The adrenaline (epinephrine) produced by the body in case of danger which increases the heart rate and pupil dilation to improve visibility and increase blood flow in the muscles to prepare the body for action.And the key to controlling the fear is to develop the confidence and ability to assess any given situation before going in, also learn to use epinephrine as a performance stimulant rather than a barrier that helps you calm while tense, this ability to evolve over time and get better day by positive thinking about self and healthy lifestyle change such as diet and exercise will help you calm under pressure.