Explain why oxygen moves from the air in the alveoli into the blood in the surrounding capillaries during gas exchange in the lungs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
During gas exchange in the human lungs, gases move between the alveoli and the blood in the capillaries that surround them. The blood arriving at the lungs has returned from the body's tissues, where it has delivered oxygen to respiring cells.
Model answer (4 marks)
Oxygen moves into the blood by diffusion, travelling down a concentration gradient.
The concentration of O₂ is higher in the alveolar air than in the blood that has just returned from the tissues.
This is because the tissue blood has used up O₂ during respiration, leaving it with a low O₂ concentration.
The alveolar and capillary walls are only one cell thick, which allows rapid diffusion and keeps the gradient intact by continuously supplying de‑oxygenated blood.
The concentration of O₂ is higher in the alveolar air than in the blood that has just returned from the tissues.
This is because the tissue blood has used up O₂ during respiration, leaving it with a low O₂ concentration.
The alveolar and capillary walls are only one cell thick, which allows rapid diffusion and keeps the gradient intact by continuously supplying de‑oxygenated blood.
Examiner tips
- Show the diffusion mechanism and concentration gradient first. Mention the low O₂ in tissue blood. Explain the thin barrier. Use the exact wording "diffusion" and "concentration gradient".
Common mistakes
- Forgetting to mention the concentration gradient. Using vague terms like "movement" instead of "diffusion". Omitting the thin‑wall explanation.
Mark scheme (4 marks)
- Oxygen moves by diffusion (down a concentration gradient)
- The concentration of oxygen is higher in the alveoli than in the blood arriving at the lungs
- This is because oxygen has been used up / removed by respiration in the body's tissues, so the returning blood has a low concentration of oxygen
- The thin walls of the alveoli / capillaries (one cell thick) allow rapid diffusion / maintain the concentration gradient by continuously supplying deoxygenated blood
Key terms in this question
Related
- All Pearson Edexcel International GCSE Biology (4BI1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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