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 · View as Markdown

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

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.

Model answer (4 marks)

Baroreceptors in the aortic arch and carotid sinus sense the drop in pressure and send increased afferent signals to the cardiovascular centre in the medulla oblongata.
The medulla increases sympathetic outflow, causing the SA node and cardiac muscle to release noradrenaline, which raises heart rate and contractility.
The sympathetic surge also stimulates the adrenal medulla to secrete adrenaline into the blood, which further boosts heart rate and induces vasoconstriction of peripheral arterioles.
The rapid neural action raises cardiac output, while the slower hormonal action sustains it and increases peripheral resistance, restoring blood pressure and demonstrating integration of the nervous and endocrine systems.

Examiner tips

  • Use the exact terms: baroreceptors, cardiovascular centre, sympathetic, noradrenaline, adrenaline, SA node, cardiac output, peripheral resistance.
  • Show the sequence: detection → medulla → sympathetic → heart + adrenal medulla → blood pressure restored.
  • Mention both fast (neural) and slower (hormonal) components to hit all marks.

Common mistakes

  • Confusing the adrenal cortex with the adrenal medulla; only the medulla releases adrenaline.
  • Omitting the role of noradrenaline on the SA node or cardiac muscle.
  • Failing to state that the hormonal response is slower but sustains the effect.

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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