fightorflight

Fight-or-flight, or the acute stress response, is a rapid physiological reaction to perceived danger that prepares the body to confront or escape. Coined by Walter Cannon in the early 20th century, the term describes a coordinated set of autonomic and hormonal changes that mobilize energy and attention for immediate action. It occurs in humans and many vertebrates.

Mechanism: The response begins in the brain with the hypothalamus activating the sympathetic nervous system and triggering the adrenal medulla to release adrenaline (epinephrine) and noradrenaline (norepinephrine). The hypothalamic-pituitary-adrenal axis also releases cortisol to sustain energy during longer stress.

Physiological effects include increased heart rate, blood pressure, and respiratory rate; dilated airways; pupil dilation; and redirecting blood to skeletal muscles. Metabolic changes raise glucose availability. Digestive processes momentarily slow, while sweating and heightened vigilance support rapid decision-making and action.

Threat processing often involves the amygdala, which can invoke rapid, automatic subcortical pathways or slower cortical appraisal. The response can be shaped by context, prior experience, and coping strategies, and may culminate in fight, flight, or, in some cases, freeze.

The fight-or-flight response is adaptive for acute threats but can contribute to health problems when chronically activated, such as hypertension, anxiety, or metabolic disturbances. It is one component of the body's broader stress response system and varies across individuals and species.