Stress depends whether you think a situation around you is worthy of making you anxious. If it does, then it's down to how your body reacts to your thought processes. This instinctive stress response to unexpected events is known as 'fight or flight'.

Flight or flight
The fight or flight response was first noted by one of the early pioneers in stress research, Walter Cannon. In 1932 he established that when an organism experiences a shock or perceives a threat, it quickly releases the adrenalin hormone to help it survive.

In humans, as in other animals, adrenalin helps us to run faster and fight harder. It increases the heart rate and blood pressure, delivering more oxygen and blood sugar to power important muscles. It increases sweating in an effort to cool these muscles, and help them stay efficient. It diverts blood away from the skin to the core of our bodies, reducing blood loss if we are injured. As well as this, adrenalin focus' our attention on the threat, to the exclusion of everything else. All of this significantly improves our ability to survive life-threatening events.

Life-threatening events are not the only ones to trigger this reaction. We experience it almost any time we come across something unexpected or something that frustrates us. When the threat is small, our response is small and we often do not notice it among the many other distractions of a stressful situation.