Feeling stressed or feeling a sense of being under threat can trigger the 'fight or flight' response which activates the sympathetic nervous system. Amongst other things, one of its primary jobs is to regulate your heart rate and breathing. When your brain perceives threat or danger, this system responds by activating the body, producing an increase in heart rate, a boost in levels of hormones including adrenalin, cortisol and glucagon and an increase in the amount of oxygen in your blood.
Those changes are priming you for action; to fight a terrible enemy or to run as fast as you can.
Unfortunately the kind of things which we fear or that stress us out aren’t always things you can hit or run away from and this is where problems can occur. If you’re not using up the extra oxygen, adrenalin etc to run away or fight for your life, you can begin to feel very unwell and unstable – frightening symptoms can include shaking, pins and needles, over breathing, a racing heart and changes in visual perception. Many people report a feeling of losing control, and in some cases people feel seriously ill.
As you might expect, people who’ve felt like this do not want to feel this way again and so they try to avoid the situations about which they have felt anxious or afraid. That isn’t always a great idea, though, because if you avoid things that make you stressed you’re telling yourself that you can’t deal with them. You can.