The "fight or flight" response has obvious evolutionary value; the more primitive parts of our brain react to a perceived threat by getting our bodies ready to fight or run away from danger: we breathe faster to take in more oxygen to fuel energy expenditure, our heart pumps more rapidly to move blood to our muscles, diverting it from digestion and other processes, and so forth. Unfortunately, this part of our brain, which is not under conscious control, does not recognize the difference between a sabre-toothed tiger and having to give a speech in class, and these preparations are not nearly so useful in the second instance. In fact they can leave us feeling dizzy, short of breath and lightheaded. Fortunately, we can interrupt the feedback loop by consciously slowing our breathing, consciously relaxing our muscles, etc. Doing this tells our brain that there is no threat.