Stress reaction and its phases

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Last update: Monday, 12 Jan 2026 at 11:24 pm.


Introduction

Stress is a physiological and psychological response of the organism to any factor that threatens homeostasis. This factor is called a stressor. Stress activates a coordinated set of neural, endocrine, and behavioral mechanisms that help the organism adapt to the challenge. Although stress is a normal and useful response, chronic or excessive stress can lead to pathological consequences.

Definition of Stress

Stress represents a non-specific response of the body to any demand placed on it. It includes activation of the autonomic nervous system, endocrine changes (especially the hypothalamic-pituitary-adrenal axis), and behavioral adaptation. The concept was first described by Hans Selye.

General Adaptation Syndrome (Selye)

Hans Selye described a three-phase model known as the General Adaptation Syndrome (GAS). It explains how the body reacts to stress over time.

The phases are:

  1. Alarm phase
  2. Resistance phase
  3. Exhaustion phase


1. Alarm Phase

The alarm phase represents the immediate reaction to a stressor.

Main characteristics-

  • Activation of the sympathetic nervous system
  • Release of catecholamines (adrenaline and noradrenaline) from the adrenal medulla
  • Increased heart rate and blood pressure
  • Pupil dilation, increased respiration
  • Mobilization of glucose and fatty acids for quick energy
  • Heightened alertness

Purpose-

To prepare the organism for “fight or flight”. This phase is short and energy-intensive.

2. Resistance Phase

If the stressor persists, the organism enters the resistance phase. The body attempts to adapt and maintain homeostasis under prolonged stress.

Main characteristics-

  • Dominant role of the HPA axis (hypothalamus–pituitary–adrenal)
  • Increased secretion of ACTH and cortisol
  • Cortisol promotes:
    • Gluconeogenesis
    • Protein catabolism
    • Lipolysis
    • Suppression of inflammation
  • The organism tries to balance energy supply and demand
  • Autonomic functions begin to normalize, but cortisol remains elevated

Purpose-

To support long-term adaptation and maintain function during ongoing stress.

3. Exhaustion Phase

If stress continues for too long and compensatory mechanisms fail, the organism enters the exhaustion phase.

Main characteristics-

  • Depletion of energy reserves
  • Persistent elevation of cortisol causes
  • Immune suppression
  • Increased susceptibility to infections
  • Impaired wound healing
  • Muscle wasting
  • Increased abdominal fat
  • Cardiovascular strain
  • Risk of burnout, anxiety, and depression

Purpose-

This phase has no adaptive value; it reflects the failure of coping mechanisms.

Types of Stress

1. Acute stress

Short-term stress, often beneficial. Produces rapid sympathetic activation.

2. Chronic stress

Long-term stress with persistent activation of the HPA axis. Associated with adverse health outcomes.

3. Eustress

Positive stress. Motivating, leads to improved performance.

4. Distress

Negative, harmful stress that exceeds coping abilities.


Physiological Mechanisms of Stress Response

1. Sympathetic–adrenal–medullary (SAM) system

  • Fast response
  • Release of adrenaline and noradrenaline
  • Increases cardiovascular and metabolic activity

2. Hypothalamic–pituitary–adrenal (HPA) axis

  • Slower, long-term response
  • Secretion of CRH → ACTH → cortisol
  • Maintains energy balance under prolonged stress

Consequences of Chronic Stress

Chronic overstimulation of the stress systems can contribute to:

  • Hypertension
  • Atherosclerosis
  • Metabolic syndrome
  • Type 2 diabetes
  • Depression and anxiety
  • Sleep disturbances
  • Decreased fertility
  • Immunosuppression

Management of Stress

Although not part of the physiological model, stress management is important for health:

  • Relaxation techniques (breathing, meditation)
  • Physical activity
  • Adequate sleep
  • Social support
  • Time management
  • Exposure reduction to chronic stressors

Conclusion

Stress is an essential biological mechanism for survival, but prolonged or excessive stress can lead to serious health consequences. Understanding the phases of the stress response—alarm, resistance, and exhaustion—helps explain how the body adapts to challenges and why chronic stress is harmful.

References

  • Selye, H. The Stress of Life. McGraw-Hill.
  • Guyton & Hall. Textbook of Medical Physiology.
  • McEwen BS. Stress, adaptation, and disease. Ann N Y Acad Sci.
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