Your body is the most fascinating machine. Its reaction to a stressor is called the ‘fight or flight’ syndrome. This is best explained by the
General Adaptation Syndrome (GAS) model by Hans Selye.

 
He explains it as a three step process:
 
The Alarm Stage
The minute your body senses the presence of a stressor, signals are sent to your brain about the stressor.
The brain consists of many parts, one part called the hypothalamus. The hypothalamus then arouses the autonomic nervous system (ANS).
The autonomic nervous system basically controls organs like the heart and blood vessels and lungs.
The ANS is actually further divided into 2 subparts, the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS).
The SNS is the part of the ANS that reacts during a stressful situation.
The SNS must act quickly when your body senses the presence of a stressor.
So, the nerve endings release neurotransmitters. You might have heard of these neurotransmitters called adrenalin. Adrenalin is transported to the SNS and this
enables your body to:

• Make your muscles stronger (So you can lift heavy things or run faster)
• Make your heart beat faster (so that enough oxygen is pumped to the muscles that are working harder)
• Increase sugar levels (which in turn increases energy)

 
But that’s not all! The SNS also carries a myriad of functions like dilating the pupils, constricting peripheral blood vessels,
slowing down bowel movements and of course making you think faster.
 
The Resistance Stage
While the body is still fighting the stressor during the resistance stage, simultaneously it is allocating energy to repair the tissues that are destroyed
during the alarm stage and also energy is being allocated to decrease the secretion of stress hormones. So the response of your body is not as strong as it
was during the alarm stage.
 
Hence, ideally the stressor should be eliminated or removed in the resistance stage.
 

The Exhaustion Stage
At this stage, your body has run out of all the adaptive energy required to deal with the stressor. However, if the stressor
still persists, well, your body gives up and this could result in major physical or mental health complications.