Definition
The general adaptation syndrome (GAS) is Hans Selye's three-stage model of how the body responds to any prolonged stressor, regardless of its specific nature. The first stage, alarm, is the immediate fight-or-flight surge — adrenaline, cortisol, raised heart rate, mobilised energy. The second, resistance, is the body's attempt to cope by maintaining elevated arousal at a sustainable level. The third, exhaustion, sets in when the resources required to sustain resistance are depleted — and it is in this stage that the stress response itself becomes the source of disease, as cumulative wear on cardiovascular, immune, and metabolic systems produces lasting harm.
Why it matters
How it works
When the brain registers a threat, the hypothalamus triggers the sympathetic nervous system to release adrenaline, producing the rapid changes characteristic of alarm — increased heart rate and blood pressure, redirected blood flow to muscle, sharpened attention. In parallel, the hypothalamic-pituitary-adrenal (HPA) axis releases cortisol, which mobilises glucose, suppresses non-essential systems like digestion and immunity, and sustains the body's heightened state. If the stressor passes, these systems return to baseline. If the stressor persists, the body shifts into resistance — operating above baseline but trying to function normally.
Resistance can be sustained for a while, but at a cost. Cortisol that protects in the short term damages in the long term: it impairs immune response, accelerates cardiovascular disease, contributes to depression, and remodels brain regions like the hippocampus that are important for memory. Exhaustion is the stage at which these costs surface as disease. Selye's insight was that for many illnesses, the relevant causal arrow runs from chronic adaptation back to the body rather than from the stressor directly. Modern research has extended this with the allostatic load model, which quantifies cumulative physiological cost across the systems the stress response engages.