Explain how the nervous system and the endocrine system are integrated during the fight-or-flight response to produce coordinated physiological changes in the body.

IB DP Biology Higher Level (2023 syllabus) — C3.1 Integration of body systems · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The hypothalamus senses a stressor and activates the sympathetic nervous system, which directly stimulates the adrenal medulla.
The adrenal medulla releases adrenaline (epinephrine) into the bloodstream – the endocrine part of the response.
Adrenaline binds to receptors on target organs, causing rapid, widespread effects such as increased heart rate, bronchiolar dilation and redirection of blood flow to skeletal muscle.
Integration occurs because the nervous system gives a fast, short‑duration, localised signal, while the endocrine system provides a slower, sustained, widespread signal, allowing an immediate and prolonged response.

Examiner tips

  • Use the word ‘activate’ for the hypothalamus‑sympathetic link. Mention adrenaline as the hormone. Show the two effects (rapid vs sustained) to hit all marks. Keep the answer concise and use the exact terminology.
  • common_mistakes
  • :
  • Confusing the adrenal cortex with the adrenal medulla. Forgetting to mention the hypothalamus as the initiator. Mixing up adrenaline with noradrenaline or omitting the sustained endocrine effect.

Mark scheme (4 marks)

  1. The hypothalamus detects a stressor and activates the sympathetic nervous system, which directly stimulates the adrenal medulla.
  2. The adrenal medulla secretes adrenaline (epinephrine) into the blood, representing the endocrine component of the response.
  3. Adrenaline binds to receptors on target organs, producing rapid, widespread effects such as increased heart rate, dilation of bronchioles, or redirection of blood flow to skeletal muscle.
  4. Integration is achieved because the nervous system provides rapid, short-duration, localised signals whereas the endocrine system provides slower, sustained, widespread effects, allowing the body to respond both immediately and over a prolonged period.

Key terms in this question

fight-or-flight response

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