fight-or-flight response

The fight-or-flight response is the body's instant alarm: in the moment you sense danger, your heart pounds, your breathing quickens, your pupils widen, and a surge of energy floods your muscles, all before you have consciously decided what to do. It is the split-second preparation either to confront a threat or to run from it.

This rapid reaction is driven by the sympathetic nervous system acting together with the adrenal medulla, which together release the catecholamines adrenaline (epinephrine) and noradrenaline. These hormones speed the heart, divert blood from gut and skin to skeletal muscle, dilate the airways, and break down stored glycogen to raise blood glucose, all within seconds. Digestion, reproduction, and other non-urgent functions are temporarily suppressed.

The term was coined by physiologist Walter Cannon in 1915. Modern accounts often add freezing and a tend-and-befriend pattern to the original two options, recognizing that real responses to threat are more varied than fighting or fleeing alone.