Your blood picks up oxygen in the lungs, as you saw in The Heart and Blood. How does air get in, and how does oxygen cross into the blood?
Key idea
The diaphragm, a dome of muscle under the lungs, shortens and flattens; the chest gets bigger and air rushes in. Air travels nose → trachea → bronchi → lungs, into millions of tiny air sacs called alveoli. There oxygen diffuses into the blood and carbon dioxide diffuses out.
Why it works
Gas exchange is diffusion. Air in the alveoli has more oxygen than the blood arriving from the body, so oxygen moves into the blood. That blood has more carbon dioxide than the air, so carbon dioxide moves out. The walls of the alveoli and capillaries are each one cell thick, and the alveoli give the lungs a very large surface.
| Gas | Air breathed in | Air breathed out |
|---|---|---|
| Nitrogen | about 78% | about 78% |
| Oxygen | about 21% | about 16% |
| Carbon dioxide | about 0.04% | about 4% |
| Water vapor | varies | more (saturated) |
Worked example: Read the table
How much of the oxygen you breathe in do you use? Use the table.
- In: about 21%. Out: about 16%.
- Used: 21 − 16 = 5 percentage points.
- As a share of what came in: 5 ÷ 21 ≈ 0.24, about a quarter.
Answer: About a quarter of it; most of the oxygen is breathed back out.
Common mistake
Students think we breathe in only oxygen and breathe out only carbon dioxide. Both kinds of air are mostly nitrogen. Air out still has about 16% oxygen and only about 4% carbon dioxide.