Explain how carbon dioxide produced in respiring muscle cells reaches the alveoli to be breathed out.

WJEC GCSE Biology (Wales) — 1.2.2 The respiratory system · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Carbon dioxide diffuses out of the muscle cells into the blood in the capillaries.
Blood carries the carbon dioxide to the lungs.
In the lungs, carbon dioxide diffuses from the blood in the capillaries surrounding the alveoli into the alveolar air.
Diffusion occurs because the concentration of carbon dioxide is higher in the blood than in the alveoli, creating a concentration gradient.

Examiner tips

Use the word ‘diffuses’ to show the movement of CO₂. Mention the capillaries as the site of transport. Explain the concentration gradient as the driving force. Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

Failing to mention the capillaries as the transport route. Using ‘moves’ instead of ‘diffuses’. Omitting the concentration gradient explanation.

Mark scheme (4 marks)

Carbon dioxide diffuses out of the muscle cells into the blood (in the capillaries / bloodstream)
Blood carries / transports carbon dioxide to the lungs
Carbon dioxide diffuses from the blood (capillaries surrounding the alveoli) into the alveoli
Diffusion occurs because there is a higher concentration of carbon dioxide in the blood than in the alveoli (concentration gradient)

Key terms in this question

alveoli