Fight Or Flight
The stress response likely developed in our early ancestors to help them survive. A real or perceived threat causes a cascade of stress hormones to be released throughout the body. Glucose, for energy, is released into the blood stream. Muscles prepare to work hard. Systems unnecessary for our short-term survival, such as digestion or reproduction, slow down. These physical changes give a person enough energy to run away from a fierce predator, or fight another person for food.