NOTE: we need to know like how alveoli does the gas exchange and stuff but i can't really find it in the spec so I'm just going to put it here in red (if you already know it, skip past the red)
Okay so the lungs contain millions and millions of alveoli. The blood passing next to the alveoli has just returned back to the lungs (from going around the body). This blood contains lots of CO2 and little O2. Oxygen (from the air you've just breathed in) diffuses out the alveolus into the blood (from a high concentration, in the alveolus, to a low concentration, in the blood). CO2 diffuses out of the blood (high concentration) into the alveolus (low concentration) to be breathed out.
How are alveoli specialised for gas exchange...
- Millions of them (gives a really big surface area)
- Moist lining for O2 and CO2 to dissolve in
- The alveoli walls are only 1 cell thick (which is very thin) - meaning the gas doesn't have far to diffuse
- Their walls are permeable - meaning O2 and CO2 can easily diffuse across them
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