Humanity has survived and thrived on this earth for hundreds of thousands of years, through some of the most difficult and stressful of times.  One of the factors that has enabled us to survive is a system called the fight/flight response.  Whenever we perceive we are physically or psychologically threatened an inbuilt, reflex, alarm-system in our brain triggers the release of electrical impulses and a variety of hormones. There is a complex hormonal cascade of over 30 stress hormones, such as adrenaline, noradrenaline and cortisol, which have a powerful and widespread effect on our body's biochemistry, physiology and psychology, giving us the extra strength and speed we need to deal with the threatening situation.  If a wild animal attacks us, we can either run away (flight) or if trapped, stand our ground and fight. 

The fight/flight response is a 40,000-year-old model, its alarm reaction is designed for short-term use to deal with physical threats in which the emergency resolves very quickly in a few seconds or minutes; either we kill the wild animal or it kills us!  But life in the 21st Century is infinitely far more complex than it was 40,000 years ago (or even 50 years ago for that matter). Many of the stressors today are psychological in origin, and they are chronic, lasting days, weeks, months, even years in some cases.  Modern stressful events such as financial problems, health worries, work problems, difficult neighbours, relationship problems, etc. can not be resolved by fighting or running away; never the less these psychological stressors still trigger the fight/flight response.

The fight/flight response is designed to be triggered occasionally.  However modern living keeps tripping it, making it overactive and this can be a factor in causing stress related health problems.  These problems are then made worse by the typical western diet, full of fat and sugar which can cause increased blood clotting, increased blood pressure and can stimulate higher levels of stress hormones in the blood stream.

This is then complicated by the fact that we do not get enough physical exercise.  Exercise can help to counter the stress response by - reducing blood clotting, boosting immune function, reducing blood pressure, relaxing muscles, increasing metabolism which burns up stress hormones and making the sympathetic nervous system (which triggers the stress response) less sensitive. 

When the fight/flight response is triggered over 1400 different physiological and biochemical changes occur in the body.  But there are also psychological effects making us more alert, aggressive, angry, fearful etc., which all motivate us when we are physically threatened, but have to be suppressed during a meeting with the Bank Manager that doesn't go so well!

Having said this, the fight/flight response is still an important part of our body's vital defence systems.  Even though the majority of physical threats have gone, there are still situations today where the fight/flight response can be life saving; such as escaping a house fire or speeding up the reflexes to avoid an accident on the motorway.

For example, a few years ago a farm worker had his arm torn off by a bailing machine, yet he was able to pick up his severed arm, carrying it for two miles, to get help.  It was the fight/flight response that enabled him to do this.

The Two Stages of the Fight/Flight Response

There are 2 co-ordinated stages to the fight/flight response:

1.  Short Term Fight/Flight Response

2.  Long term Fight/Flight Response

1.   Short Term Fight/Flight Response (SAM Sympathetic Adreno Medullary) Axis

The SAM or Short-term response is the primary system that is triggered within us in response to short-term threats.  This is a reflex response, which is electrically triggered.  Electrical impulses from the hypothalamus, a gland located in the brain, travel along nerves that directly connect to the adrenal glands (these sit on top of the kidneys) and stimulate the release of stress hormones adrenaline and noradrenaline.  The body cant sustain this short-term fight/flight response for long because it would become exhausted.  If the stressor is a more chronic one then this triggers the secondary, longer-term fight/flight response to take over. 

2.  Long Term Fight/Flight Response (HPAC Hypothalamic Pituitary Adreno Cortical) Axis

The longer-term fight/flight response is triggered hormonally.  This time the hypothalamus secretes a hormone called CRF (Corticotrophin Releasing Factor), which stimulates the pituitary gland (also located in the brain) to produce ACTH (Adrenocorticotropic Hormone), which in turn stimulates the adrenal cortex (outer part of the adrenal glands) to release stress hormones like cortisol.  This longer-term fight/flight response is affected by our perception of the event, which decides the type and amount of stress hormones that are secreted.  Research has shown that chronic activation of this longer-term HPAC fight/flight response can be a factor in causing a number of psychological and physiological health problems.

Nerve impulses travel at 150 metres per second so the first stage of the fight/flight response the short-term response - occurs very quickly, literally in milliseconds; whereas the second, long-term (HPAC) response takes several minutes to kick in and lasts longer. 

Fight/Flight Response and The Autonomic Nervous System

The fight/flight response is controlled by the Autonomic Nervous System (ANS) which is the part of our nervous system that controls the automatic functions of the body (those not under our conscious control such as regulating heart beat, digestion etc.)

The Autonomic Nervous System is made up of two branches:

1. The Sympathetic Nervous System (SNS).  This is the system which triggers the biochemical and physiological changes brought about by fight/flight.  Think of it as the accelerator on a car or a supercharger on an engine which pump in more air and fuel to increase speed.

2.  Parasympathetic Nervous System (PNS).  The second part of the ANS is the Parasympathetic Nervous System, which helps to switch off the fight/flight response and return all hormones, organs and systems, back to pre-stress levels.

Our body's organs such as the brain and heart, and systems such as the endocrine, immune and digestive systems, are all hard-wired into the Autonomic Nervous System.  As a result the Sympathetic branch can speed-up an organ or system and the Parasympathetic branch can slow-down an organ or system. 

When the Stress is Over

The fight/flight response is triggered by the Sympathetic branch of the Nervous System, but three minutes after we perceive that the threat is over, the brain, via the hypothalamus, stimulates the Parasympathetic nervous system which switches off the fight/flight response.  The stress hormones fall back to pre-stress levels, reversing all the biochemical and physiological responses; blood pressure falls, heart rate slows down, digestion is stimulated again.  However if the stress becomes chronic the fight/flight response can become overactive and maintaining factors can leave us with heightened stress levels even though the original stress may be over.

Threats Real or Imagined

The brain cannot distinguish between a real or potential threat.  It can only respond to both, by triggering the fight/flight response.  For example research has shown that our levels of stress hormones rise when we watch a horror film even though we are not physically experiencing the stressor.

Biochemical and Physiological Changes caused by the Fight/Flight Response

All the biological, psychological, chemical and physiological changes brought about by the fight/flight response are designed to give us extra strength and speed to help us fight or run away.  They are vital in the short-term but in the long-term they can have a potentially negative impact on our physical, psychological and social well-being.  Some of the changes that occur include:

• Blood Flow

Our muscles are a vital part of fight-flight.  Blood flow to our muscles is increased by 300%, by being diverted from less important areas like the skin.

• Blood Pressure

In order to pump the extra blood, oxygen, fats and sugars to the muscles to supply energy, our blood pressure and heart rate increase.

• Blood Sugar and Fats Increase

Stored reserves of fats and sugars are converted and released into our blood stream to supply extra energy to fight or run away.

• Red Blood Cells Increase

The spleen manufactures more of the oxygen-carrying, red blood cells and releases them into the blood stream in order to get more oxygen to our muscles.

• Breathing Rate Increases

The muscles need extra oxygen for fight-flight.  To supply this extra oxygen requirement our breathing rate speeds up and the airways in the lungs widen.  Breathing switches from relaxed, slow, diaphragmatic breathing, to fast, shallow, chest breathing.

• Muscles Tense

In order to give us extra speed and strength the muscles of our body tense.

• Digestion slows

Blood is diverted from less vital areas such as digestion, to more vital areas such as muscles; digestion slows and stomach acidity increases.

• Pupils Dilate

In order to help us see more clearly, our pupils widen to let in more light.

• Hearing

Our hearing becomes sharper.

• Perspiration Increases

During fight/flight our metabolic rate increases and so we get hotter because of all the biochemical reactions going on in the body; to help prevent us from over-heating we need to cool down so perspiration increases.

• Dry Mouth

Our mouth dries up and digestive juices reduce as blood is diverted from less important areas to provide energy needed elsewhere in the body.

• Fear/Anxiety

The stress hormone adrenaline, primes an area of the brain called the Amygdala to feel increased anxiety and fear, our thoughts race and we can think more quickly. 

Factors which Influence the Fight/Flight Response

Stress is not simply a case of cause and effect.  There are a variety of factors that influence - whether the fight/flight response is triggered, how long it remains switched on and the degree to which it has a negative effect on us.  We can have an affect on these influencing factors and so therefore can have a major influence over how severely stress affects us.

Factors which influence the fight/flight response, some of which we can influence to reduce our stress, include: