Lung Disease and Gas Exchange
How fibrosis, asthma, emphysema and tuberculosis change diffusion distance, surface area, elasticity and ventilation. Includes the 4-mark exam question, a worked model answer and the examiner trap on saying emphysema thickens the wall.
Chapters (9)
Exam question [4 marks]
Fibrosis and emphysema both make a person breathless. Explain how each disease reduces gas exchange in the lungs.
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 | Fibrosis: scar tissue thickens the alveolus wall, so the diffusion distance is greater / diffusion is slower | "thicker walls", "longer diffusion pathway", "scarring" | "lungs are damaged"; "less oxygen gets in" with no cause; "wall is thinner" |
| 2 | Fibrosis: lungs are less elastic / cannot stretch, so less air is breathed in (ventilation reduced) | "lungs cannot expand fully", "smaller volume of air inhaled" | "lungs do not work"; "less oxygen" alone |
| 3 | Emphysema: alveolus walls are destroyed / alveoli merge, so the surface area for gas exchange is smaller | "fewer, larger air spaces", "less surface area" | "alveoli burst" with no effect on area; "thicker walls" |
| 4 | Emphysema: loss of elastic recoil, so air is not forced out / less fresh air drawn in, so ventilation is reduced / concentration gradient is weaker | "air trapped in lungs", "cannot breathe out fully", "less effective ventilation", "loss of elastin / elastase breaks down elastin so lungs cannot recoil" | "lungs lose elasticity" with no consequence stated; "ventilation reduced" with no mention of recoil or elasticity |
Marks 1 and 2 are for fibrosis, 3 and 4 for emphysema. Each mark names the change (wall, surface, elasticity) and its effect on exchange. Saying "less oxygen gets in" is the result, not the explanation. Do not credit thickened walls to emphysema. Do not credit surface area for fibrosis.
Model answer (4/4)
In fibrosis, scar tissue thickens the alveolus walls, so the diffusion distance is greater and diffusion is slower. The scarred lungs are less elastic and cannot stretch, so less air is breathed in and ventilation is reduced. In emphysema, the alveolus walls are destroyed, so there is a smaller surface area for gas exchange. The lungs also lose elastic recoil, so air is trapped and less fresh air is drawn in, which weakens the concentration gradient.
Examiner trap
- Not linking a symptom to a change in diffusion distance, surface area or ventilation
- Confusing disease names with effects
Key terms
- fibrosis
- asthma
- emphysema
- tuberculosis
- lung cancer
- elasticity
- diffusion distance
- ventilation rate
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:29A healthy alveolus
First, a healthy alveolus. A very thin wall, a capillary right beside it, and a large surface area. Gases diffuse quickly because the distance is short and the surface is big. Ventilation keeps fresh air coming in, so the concentration gradient stays steep. Disease damages one or more of those three things.
0:51Fibrosis: thicker wall
Fibrosis first. Scar tissue builds up in the lung tissue, and the alveolus walls get thicker. So the diffusion distance increases, and gases diffuse more slowly. Scarred lungs are also less elastic, so they cannot stretch as far. Less air is breathed in each time, so ventilation is reduced. As a result, less oxygen reaches the blood, and the person is breathless.
1:18Emphysema: lost surface area
Emphysema is different. Here the alveolus walls are destroyed, and neighbouring alveoli merge into fewer, larger air spaces. That means the surface area for gas exchange is much smaller. So less oxygen diffuses in each second, even though the walls are not thicker.
1:38Lung elasticity
There is a second effect. The lungs lose their elasticity. Normally, elastic recoil helps push air out. In emphysema, the lungs do not spring back, so air is trapped, and less fresh air is drawn in. Ventilation is less effective, so the concentration gradient is weaker.
1:58Asthma and tuberculosis
You may meet other diseases in a data question. In asthma, the airway walls thicken and mucus builds up, so the airways become narrow, and air flows in and out more slowly. Ventilation falls. In tuberculosis, the immune system walls off the bacteria in small lumps called tubercles. The tissue is damaged and scarred, which can lead to fibrosis. Lung cancer is a growth of abnormal cells, a tumour, which can block airways and reduce the working lung tissue.
Whatever the disease, the exam wants the effect on exchange, not just the name.
2:36Disease comparison table
A quick table to hold it together. Fibrosis: thicker walls, so a longer diffusion distance, and less stretch, so less ventilation. Emphysema: destroyed walls, so a smaller surface area, and no recoil, so less ventilation. Asthma: narrow airways, so less ventilation.
3:37Examiner trap and recap
The trap. Do not mix up the diseases. Thickened walls belong to fibrosis, destroyed walls belong to emphysema.
And always link the damage to diffusion distance, surface area or ventilation.
Recap. Fibrosis lengthens the diffusion distance. Emphysema shrinks the surface area. Both reduce ventilation.
Next time, we ask what causes these diseases. Smoking, pollution, and the difference between correlation and causation.
Watch first
This lesson builds on:
- Human Gas Exchange System and Alveoli · AQA 7402 · 3.3.2
- Ventilation: How Breathing Works · AQA 7402 · 3.3.2
Keep going
More from 3.3 Exchange with the environment
- 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
- Gas Exchange vs Water Loss: Xerophytes · AQA 7402 · 3.3.2
All 10 3.3 Exchange with the environment lessons Whole playlist on YouTube
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-059.