Gas exchange in the alveoli
Introduced| English | Meaning |
|---|---|
| diffusion/dɪˈfjuːʒn/ | The net movement of particles from higher to lower concentration due to their random motion. |
| surface area/ˈsɜːfɪs ˈeərɪə/ | The total area of the outer surfaces of a three-dimensional object. |
| gas exchange/ɡæs eksˈtʃeɪndʒ/ | The movement of gases, such as oxygen and carbon dioxide, between an organism and its environment or between internal regions. |
| concentration gradient/ˌkɒnsənˈtreɪʃn ˈɡreɪdɪənt/ | A change in a substance's concentration across a distance or between regions. |
Where the air meets the blood
- The alveoli are where oxygen enters the blood and carbon dioxide leaves it.
- Their structure makes them brilliant exchange surfaces.
- The swap happens entirely by diffusion 扩散.
Alveoli have a large surface area and very thin walls for fast gas exchange.
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A large surface area, thin walls, a rich blood supply and a moist lining all speed up gas exchange.
Why alveoli are ideal
- a very large total surface area 表面积 (millions of tiny sacs),
- very thin walls — alveolus and capillary each one cell thick → a tiny distance to cross,
- a rich blood supply from the capillary network,
- a moist lining, so gases dissolve before crossing.
Gas exchange 气体交换 between an alveolus and a capillary
At the alveolus
Tap each part. Oxygen and carbon dioxide swap across a wall just one cell thick, between the air and the blood.
Which feature makes alveoli good for gas exchange?
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Large surface area, thin walls, a rich blood supply and a moist lining all speed up gas exchange.
Match each alveolar feature to why it helps.
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Every alveolar adaptation makes diffusion faster — more area, shorter distance, steeper gradient.
Diffusion of the gases
- Oxygen is high in the alveolar air, low in the blood — so it diffuses air → blood.
- Carbon dioxide is high in the blood, low in the air — so it diffuses blood → air to be breathed out.
- Both move down their concentration gradients 浓度梯度.
Oxygen diffuses:
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Oxygen is higher in the alveolar air than in the blood, so it diffuses air → blood.
Carbon dioxide diffuses from the blood into the alveolar ______, to be breathed out.
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CO₂ is higher in the blood, so it diffuses blood → air and is then exhaled.
Keeping exchange fast
- Breathing brings fresh air, keeping alveolar oxygen high and CO₂ low.
- Flowing blood carries gases away as fast as they cross.
- Both keep the concentration gradients steep, so diffusion stays rapid.
Breathing and the flow of blood keep gas exchange fast because they:
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Fresh air and moving blood keep concentrations different on each side, so the gradients stay steep.
You've got it
- alveoli: large surface area, thin walls (one cell each), rich blood supply, moist lining
- oxygen diffuses air → blood; carbon dioxide diffuses blood → air (down the gradients)
- breathing + blood flow keep the gradients steep, so exchange stays fast
Write from oxygen entering the blood
- Alveolar air is rich in oxygen.
- Blood arriving at the lungs has a lower oxygen concentration. The alveolar wall is very thin.
Which way does oxygen diffuse?
Oxygen moves down its concentration gradient into the blood.
Where does oxygen go at an alveolus?
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Alveolar air has the higher oxygen concentration.