The big idea: The lungs exchange gases at millions of tiny air sacs called alveoli. Their thin walls and huge total surface area let oxygen diffuse quickly into the blood.
Emphysema is a lung disease in which the walls between alveoli are destroyed.
Neighbouring sacs merge into fewer, larger air spaces, so the lung has a much smaller surface area for gas exchange — and gas exchange becomes far less efficient.
A healthy alveolus: a small, thin-walled air sac pressed against a capillary — many of these give a huge surface area for fast gas exchange. Emphysema destroys these walls, so the sacs merge into fewer, larger spaces with far less surface area.
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- Alveolus (plural: alveoli)
- A tiny air sac in the lung where gas exchange happens; its wall is just one cell thick. Millions of alveoli give the lung a very large surface area.
- Surface area for gas exchange
- The total area of alveolar wall across which oxygen and carbon dioxide can diffuse. The larger it is, the faster gases can be exchanged.
- Emphysema
- A lung disease in which the walls between alveoli are broken down, so the sacs merge into fewer, larger spaces with a much smaller surface area for gas exchange.
- Elastic recoil
- The ability of healthy lung tissue to spring back to its smaller shape, helping to push air out when you breathe out. Emphysema destroys this.
Why this matters: Gas exchange depends on a large surface area, a short diffusion distance and a steep concentration gradient.
Emphysema attacks the first two: it shrinks the surface area and damages the thin walls, so oxygen can no longer diffuse into the blood fast enough.
Emphysema is usually caused by long-term irritation of the lungs, most often from cigarette smoke (and from air pollution).
The damage follows a clear chain: the irritation breaks down alveolar walls, which reduces the surface area, which slows oxygen uptake, which leaves the person breathless.
The cause-and-effect chain: 1. Walls destroyed → the thin walls between neighbouring alveoli break down.
2. Sacs merge → many small sacs become fewer, larger air spaces.
3. Surface area falls → there is far less area for oxygen to diffuse across.
4. Recoil lost → the lung can no longer spring back, so air is trapped and breathing out is hard.
5. Result → oxygen reaches the blood too slowly, so the person is breathless and tires quickly.
| Step in emphysema | What happens | Why it impairs gas exchange |
|---|---|---|
| 1. Trigger | Long-term irritation, most often from cigarette smoke | Causes inflammation that damages the lung tissue |
| 2. Walls destroyed | The thin walls between neighbouring alveoli are broken down | Small sacs merge into fewer, larger air spaces |
| 3. Surface area falls | Total surface area for diffusion is greatly reduced | Less area means less oxygen can diffuse per breath |
| 4. Recoil lost | The lung loses its elastic recoil | Air is trapped; it is harder to push old air out and draw fresh air in |
| 5. Outcome | Oxygen uptake into the blood is too slow | The person becomes breathless and cannot exercise as before |
Two effects, not one: Emphysema hurts gas exchange in two ways at once:
It reduces the surface area (fewer, larger sacs), and it loses the elastic recoil (so stale, low-oxygen air is trapped in the lungs).
Both mean less oxygen enters the blood per breath — a strong answer names both.
Healthy lung
- Many small alveoli → large surface area
- Walls one cell thick → short diffusion distance
- Good elastic recoil → air pushed out easily
- Oxygen uptake keeps up with demand
Lung with emphysema
- Fewer, larger spaces → small surface area
- Walls damaged → longer / poorer diffusion
- Lost elastic recoil → air trapped, hard to exhale
- Oxygen uptake too slow → breathlessness
A memory hook: Think 'fewer, bigger, slower': emphysema makes fewer, bigger air sacs, so gas exchange becomes slower.
Less surface area + trapped air = not enough oxygen.
Risk factors and prevention: The biggest cause of emphysema is smoking; air pollution also contributes.
So the public-health changes most likely to reduce emphysema target these: anti-smoking laws, stop-smoking support, and cutting air pollution.
This 'how to reduce incidence' angle is a favourite quick exam point.
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How this is tested: On Paper 2 a 4-mark Explain question asks you to explain the changes in lung function seen in emphysema — score by chaining cause to effect (walls destroyed → fewer, larger sacs → less surface area + lost recoil → slower oxygen uptake → breathlessness).
On Paper 3 a 2-mark Predict question asks how emphysema would change a person's response to moderate exercise — they cannot raise oxygen uptake enough, so they become breathless and tire quickly.
On Paper 1A a single-mark item may show a micrograph and ask for the direct effect of alveolar destruction (reduced surface area), or ask which national changes would most reduce emphysema (reduce smoking / pollution).
IB-style question — explain the changes in lung function
Explain the changes in lung function seen in a person with emphysema. [4]
How to score all four marks
- Start with the damage. The walls between alveoli are broken down, so many small sacs merge into fewer, larger air spaces.
- Link to surface area. This reduces the total surface area available for gas exchange.
- Link to diffusion. With less surface area (and a longer / damaged diffusion path), oxygen diffuses into the blood more slowly, so the blood is less well oxygenated.
- Add the recoil effect. The lung also loses its elastic recoil, so air is trapped and it is harder to exhale — together these make the person breathless and easily tired. (Award 1 mark per distinct point, up to 4.)
Final answer
Alveolar walls are destroyed, so sacs merge into fewer, larger spaces; this reduces the surface area for gas exchange, slowing oxygen uptake into the blood; the lung also loses elastic recoil, trapping air and making exhaling harder — so the person becomes breathless.
✓ Why this scores full marks: Each step is a separate scoring point in a logical chain — damaged walls, less surface area, slower diffusion, lost recoil.
An Explain [4] needs the links between cause and effect, not just 'the lungs are damaged' written four ways.
| Feature | Healthy lung | Lung with emphysema |
|---|---|---|
| Alveoli | Many small, separate air sacs | Walls broken down — sacs merge into fewer, larger spaces |
| Surface area for gas exchange | Very large | Greatly reduced |
| Diffusion distance | Short (walls one cell thick) | Longer / damaged in places |
| Elastic recoil (springs back to push air out) | Good — exhaling is easy | Lost — air is trapped, exhaling is hard |
| Oxygen uptake into the blood | Fast enough to meet demand | Too slow — blood is poorly oxygenated |
| Effect on the person | Normal breathing and exercise | Breathlessness, tires quickly, limited exercise |