The fight-or-flight response (or acute stress response) is a set of physiological changes that occur when an animal is threatened. The changes include increased heart rate, breathing rate and blood pressure.
This response was first described by W.B Cannon. He found that animals react to threats with a general discharge of the sympathetic nervous system. This leads to changes such as those mentioned above. The changes prepare the animal for fighting or fleeing. This response is the first stage of a general adaptation that regulates stress responses among vertebrates and other organisms.