There are two names for the fight or flight response. There is that and the acute stress response. Every animal has this response for fighting or fleeing. There are nine parts You breath harder, your heart beats faster, some parts of your body gets smaller, your blood vessels get bigger, your muscles get bigger, your bladder gets relaxed (so that's why cartoon characters pee themselves when they get frightened) and your pupils get bigger. Imagine a zebra eating the grass, when it sees a lion, it sprints away.