VCE Psychology (Year 12) · Nervous System and Stress Response · 13m read

Stress — Acute, Chronic and the GAS Model

Have you ever noticed your body feels completely different when you're stressed for five minutes versus five months? Hans Selye's General Adaptation Syndrome (GAS) explains exactly what's happening: your body moves through three stages—alarm, resistance, and exhaustion—each triggering different physical responses. Understanding this model helps you recognize why short-term stress can actually help you perform, but prolonged stress wears your immune system down and contributes to serious health problems you'll need to know for VCE psychology.

Learning objective: Describe physiological responses to stress and apply Selye's General Adaptation Syndrome (GAS).

By the end of this lesson you can:

• I describe the role of cortisol in chronic stress.
• I identify the three stages of the GAS model.
• I explain why chronic stress can cause illness.

Stress is an alarm system. Acute stress is a quick alarm; chronic stress is an alarm that never switches off and wears the system down.

Acute vs chronic, and the GAS stages

Acute stress is short-term. Sympathetic nervous system → adrenal medulla → adrenaline + noradrenaline into bloodstream. Fast but short-lived response (seconds to minutes).

Chronic stress is prolonged. Hypothalamic-Pituitary-Adrenal (HPA) axis activates: hypothalamus → pituitary → adrenal CORTEX → cortisol. Cortisol keeps glucose available and suppresses non-essential functions. Useful short-term, damaging long-term.

Selye's General Adaptation Syndrome (GAS) — 3 stages:
1. Alarm — initial shock (resistance briefly below normal), then countershock (SNS activation, adrenaline).
2. Resistance — body adapts, cortisol remains high. Appears to cope, but resources drain.
3. Exhaustion — if stressor persists, resources deplete, immune function falls, illness emerges (hypertension, infections, depression).

Chronic cortisol suppresses the immune system — one reason chronically stressed people get colds more often.

Stress essentials

• Acute: Fast. Sympathetic + adrenaline.
• Chronic: Slow. HPA axis + cortisol.
• GAS stages: Alarm → Resistance → Exhaustion.
• Cortisol long-term: Suppresses immune function, damages health.

Quick reference

| GAS stage | Description |
|---|---|
| Alarm reaction (shock) | Resistance drops |
| Alarm reaction (countershock) | Resistance rises |
| Resistance | Sustained cortisol |
| Exhaustion | Resources depleted |
| Cortisol | Stress hormone |

Worked example (Medium)

Ella has been under intense pressure finishing Year 12 exams for weeks. She falls ill with tonsillitis in the last week. Use GAS to explain.

1. Identify the stressor. Prolonged exam pressure = chronic stressor.
2. Map to stage. Weeks of high cortisol → resistance stage, progressing to exhaustion.
3. Explain immune link. Chronic cortisol suppresses immune system; previously dormant infection flares up.

Answer: Chronic exam stress activated the HPA axis; sustained cortisol kept Ella functioning (resistance) but suppressed her immune system. As her reserves depleted (exhaustion), her body couldn't fight off infection, resulting in tonsillitis.

Worked example (Hard)

Maria is a Year 12 student experiencing chronic stress from academic pressure. Over several months she develops frequent colds, difficulty sleeping, and stomach problems. Use the GAS model to explain her physiological decline.

1. 1. Alarm reaction stage. When Maria first perceived academic pressure as threatening, her body entered the alarm reaction. The hypothalamus activated the sympathetic nervous system and HPA axis, releasing adrenaline and cortisol. Heart rate increased, energy was mobilised, and non-essential functions (digestion, immune response) were suppressed.
2. 2. Resistance stage. As the academic pressure continued over weeks, Maria's body entered the resistance stage. Cortisol levels remained elevated to maintain energy supply. She appeared to cope on the surface, but her body was working at an unsustainable level — immune function remained suppressed and sleep hormones were disrupted by sustained cortisol.
3. 3. Exhaustion stage. After months of chronic stress, Maria's body's resources became depleted. She entered the exhaustion stage: her immune system could no longer fight infections (frequent colds), sustained cortisol disrupted her circadian rhythm (insomnia), and suppressed digestive function manifested as stomach problems.
4. 4. Link to the biopsychosocial model. Maria's experience illustrates the biological consequences of chronic psychological stress — and highlights why interventions must address not just symptoms (colds, insomnia) but the source of stress (academic pressure, coping strategies, social support).

Answer: GAS stages: Alarm (sympathetic activation, cortisol release) → Resistance (sustained cortisol, coping but at cost) → Exhaustion (resource depletion, immune suppression, illness). Chronic stress depletes the body's ability to maintain homeostasis.

Common mistakes

• Confusing acute and chronic stress responses Acute stress is short-term and adaptive (e.g. dodging a car). Chronic stress is prolonged and harmful. Students often describe chronic stress symptoms for an acute scenario or vice versa. Fix: Check the timeframe in the question. If the stressor is brief and immediate = acute (fight-or-flight). If the stressor persists over weeks/months = chronic (GAS model, cortisol effects).
• Describing GAS stages without linking to specific physiological changes Simply naming "alarm, resistance, exhaustion" earns minimal marks. The examiner wants specific physiological detail at each stage. Fix: For each stage, name at least one hormone (cortisol, adrenaline) and one bodily change (heart rate, immune suppression, cortisol depletion). Link each change to the symptoms described in the scenario.
• Saying stress is always harmful Acute stress is adaptive — it mobilises energy and sharpens focus for immediate threats. Only chronic or unmanaged stress leads to negative health outcomes. Fix: Distinguish between eustress (positive, motivating stress) and distress (negative, harmful stress). Note that the fight-or-flight response evolved as a survival mechanism.

The HPA axis — the body's long-term stress response system

While the sympathetic nervous system produces the immediate fight-or-flight response (via adrenaline from the adrenal medulla), the HPA axis (hypothalamic-pituitary-adrenal axis) manages the body's sustained stress response through cortisol.

The process works as follows: (1) The hypothalamus detects a stressor and releases CRH (corticotropin-releasing hormone). (2) CRH stimulates the anterior pituitary gland to release ACTH (adrenocorticotropic hormone). (3) ACTH travels through the bloodstream to the adrenal cortex, which releases cortisol.

Cortisol has several functions: it maintains elevated blood sugar for energy, suppresses the immune system (to redirect resources), and inhibits non-essential functions like growth and reproduction. In the short term, these effects are adaptive. In the long term, they become destructive.

Chronic cortisol elevation is linked to: hippocampal damage (impairing memory consolidation), immune suppression (increased susceptibility to illness), disrupted sleep architecture (difficulty entering deep sleep), and metabolic changes (increased abdominal fat storage).

The HPA axis also has a negative feedback loop: when cortisol levels rise sufficiently, the hypothalamus and pituitary reduce CRH and ACTH production. However, chronic stress can dysregulate this feedback loop, leading to either persistently elevated cortisol or a blunted cortisol response — both of which are associated with mental health disorders.

For VCAA, ensure you can distinguish between the fast pathway (sympathetic NS → adrenal medulla → adrenaline) and the slow pathway (HPA axis → adrenal cortex → cortisol).

Practice questions

1. Cortisol is primarily released during:

   1. Chronic stress (HPA axis)
   2. Acute stress (sympathetic)
   3. Parasympathetic activation
   4. Reflex responses

   (2 marks)

2. In which stage of GAS does the immune system begin to fail, leading to illness?

   1. Exhaustion
   2. Alarm
   3. Resistance
   4. Countershock

   (2 marks)

3. Evaluate one strength and one limitation of Selye's GAS model.

   (4 marks)

Summary

• Describe the role of cortisol in chronic stress.
• Identify the three stages of the GAS model.
• Explain why chronic stress can cause illness.