The lungs end in millions of tiny air sacs called alveoli. Each is wrapped in capillaries, and the wall between air and blood is so thin that oxygen and carbon dioxide diffuse across in a fraction of a second.
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Labelled parts
Key points
Gases cross by simple diffusion, down their concentration gradients: O₂ into the blood, CO₂ out.
The blood-air barrier is often only about half a micrometre thick.
Surfactant from type II cells reduces surface tension so the alveoli do not collapse.
