Fight and flight reaction in human body is brought about by the activation of the sympathetic nervous system. The sympathetic drive causes release of adrenaline (epinephrine) from the adrenal medulla and noradrenaline from sympathetic nerve endings. The result is an increase in the heart rate and blood pressure, decreased salivation, dilatation of pupils and increase in the blood level of glucose etc.
The aim of the response is to prepare the body to provide with blood to the important organs (muscles to give power, brain to plan) to fight or run. Dilated pupils makes the field of vision more broader. Diversion of blood from the skin reduces bleeding during the response even if injured.
This response was an important response for survival evolutionarily but currently the use of the response is limited to conflicts (e.g. - a fight between two kids to battle field)and sports.