Human Gas Exchange System and Alveoli
The pathway of air from trachea to alveoli, and the features of the alveolar epithelium that make it an efficient gas exchange surface. Includes the 4-mark exam question, a worked model answer and the examiner trap on describing the alveolar wall as one cell thick.
Chapters (9)
Exam question [4 marks]
Describe the pathway taken by air from the atmosphere to the alveoli, and explain how the alveoli are adapted for gas exchange.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Air passes through the trachea, then bronchi, then bronchioles, to the alveoli (correct order) | "nose/mouth, trachea, bronchi, bronchioles, alveoli" | Wrong order; "trachea to lungs to alveoli" with no bronchi or bronchioles |
| 2 | Alveolar epithelium is very thin / one cell thick (squamous), so short diffusion distance | "flattened cells", "thin wall, so fast diffusion", "thin capillary walls also shorten the distance" | "one cell thick" with no consequence; "thin so oxygen can get in" |
| 3 | Many alveoli give a large surface area | "millions of alveoli", "large surface area for diffusion" | "large lungs"; "large surface area" with no reference to alveoli |
| 4 | Dense capillary network / blood flow removes oxygen and brings carbon dioxide, maintaining a steep concentration gradient (breathing also replaces the air) | "blood flow keeps the gradient steep", "ventilation maintains gradient" | "capillaries are thin" alone; "gas moves down a steep gradient" with no mechanism |
Mark 1 is for the correct sequence. Marks 2 to 4 each pair a feature with its effect on diffusion; a feature with no effect scores nothing. "One cell thick" must be linked to a short diffusion distance. Do not credit gas exchange in the bronchioles. The ventilation mechanism is not required here.
Model answer (4/4)
Air passes through the trachea, then the bronchi, then the bronchioles, and reaches the alveoli. The alveolar epithelium is very thin, a single layer of flat squamous cells, so there is a short diffusion distance. There are many alveoli, which gives a large surface area for gas exchange. A dense network of capillaries carries blood that removes oxygen and brings carbon dioxide, and breathing replaces the air, so a steep concentration gradient is maintained.
Examiner trap
- Drawing the bronchioles before the bronchi in order
- Saying the alveolar wall is 'one cell thick' without 'thin so short diffusion distance'
- Not naming the squamous epithelium and capillary
Key terms
- trachea
- bronchus
- bronchiole
- alveolus
- alveolar epithelium
- capillary
- large surface area
- thin
- steep concentration gradient
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:24The trachea
Air enters through the nose or mouth and passes down the trachea. The trachea is a wide tube, held open by rings of cartilage, so it never collapses when you breathe in. Trachea first.
0:36Bronchi and bronchioles
At the bottom, the trachea divides into two bronchi, one going to each lung. One tube is a bronchus, two are bronchi. Inside the lung, each bronchus divides again and again into narrower tubes called bronchioles. So the order is trachea, bronchi, bronchioles. Think of an upside down tree. The trunk is the trachea, the main branches are the bronchi, and the twigs are the bronchioles.
Get that order wrong in an exam, and you lose the mark.
1:09Alveoli
The finest bronchioles end in clusters of tiny air sacs. These are the alveoli. There are millions of them in each lung, and this is where gas exchange happens. So the full pathway is trachea, bronchi, bronchioles, alveoli. Examiners often ask for exactly this, so learn that sequence as one chain, and say all four names.
1:33One alveolus up close
Now zoom in on one alveolus. First adaptation, a thin wall. Its wall is a single layer of flat cells, called squamous epithelium. Wrapped around it is a dense network of capillaries, whose walls are also very thin. So oxygen only has to cross a very short distance to get from the air into the blood.
Gases diffuse faster when the distance is shorter, so a thin barrier means fast diffusion. Oxygen goes from the air in the alveolus into the blood, and carbon dioxide goes the other way.
2:08Large surface area
Second adaptation. There are millions of alveoli, and together their walls give a very large surface area. More surface means more room for oxygen and carbon dioxide to diffuse across at once. One big balloon would have far less surface for the same volume of air.
2:26Steep concentration gradient
Third, the concentration gradient needs to stay steep. Blood flowing through the capillaries carries oxygen away, and brings carbon dioxide in. Breathing keeps replacing the air in the alveoli. So oxygen stays higher in the alveoli than in the blood, and carbon dioxide stays higher in the blood than in the alveoli.
That keeps diffusion going.
3:28Examiner trap and recap
The trap. Saying the wall is one cell thick is not enough. You must add that it is thin, so the diffusion distance is short.
Recap. Trachea, bronchi, bronchioles, alveoli. Thin wall, large surface area, steep gradient.
Next time, we look at how breathing actually moves air in and out of the lungs.
Watch first
This lesson builds on:
- Surface Area to Volume Ratio Explained · AQA 7402 · 3.3.1
Keep going
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- Gas Exchange: Single Cells and Insects · AQA 7402 · 3.3.2
- Fish Gills and the Counter-current Principle · AQA 7402 · 3.3.2
- Gas Exchange in Leaves: Mesophyll and Stomata · AQA 7402 · 3.3.2
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Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-057.