Alarmed, your pituitary and adrenal and adrenal glands flood your bloodstream with their excited hormones. Your pulse quickens. Your lungs suck in more oxygen to fuel your muscles. Your blood sugar soars to provide a burst of energy. Digestion slows. You break into a sweat. All of these biological responses happen in an instant, triggered by stress. They prepare you - in the same way they prepared our ancestors - for fight or flight.