Features of gas exchange surfacesSpec B11.1.3
In short
A good gas exchange surface in humans has four features: a large surface area, a thin surface, a good blood supply and good ventilation with air. In the alveoli, oxygen diffuses into the blood and carbon dioxide diffuses out. Millions of alveoli give a large surface area, and walls one cell thick give a short diffusion distance.
Gas exchange happens at the surface of the alveoli. Oxygen moves from the air in the alveoli into the blood, and carbon dioxide moves from the blood into the air in the alveoli. Both gases move by diffusion.
A good gas exchange surface in humans has four features: a large surface area, a thin surface, a good blood supply and good ventilation with air.
| Feature | Why it helps |
|---|---|
| Large surface area (millions of alveoli) | More space for oxygen and carbon dioxide to diffuse across, so more gas can be exchanged at once |
| Thin surface (walls of the alveoli and capillaries are one cell thick) | Short distance for the gases to diffuse, so diffusion is fast |
| Good blood supply (network of capillaries around each alveolus) | Blood carries oxygen away and brings carbon dioxide, which keeps a steep concentration gradient |
| Good ventilation with air (breathing in and out) | Fresh air keeps the oxygen concentration high and the carbon dioxide concentration low in the alveoli, which keeps a steep concentration gradient |
Do not stop at the feature. Link it to its effect: for example, thin walls give a short diffusion distance, so diffusion is faster.
(opens full size in a new tab)Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026