Alveoli and gas exchangeSpec 8.3
In short
Alveoli are tiny air sacs in the lungs where oxygen diffuses into the blood and carbon dioxide diffuses out. Millions of alveoli give a very large surface area, and walls one cell thick give a short diffusion distance. A rich capillary network and ventilation keep a steep concentration gradient, and the moist lining helps gases dissolve.
The lungs are the exchange surface for oxygen and carbon dioxide. Each lung contains millions of tiny air sacs called alveoli. Gases move between the air in the alveoli and the blood in the capillaries around them by diffusion.
- Oxygen diffuses from the air in the alveoli (high concentration) into the blood (low concentration).
- Carbon dioxide diffuses from the blood (high concentration) into the air in the alveoli (low concentration), and is breathed out.
| Adaptation | Why it helps |
|---|---|
| Millions of alveoli | Gives a very large surface area for diffusion |
| Walls only one cell thick | Short diffusion distance, so gases diffuse quickly |
| Capillary walls only one cell thick | Short diffusion distance between the air and the blood |
| Rich network of capillaries (good blood supply) | Blood carries oxygen away and brings carbon dioxide, keeping a steep concentration gradient |
| Moist lining | Gases dissolve, which helps them to diffuse |
| Ventilation (breathing in and out) | Keeps a high oxygen concentration in the alveoli and a low carbon dioxide concentration, which maintains the concentration gradient |
Do not stop at the adaptation. Link every feature to its effect: for example, thin walls means a short diffusion distance, so faster diffusion.
(opens full size in a new tab)Quick check
Why must urea be transported out of the body's cells?
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It is a waste product that is carried in the blood to the kidneys and removed in urine.
Does a large animal have a larger or smaller surface area : volume ratio than a single cell?
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Smaller.
Why do multicellular organisms need a transport system?
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Many cells are far from the surface, so diffusion alone is too slow to supply them.
State two ways alveoli are adapted for gas exchange.
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Any two of: large surface area, walls one cell thick, moist lining, good blood supply, ventilation.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026