The Alveoli
Oxygen enters the blood through the alveoli – bumpy air sacs. In I/GCSE Biology, the structure of the alveoli increases surface area, which also increases gas exchange. It also provides a concentration gradient for diffusion as oxygen will move from higher to lower concentration. There are about 700 million of these cuties in our body, giving a total surface area of 60m2.
Diagram: the structure of alveoli
Gas | Atmospheric Air (%) | Exhaled Air (%) |
Nitrogen | 78 | 79 |
Oxygen | 21 | 16 |
Carbon Dioxide | 0.04 | 4 |
Other Gases | 1 | 1 |
- Blood is pumped from the heart to the lungs and passes through the capillaries that surround the alveoli.
- In I/GCSE Biology, the blood has come from respiring tissues of the body, where it has given up some of the oxygen to the cells, and gained carbon dioxide.
- Around the lungs, the blood is separated from the air inside each alveolus by only two cell layers; the wall of the alveoli and the capillary wall. It is thin so oxygen can easily diffuse to capillaries and carbon dioxide, from the capillaries.
Because the air in alveolus has a higher concentration of oxygen than the blood entering the capillary network, oxygen diffuses across the alveolus wall and into the blood. At the same time, there is more carbon dioxide in blood than there is in the air so it diffuses out of the blood and into the alveolus. The capillaries containing the oxygen rich blood then flow to the heart, where it can be pumped around the body!
Note: Surfactant is a chemical that keeps lungs moist. When air is exhaled, lungs get drier as water escapes. Therefore, to stop the lungs from collapsing, the surfactant hardens. Finally, moisture is useful as it speeds diffusion.
That's the end of the topic!
Drafted by Joey (Biology)