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Biology AS CIE

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  1. 1-cell-structure
    10 主题
  2. 2-biological-molecules
    19 主题
  3. 3-enzymes
    13 主题
  4. 4-cell-membranes-and-transport
    16 主题
  5. 5-the-mitotic-cell-cycle
    8 主题
  6. 6-nucleic-acids-and-protein-synthesis
    9 主题
  7. 7-transport-in-plants
    11 主题
  8. 8-transport-in-mammals
    16 主题
  9. 9-gas-exchange
    6 主题
  10. 10-infectious-diseases
    6 主题
  11. 11-immunity
    10 主题
课 9, 主题 1
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9-1-the-gas-exchange-system AS gas-exchange-processes

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Exam code:9700

Gas exchange processes

  • The exchange of oxygen and carbon dioxide occurs between the alveoli and the capillaries in the lungs

  • Oxygen and carbon dioxide are exchanged in a process of simple diffusion

    • This is a passive movement from high to low concentration

  • The air in the alveoli contains a high concentration of oxygen

    • The oxygen diffuses from the alveoli and into the blood capillaries, before being carried away to the rest of the body for aerobic respiration

  • The blood in the capillaries has a relatively low concentration of oxygen and a high concentration of carbon dioxide

    • The carbon dioxide diffuses from the blood and into the alveoli and is then exhaled

Diagram of an alveolar sac showing oxygen diffusion into blood. Labels include alveolar sac, liquid surfactant, and blood capillary with arrows.
The movement of blood through the capillaries surrounding the alveoli maintains a concentration gradient to drive diffusion of oxygen into the blood

Feature of alveoli

Importance

Large surface area

The average human adult has around 480-500 million alveoli in their lungs

This equals a surface area of 40-75m2

This enormous surface area increases the space available for oxygen and carbon dioxide to diffuse

Thin walls

The walls of the alveoli are only one cell thick

This means that gases have a very short diffusion distance and so gas exchange is as quick and efficient as possible

Examiner Tips and Tricks

Remember that blood arriving to the lungs in capillaries is deoxygenated, while blood leaving the capillaries is oxygenated and returns to the left side of the heart.

You don’t need to know how many alveoli there are for your exam, or their total surface area—these are only included here to show you just how large a surface area they have in total!

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