# 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

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

## 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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