The body has a protection system that has not yet evolved and acts the same way it did thousands of years ago. When it senses danger, it triggers a stress response intended to keep you alive. The type of danger was generally from animals or other humans, requiring physical action. Just imagine being chased by a sabre tooth tiger or attacked by a member of your tribe; running or fighting would be the only way to survive. Therefore, this response is called the ‘fight or flight’ response.
This response is carried out by the subconscious part of your brain, also known as the unconscious. The subconscious mind works much faster than the conscious mind; it is approximately 1 million times more powerful. The subconscious part of your brain reacts immediately in this situation because its purpose is to keep you alive.
When your protection system senses a threat, it triggers a series of reactions in your body that ultimately create and pump adrenaline around your body. This prepares you for the immediate action of fighting or fleeing the situation. The adrenaline causes a change in body temperature and blood pressure. Blood is diverted away from parts of the body, such as digestion and frontal lobes, and towards the arms and legs. There is no need for the digestive system to work hence the dry mouth, or for the brain to think logically at that exact moment because all the body cares about is enabling you to fight or run away. It just needs your arms and legs to work.
If the stress response is triggered by a physical threat, running or fighting would utilise the adrenaline. In today’s society, the danger is usually perceived and not physical, which means the adrenaline and other chemicals the body creates are not used up. The effects of adrenaline running through the system include dry mouth, funny tummy, rapid heartbeat, anxiety, sweating and palpitations.
The subconscious can also decide to freeze when faced with a dangerous and stressful situation because any noise or movement you make may put you in more danger, so the safest option may be to freeze.
A more details explanation of the stress response can be found here.