# Explain how the respiratory system and the cardiovascular system are integrated to maintain gas exchange during physical exercise.

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

## Mark scheme (4 marks)

1. Increased muscle activity during exercise raises carbon dioxide levels / lowers oxygen levels in the blood.
2. Chemoreceptors detect the change in blood CO₂ / pH and send nerve impulses to the medulla oblongata, which increases breathing rate and depth.
3. Increased ventilation brings more O₂ into the alveoli and removes CO₂, steepening the diffusion gradient for gas exchange across the alveolar capillary membrane.
4. The heart rate increases (via sympathetic nervous stimulation / adrenaline) so that oxygenated blood is delivered to respiring muscles more rapidly and CO₂-laden blood is returned to the lungs more quickly.

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