Fight or Flight Response

The Fight-or-Flight Response

When an organism perceives a threat — whether physical (a predator) or psychological (an examination) — a coordinated physiological response is triggered that prepares the body for vigorous action: the fight-or-flight response. This response involves both the nervous system and the endocrine system, operating via two partially overlapping pathways that differ in their speed and duration: the sympathetic-adrenal-medullary (SAM) axis and the hypothalamic-pituitary-adrenal (HPA) axis.

The SAM Axis: The Rapid Response

The SAM axis provides the immediate, fast-acting component of the fight-or-flight response. When the brain detects a stressor, the hypothalamus — acting as the integration centre — activates the sympathetic division of the ANS. The sympathetic nervous system directly stimulates the adrenal medulla (the inner layer of the adrenal glands) to release adrenaline and noradrenaline into the bloodstream within seconds.

Adrenaline and noradrenaline are both neurotransmitters and hormones (sometimes called catecholamines). Their release amplifies and sustains the direct sympathetic effects, producing a rapid, whole-body mobilisation for action:

• Increased heart rate and blood pressure — to deliver more oxygen and glucose to muscles and brain
• Increased breathing rate and bronchodilation — to maximise oxygen intake
• Mobilisation of blood glucose from glycogen stores — providing fuel for action
• Dilated pupils — improving visual sensitivity to the threat
• Inhibited digestion and reduced immune activity — redirecting energy away from non-urgent processes
• Increased sweating and muscle tension — preparing for physical exertion and thermoregulation

The HPA Axis: The Sustained Response

The HPA axis activates more slowly (minutes rather than seconds) and provides the sustained component of the stress response. The hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH) into the bloodstream. ACTH travels to the adrenal cortex (the outer layer of the adrenal glands) and stimulates the release of cortisol (a glucocorticoid hormone).

Cortisol has several important functions in sustaining the stress response: it maintains elevated blood glucose (by stimulating gluconeogenesis — the production of glucose from non-carbohydrate sources); suppresses immune and inflammatory responses; and maintains the heightened physiological state of arousal. Cortisol levels remain elevated for much longer than adrenaline — sometimes hours — providing sustained energy mobilisation during prolonged stress.

Negative Feedback and Recovery

Once the stressor has passed, negative feedback mechanisms restore homeostasis. Elevated cortisol feeds back to the hypothalamus and anterior pituitary, reducing CRH and ACTH release — thereby reducing further cortisol production. The parasympathetic division of the ANS becomes relatively more active, counteracting sympathetic activation and restoring the body to its resting state: heart rate slows, digestion resumes, and muscle tension dissipates.

The Adaptive Value and Limitations of the Response

The fight-or-flight response evolved as an adaptive mechanism for responding to acute physical threats in the ancestral environment. The physiological changes it produces — increased energy, strength, alertness, and speed — are highly adaptive when the threat requires immediate physical action.

However, in modern life, the same response is triggered by psychological stressors (work pressure, social threats, financial worry) that do not require physical action. Chronic activation of the stress response — with persistently elevated cortisol and sympathetic activity — has significant health consequences: cardiovascular disease (high blood pressure, atherosclerosis), impaired immune function, disrupted sleep, cognitive impairment (hippocampal damage from prolonged cortisol exposure), and increased risk of anxiety and depression. This mismatch between the evolved stress response and the demands of modern psychological stressors is a central theme in health psychology.