Explain how the nervous system and the endocrine system are integrated in the regulation of heart rate during a sudden stressful event.

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).

When an organism perceives a threat, multiple body systems act in a coordinated sequence to prepare the body for rapid physical action.

Model answer (4 marks)

The nervous system first detects the threat via sensory receptors and relays the signal to the hypothalamus. From there, the sympathetic nervous system sends rapid nerve impulses to the adrenal medulla, causing it to secrete adrenaline into the bloodstream. The adrenal medulla’s release of adrenaline is the endocrine component of the response.

Adrenaline acts on the heart in two ways. Firstly, the SA node receives direct sympathetic nerve stimulation, which immediately increases the frequency of its electrical impulses, raising heart rate. Secondly, adrenaline binds to β‑adrenergic receptors on the SA node and on cardiac muscle cells, reinforcing and prolonging the increase in heart rate beyond the brief neural stimulus. This dual action shows how the nervous and endocrine systems are integrated to regulate heart rate during stress.

Examiner tips

  • Mention the sensory detection and hypothalamus first to show the start of the pathway.
  • Show the two mechanisms of adrenaline action (neural and hormonal) to cover all points.
  • Use the exact terms: sympathetic nervous system, adrenal medulla, adrenaline, SA node, β‑adrenergic receptors.
  • Keep the answer concise – 4 marks, so one sentence per point is sufficient.

Common mistakes

  • Mixing up the adrenal cortex with the adrenal medulla; only the medulla releases adrenaline.
  • Failing to mention the SA node as the site of increased impulse frequency.
  • Omitting the endocrine component (adrenaline release) and describing only the nervous system.”

Mark scheme (4 marks)

  1. The nervous system detects the stressor via sensory receptors / the hypothalamus and rapidly transmits nerve impulses along the sympathetic nervous system to the adrenal medulla.
  2. The adrenal medulla secretes adrenaline (epinephrine) into the blood, representing the endocrine component of the response.
  3. The sinoatrial (SA) node in the heart receives direct sympathetic nerve stimulation, increasing the frequency of electrical impulses and thereby increasing heart rate immediately.
  4. Adrenaline also binds to receptors on heart muscle cells / the SA node, reinforcing and prolonging the increase in heart rate beyond the immediate neural response, illustrating the complementary integration of both systems.

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