annual Neuroscience Day this year, Prof Nola Dippenaar, a Professor Emeritus at the University, said that in trying to understand the body’s response to stress, one should start by asking the question, ‘What is the human body actually designed for?’ To answer this question we need to go back to a time when life was a lot simpler.

Thousands of years ago, our ancestors were hunter-gatherers who lived on plants and berries, and occasionally feasted on lean meat. Their main aim in life was to find food every day and to remain safe. It is estimated that in order to do this, they walked between 10 and 16 kilometres a day. This is essentially what the human body was designed for. Built into this ‘design’ was an emergency mechanism for when they could not find food or were not safe. This is what is known as the ‘fight or flight’ response and is what we often refer to nowadays as stress.

This mechanism was of course very well designed for those days, but in the modern world we very seldom experience situations where we actually have to fight or flee. The problem now, says Prof Dippenaar, is that the flight-or-fight reaction hasn’t actually evolved and still functions in the same way as it did in our ancestors. That being said, although the “threats” we face today often take more benign forms compared to those our ancestors faced, they can be equally taxing on our bodies.

Some stress can, of course, be beneficial and the pressure it exerts can be an incentive to accomplish necessary goals. Often, however, stress reaches chronic, harmful levels, which has an undesirable effect on our bodies. This effect can include anything from a compromised immune system to weight gain. In short, brief periods of stress can work to our advantage, but chronic, long-lasting stress can be very harmful. Proper stress management takes on great importance given the wide range of bodily systems impacted by stress hormones.

A Canadian physiologist, Dr Hans Selye, did some ground breaking research on rats in 1936, where he found that when he exposed them to different stimuli, which included anything from prolonged food deprivation to an exhaustive muscular workout, there was always a generalised response. Dr Selye called this the General Adaptation Syndrome (GAS; sometimes also referred to as the stress syndrome), which, as Selye explained, causes the body to pass through three phases of coping.

First there is an ‘alarm reaction’, in which the body prepares itself for ‘fight or flight’ and the sympathetic nervous system in the body is highly activated. In addition, the adrenal glands secrete a large amount of adrenalin from the adrenal medulla into the blood stream in support of the sympathetic nervous system. This is a very short and acute phase and is essential to the survival of the person. No organism can sustain this condition of excitement for very long, however, and provided it survives whatever caused the first stage, a second phase of resistance ensues. In this second stage, certain re-adjustments occur in the body to ensure the survival of the organism. This takes place over a much longer period. Finally, because the re-adjustments made by the body in the second phase are not able to restore homeostasis in the body, a sufficiently long period of stress will cause the body to enter a stage of exhaustion.

Professor Dippenaar explained that when we examine each of these phases to understand exactly what happens inside the body during each of them, we see that the alarm reaction is actually a massive activation of the sympathetic nervous system that takes place when a threat is perceived. This action results in the ‘fight or flight’ response, which can be described in physiological terms as an increase in heart rate, dilation of bronchial airways to enhance the lungs’ capacity for oxygen, and enhancement of the metabolic rate so that more of the stored energy in the body can be used to ensure survival. Most physiologists agree that this phase rarely lasts longer than around 20 minutes.

In the second phase (also known as the resistance phase) the body secretes more hormones, in particular the chronic stress hormone, cortisol, which increases blood sugar levels to sustain energy and raise blood pressure. Although the body begins to try to adapt to the strains or demands of the environment, it cannot keep this up indefinitely and its resources are gradually depleted until the body enters the exhaustion phase where it basically suffers from ‘adrenal exhaustion’. At this stage the blood sugar levels decrease as the adrenals become depleted, leading to decreased stress tolerance, progressive mental and physical exhaustion, illness and eventually collapse.

It is interesting to note that, whether the stress is physical or emotional, the response is exactly the same. Chronic stress, such as that experienced by many people in our modern world, remains in the resistance phase, with the hypothalamus continuing to signal to the adrenals to produce cortisol. This increased cortisol production keeps the body in ‘crisis’ mode, which can lead to weight gain, poor memory, fatigue and depression. Continually high cortisol levels also lead to suppression of the immune system and makes the body more susceptible to everything from colds and flu to cancer. Fortunately, according to Prof Dippenaar, we can reduce cortisol levels in our bodies by making some relatively simple adjustments to our lifestyles.

The most effective way to reduce cortisol levels in our bodies, she says, is regular exercise. A daily exercise routine that gets your heart pumping, such as running, brisk walking or cycling, increases the body’s production of endorphins and makes you feel happy and content. In addition, Prof Dippenaar suggests stopping three times a day to eat a balanced meal that includes healthy carbohydrates, fats and proteins. If you combine the above with enough sleep (Prof Dippenaar suggests at least 7–8 hours per night), you should be well on your way to coping better with the pile of stress you are carrying around with you! A good sense of humour and making time to share with good friends and family, will of course also contribute.

Prof Dippenaar concluded her talk at Neuroscience Day 2015 by saying that we should always bear in mind that stress is always an internal experience, which implies that it is also highly personal and subjective. We need to identify what our pile of stress is made up of, and actively do something to reduce the chronic cortisol levels in our body, in order to stay healthy, productive and successful.   Find what works for you and remember: Life is short. Enjoy it!

- Author Ansa Heyl

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