Stress is a threat to the safety and wellbeing of the body. When we lived in caves, the physical stress response was a means of survival: it prepared us for fight or flight. What is this so-called fight or flight reaction? It is instinctive and consists of messages sent all over the body to and from the brain. These messages alert the body of a perceived threat. In a threatening situation, we are given two options: either we can stand our ground and fight the threat or we can run away from it. The choice is made based on our perception of the situation, if we feel we have a chance of overcoming the danger (e.g. winning the fight) or not. For a list of changes that occur in your body in preparation for fight of flight, please see our stress-response diagram.

To prepare our body for either fight or flight, changes occur which involve the whole body. It's important to understand that this is a massive operation and a vital survival mechanism. Not all the changes that are mentioned here occur in everyone for every stressor that comes along, but all bodies have the potential for these survival responses. The amount of preparation depends on how threatening the situation is perceived.

Information about the environment enters the body through our senses. This information is translated into electrical impulses and chemical substances. For example, the nerves in the body carry the news of threat to the brain. The brain then can communicate with all our organs, preparing them for fight or flight. (see the biochemistry of emotion for greater detail of chemistry). Below is a diagram that shows how the nerves transmit this information, by means of an impulse:

When the body perceives a threat and the brain sends out the message, the endocrine (hormonal) system moves into action. It deals with two different hormonal systems: the catecholamine and neuroendocrine systems.

• The catecholamine system is the first to react when the body is stressed and responds by releasing adrenaline and noradrenaline. The latter hormone makes the sensory circuits in the brain more sensitive, keeping all senses on guard. The release of these two hormones chiefly affects the cardiovascular (heart) system. They cause the heart rate to increase, blood pressure to increase and the reduction of the immune system's response (for example, the immune system's repair function is put on hold while the body redirects its resources elsewhere to respond to the stressor).
• The neuroendocrine system works "under" the hypothalamus and the pituitary glands, both in the brain. It releases the cortisol hormone, adrenocorticotropic hormone, growth hormone and beta endorphins.

These hormones are sending the message throughout the body to get ready to fight or run for your life. Therefore, our body has to prepare for some intensive physical exercise. The blood flow to the kidneys, intestines and skin decreases so that more blood can go pumping to the muscles. More sugar and free fatty acids are released into the blood because of this increased demand of energy. Our breathing becomes rapid and shallow and the body perspires to cool down. The pupils in our eyes dilate (become larger) so that we can see more of the surroundings in case of any more threats.

These physical reactions in the body are appropriate when you're facing a saber-toothed tiger or fire, they are not helpful to a person if they are in response to an emotional threat (such as the fear of being rejected at a movie audition or failing an exam).