The hypothalamic-pituitary-adrenal (HPA) axis is the body's primary stress response system. When a threat is perceived, the hypothalamus releases corticotropin-releasing hormone (CRH), which triggers the pituitary to release ACTH, which in turn stimulates the adrenal glands to produce cortisol.
Cortisol mobilizes energy, suppresses inflammation, and sharpens focus — adaptive in the short term. Chronic activation leads to allostatic overload: disrupted sleep, immune suppression, hippocampal atrophy, and metabolic syndrome. The HRV (heart rate variability) is a sensitive marker of autonomic balance and resilience.
Selye's General Adaptation Syndrome: Alarm → Resistance → Exhaustion. Recovery restores baseline through glucocorticoid negative feedback on both hypothalamus and pituitary.