Where gases swap
A balloon is thin and stretchy. An alveolus is a tiny balloon of air. Many are called alveoli.
Step 1 of 4
- One alveolus: a tiny air sac with a thin wall.
- Blood vessels wrap round it.
- Oxygen moves from the air into the blood.
- Carbon dioxide moves from the blood into the air.
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Slide to cut one big sac into many small ones.
The rule
Alveoli are tiny balloon-like sacs with thin walls. Blood vessels wrap round them. Oxygen passes into the blood. Carbon dioxide passes out. A thin wall is easy to cross, so gases pass fast.
The gases move by diffusion. They move from where there is more to where there is less.
There are very many alveoli. Spread flat, they would cover tens of square metres. A big surface lets more gas pass at once.
Fish swap gases at their gills, the parts they breathe with. Water holds much less oxygen than air does. So a fish must breathe fast.