Explain how oxygen moves from the air sacs (alveoli) in the lungs into the blood, and why the concentration of oxygen in the alveoli remains higher than in the blood.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The alveoli in the lungs are surrounded by a dense network of capillaries. Oxygen must move from the air inside the alveoli into the blood in these capillaries so it can be transported around the body.
Model answer (5 marks)
Oxygen moves into the blood by diffusion.
Diffusion occurs down a concentration gradient – from the high concentration of O₂ in the alveolar air to the lower concentration in the capillary blood.
The blood that reaches the alveoli has a low O₂ concentration because it has delivered O₂ to tissues and returned from the body.
Ventilation continuously replaces the air in the alveoli, keeping the O₂ concentration high.
The continuous flow of blood through the capillaries removes O₂, maintaining a low blood O₂ concentration and thus sustaining the concentration gradient.
Diffusion occurs down a concentration gradient – from the high concentration of O₂ in the alveolar air to the lower concentration in the capillary blood.
The blood that reaches the alveoli has a low O₂ concentration because it has delivered O₂ to tissues and returned from the body.
Ventilation continuously replaces the air in the alveoli, keeping the O₂ concentration high.
The continuous flow of blood through the capillaries removes O₂, maintaining a low blood O₂ concentration and thus sustaining the concentration gradient.
Examiner tips
- Use the word ‘diffusion’ and ‘concentration gradient’. Show the direction of movement (alveoli → blood). Explain why blood is low in O₂ and why alveoli stay high (ventilation + blood flow).
Common mistakes
- Confusing diffusion with active transport. Forgetting to mention ventilation or blood flow. Writing that O₂ moves ‘into the blood’ without specifying diffusion or gradient.
Mark scheme (5 marks)
- Oxygen moves by diffusion
- Diffusion occurs down a concentration gradient, from high concentration in the alveoli to low concentration in the blood
- Blood arriving at the alveoli has a low concentration of oxygen (because it has been used up by respiring cells / returned from the body)
- Breathing (ventilation) continuously replaces air in the alveoli, maintaining a high oxygen concentration there
- The continuous flow of blood through the capillaries removes oxygen, keeping blood oxygen concentration low and maintaining the concentration gradient
Key terms in this question
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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