Gas exchange in the lungs
We need to get oxygen from the air into the blood, and we need to remove waste carbon dioxide from the blood into the air. Moving gases like this is called gas exchange.
Features of the alveoli
The alveoli are adapted to make gas exchange in lungs happen easily and efficiently. Here are some features of the alveoli that allow this:
- they give the lungs a really big surface area
- they have moist, thin walls (just one cell thick)
- they have a lot of tiny blood vessels called capillaries
The gases move by diffusion from where they have a high concentration to where they have a low concentration:
- oxygen diffuses from the air in the alveoli into the blood
- carbon dioxide diffuses from the blood into the air in the alveoli
As the blood moves through the capillaries in the alveoli, oxygen diffuses into it and carbon dioxide diffuses out of it
Some water vapour is also lost from the surface of the alveoli into the lungs - we can see this condensing when we breathe out on cold days.