# Explain how the nervous system and the endocrine system are integrated to regulate blood pressure when a person stands up suddenly from a lying position.

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

> When a person moves rapidly from lying to standing, gravity causes blood to pool in the lower limbs, transiently reducing venous return to the heart and lowering blood pressure. The body must respond rapidly to restore perfusion to the brain.

## Mark scheme (4 marks)

1. Baroreceptors in the aortic arch and/or carotid sinus detect the fall in blood pressure and send increased afferent nerve impulses to the cardiovascular centre in the medulla oblongata.
2. The cardiovascular centre increases sympathetic nervous output, causing the heart to increase its rate and force of contraction (via noradrenaline acting on the SA node / cardiac muscle).
3. Sympathetic stimulation also triggers the adrenal medulla to secrete adrenaline (epinephrine) into the bloodstream, reinforcing and sustaining the increase in heart rate and causing vasoconstriction of peripheral arterioles.
4. This coordinated neural (fast, short-duration) and hormonal (slower, sustained) response raises cardiac output and peripheral resistance together, restoring blood pressure to normal — demonstrating integration of both systems.

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