Fear, once triggered, spreads far and wide in the human body. Up top, in our brains, the amygdala is responsible for registering frightful stimuli, and triggering a response in the nervous system. From there, the hypothalamus flushes the bloodstream with the stress hormones cortisol and adrenaline—remember that later. This causes your heart rate to rise and lungs to dilate, allowing more oxygen into the system. Organs that prove unnecessary in a life-or-death interaction, like the gastrointestinal system, shrink, pushing contents up or out. Every other resource is sent to the muscles, which you’ll need for a flight or fight response.