Explain alveolar gas exchange
Explain diffusion of oxygen and carbon dioxide between alveolar air and capillary blood using structural and gradient evidence.
Section 9.1 · The gas exchange system
AS Level; Cambridge International Biology 9700, 2028–2030. Written practice is self-checked; microscope recognition, biological drawing and supervised practical competence require practical work, while human subject-teacher review remains pending.
Before you begin
- Know that gas exchange is diffusion across a surface and that ventilation renews air while blood flow renews the blood side.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the direction of oxygen diffusion at a healthy alveolus?
What you will learn
- Describe oxygen and carbon-dioxide diffusion at alveoli
- Relate surface area, barrier thickness, blood supply and ventilation to exchange
Describe blood supply and ventilation at alveoli
Build the explanation
Oxygen partial pressure is higher in alveolar air than in the blood arriving in capillaries, so oxygen diffuses across the alveolar epithelium and capillary endothelium into blood. Carbon dioxide partial pressure is higher in incoming blood than in alveolar air, so carbon dioxide diffuses into the alveolus and is exhaled. Many alveoli provide a large total surface area; their thin walls and close capillary network provide a short diffusion distance and good blood supply. Ventilation renews alveolar air while blood flow renews the blood at the exchange surface.
Apply it carefully
Explain each gas separately and name its direction. Diffusion is down a partial-pressure gradient; it is not powered transport. A structural adaptation supports exchange but does not on its own prove a particular measured rate: gradient, surface area, barrier thickness and ventilation/perfusion conditions all matter. The alveolar air space and capillary lumen remain separate compartments.
Pause and explain
Why does carbon dioxide diffuse from incoming blood into the alveolus?
Worked example
In which direction does oxygen diffuse at an alveolus, and what drives it?
Show the worked solution
- Compare alveolar and incoming blood oxygen partial pressure.
- Oxygen moves down the partial-pressure gradient into the blood.
Answer Oxygen diffuses from alveolar air across the thin exchange barrier into capillary blood because its partial pressure is higher in the alveolus.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the direction of oxygen and carbon dioxide diffusion at the alveolar surface.
Give me a hint
State each partial-pressure comparison and direction.
Compare my reasoning
- Oxygen partial pressure is higher in alveoli, so oxygen enters the blood.
- Carbon dioxide partial pressure is higher in incoming blood, so carbon dioxide enters the alveolus.
Oxygen diffuses from alveolar air into capillary blood because its partial pressure is higher in the alveoli. Carbon dioxide diffuses from incoming blood into alveolar air because its partial pressure is higher in blood.
Look for these in your work
- Oxygen gradient and direction correct
- Carbon dioxide gradient and direction correct
Describe three features that support rapid gas exchange at alveoli.
Give me a hint
Include area, distance and renewal of the two sides.
Compare my reasoning
- Many alveoli create a large surface area and thin walls shorten diffusion distance.
- A capillary network and ventilation renew blood and air at the boundary.
Many alveoli provide a large total surface area; thin alveolar and capillary walls provide a short diffusion path; and ventilation plus capillary blood flow renew the air and blood at the surface, helping maintain gradients.
Look for these in your work
- Large area stated
- Short distance stated
- Ventilation and blood flow linked to gradients
Ventilation continues but blood flow through the associated capillaries is greatly reduced. Explain why exchange may fall even though air is renewed.
Give me a hint
Compare renewal on the air side with renewal on the blood side.
Compare my reasoning
- Ventilation renews alveolar air but not blood.
- Reduced blood flow limits delivery/removal at the blood side and can weaken effective gradients.
Ventilation renews the alveolar air, but it does not renew capillary blood. Reduced blood flow limits delivery of deoxygenated blood and removal of oxygenated blood, so effective exchange and gradient maintenance can fall.
Look for these in your work
- Air-side renewal distinguished
- Blood-side renewal distinguished
- Exchange consequence qualified
Common mistakes
- Reversing the direction of carbon-dioxide diffusion
- Saying alveoli pump gases into blood
- Explaining exchange using surface area but omitting partial-pressure gradients
What can you explain now?
What is the direction of oxygen diffusion at a healthy alveolus?
Compare with the explanation
From alveolar air into capillary blood
Alveolar oxygen partial pressure is higher than in blood arriving at the lungs.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
What drives oxygen and carbon-dioxide diffusion in opposite directions at an alveolus?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.