what is the fight or flight reaction
fight-or-flight response in Medicine Expand. fight-or-flight reaction n. A set of physiological changes, such as increases in heart rate, arterial blood pressure, and blood glucose, initiated by the sympathetic nervous system to mobilize body systems in response to stress. Also called emergency theory. fight-or-flight response. A physiological reaction in response to stress, characterized by an increase in heart rate and blood pressure, elevation of glucose levels in the blood, and redistribution of blood from the digestive tract to the muscles.