The changes that happen in the body when faced with a stressor have evolved to increase the chances of survival. When our prehistoric ancestors were faced with a threat it was often in the form of a wild animal and in order to survive they would either have had to run away (flight) or defend themselves (fight).

Even though we do not (commonly!) encounter wild animals in modern society our bodies still respond to stress in the same way.

The two main systems involved in triggering the stress response are the nervous system and the endocrine system. Both of these systems perform the body’s messaging service. They relay command signals from the brain to other the organs and also relay information back from the organs to the brain.

The brain continually monitors the information it receives from our sensory organs such as our eyes and ears. If the brain detects something which might be a threat it will make a split second evaluation of the situation. If the brain perceives that there is the potential for either physical or psychological harm to occur then it sends messages via the nervous system to prompt changes which prepare the body for action.

These changes include increasing the heart rate and blood pressure, diverting blood towards the muscles and away from the intestines, skin kidneys and brain. The liver is primed to release stored sugar into the blood. It will also trigger sweating and reduce production of saliva and cause the pupils to dilate so that more light reaches the back of the eyes.

The response of the nervous system is very quick but the nervous system alone is not able to maintain these changes in the body for very long. So another immediate action of the nervous system is to trigger the adrenal glands to release a hormone called adrenaline.

Adrenaline is carried in the blood to the organs which have already been activated by the nervous system to prolong the physical changes which have already occurred. This is called the fight or flight response since it prepares the body for the physical exertion needed to either run away or defend itself. If the stress is severe and prolonged the endocrine system will also release other hormones including a substance called cortisol which acts to raise blood sugar and fats in order to sustain physical activity. All of the above changes comprise what is known as the alarm stage of the stress response.

Although the above changes are a perfectly normal response to stress which happen in a healthy body to try to keep us safe, they can cause some quite unpleasant symptoms. It is far more likely in this day and age that the stress we face on a Monday morning will involve being late for work because of a traffic jam than having to run away from a pack of wild wolves. However the body still responds by priming us for action. The increased heart rate may be felt as palpitations; the priming of the skeletal muscles can cause shaking and the reduced blood flow to the intestines can cause nausea, butterflies in the stomach and diarrhoea.

Once the brain either perceives that the stress has passed or decides that it can be tolerated the nervous system sends messages to tell the body to relax and gradually the affected organs revert to their normal resting states. This is known as the resistance stage of the stress response.

The alarm and resistance stages of the stress response may recur over and over throughout an individual’s lifetime.

However, if someone encounters very frequent stressors, particularly if these are intense, the body’s mechanisms for resisting stress may be overwhelmed leading to what is known as the exhaustion stage. In the exhaustion stage the organs involved in the stress response may show signs of disease; the body is less able to fight infections and the person may feel psychological exhaustion and depression.

The phases of alarm, resistance and exhaustion, first proposed by Hans Seyle in 1956, are collectively known as the General Adaptation Syndrome and form the basis of our understanding of how psychosocial stressors can cause physical disease.