The physical effects of this kind of immediate stress range from an increased heartbeat to shallow breathing. This is because there's a greater flow of oxygen into the body. Your pupils will dilate to allow more light to enter your eye. All of these things happen because of a release of adrenaline -- the body's main stress hormone. There's also a release of cortisol, another stress hormone, by the adrenal gland. Cortisol will jack up your blood pressure and blood sugar. Your liver will begin to manufacture some glucose to provide you with extra energy as well. After your stress goes away you may feel a physical crash -- this is because of the extra glucose you've burned off. It essentially leaves you with a low supply of blood sugar, like when you haven't had anything to eat all day.