The Stress Response Glossary: Key Terms Explained Simply

Stressor

Any internal or external demand that disrupts the body's equilibrium. Stressors can be physical (injury, illness), psychological (deadline pressure, conflict), or social. The body responds to perceived stressors even when the threat is not physically present.

Fight-or-Flight Response

An automatic physiological reaction — first described by Walter Cannon — in which the body prepares to confront or escape a perceived threat. Heart rate, breathing, and blood sugar rise; digestion and non-essential functions slow. The response is orchestrated by the sympathetic nervous system.

HPA Axis

The Hypothalamic-Pituitary-Adrenal axis is a three-part hormonal cascade the brain uses to regulate the stress response. The hypothalamus signals the pituitary gland, which in turn prompts the adrenal glands to release cortisol. It functions like a thermostat — with feedback loops designed to return the system to baseline.

Cortisol

Often called the primary stress hormone, cortisol is released by the adrenal glands in response to HPA axis activation. In short bursts it helps mobilise energy and sharpen focus; chronically elevated levels are linked to disrupted sleep, immune suppression, and mood changes.

Adrenaline (Epinephrine)

A fast-acting hormone and neurotransmitter released from the adrenal medulla within seconds of perceiving a threat. Adrenaline accelerates heart rate, dilates airways, and redirects blood to muscles — the immediate fuel of the fight-or-flight response.

Sympathetic Nervous System

The branch of the autonomic nervous system responsible for activating the fight-or-flight response. It operates largely outside conscious control and can be thought of as the body's accelerator pedal during stress.

Parasympathetic Nervous System

The counterpart to the sympathetic system, often called 'rest-and-digest.' It promotes recovery, slows heart rate, and supports digestion. Practices like slow diaphragmatic breathing are thought to stimulate this system.

Allostatic Load

A cumulative measure of physiological wear and tear resulting from repeated or prolonged stress exposure. High allostatic load is associated with increased risk for heart disease, metabolic disorders, and mental health conditions. The concept was developed by researchers Bruce McEwen and Eliot Stellar.

Acute vs. Chronic Stress

Acute stress is short-lived and often resolves once the stressor passes. Chronic stress persists over weeks, months, or years, keeping the HPA axis activated and raising allostatic load. The body adapts differently to each type, with chronic stress carrying greater long-term health implications.

Neuroplasticity

The brain's capacity to reorganise its structure and function in response to experience. Chronic stress can alter areas like the prefrontal cortex and hippocampus, affecting memory and emotion regulation. Conversely, stress-management practices may support positive neuroplastic changes.

Dysregulation

In mental health contexts, dysregulation refers to difficulty managing emotional or physiological responses — for example, an overactive stress response that fires in the absence of real danger. It is distinct from a clinical diagnosis and describes a pattern rather than a permanent state.

Window of Tolerance

A concept developed by psychiatrist Dan Siegel describing the optimal zone of arousal in which a person can function effectively. When stress pushes someone outside this window — into hyperarousal or hypoarousal — thinking and coping become harder.