Alveoli: Adaptations for Gas Exchange

• Process — diffusion (1). Gases move from a higher to a lower concentration; no energy is used.
• Direction of oxygen (1): from the air in the alveolus into the blood.

Any two of the following (1 mark each):

• Large surface area (millions of alveoli).
• Thin walls — only one cell thick.
• Moist surface lining.
• Rich blood supply (network of capillaries).

The alveoli are adapted in several ways, each speeding up diffusion:

• There are millions of alveoli, giving a large surface area so that more diffusion can take place at the same time.
• The walls are only one cell thick, and lie next to thin-walled capillaries, giving a short diffusion distance so gases cross quickly.
• The inner surface is moist, allowing the gases to dissolve before diffusing across.
• Each alveolus has a rich blood supply (a dense capillary network) which maintains a steep concentration gradient by carrying oxygen away and bringing carbon dioxide.

As a result, oxygen diffuses into the blood and carbon dioxide diffuses out rapidly.

Gas exchange happens by diffusion, which is fastest when the concentration gradient is steep. Two systems keep that gradient steep:

Ventilation (breathing):

• Breathing in brings fresh air into the alveoli, which is high in oxygen and low in carbon dioxide.
• Breathing out removes the stale air, preventing oxygen levels in the alveoli from falling.

Blood supply (capillaries):

• Blood flows past the alveoli constantly, carrying oxygen away as soon as it diffuses in, so the blood next to the alveolus stays low in oxygen.
• The blood delivers carbon dioxide, so the blood stays high in carbon dioxide.

Together this keeps a large difference in concentration across the alveolar wall for both gases, so oxygen keeps diffusing into the blood and carbon dioxide keeps diffusing out at a high rate.

Gas exchange in the lungs occurs by diffusion across the alveoli, the tiny air sacs at the ends of the airways. Their structure is adapted to make this passive process as fast as possible:

• Large surface area — there are millions of alveoli, giving a huge total area over which gases can diffuse, so more diffusion can happen at once.
• Thin walls — each alveolus wall is one cell thick, and the capillary walls are also one cell thick, so the diffusion distance is very short and gases cross rapidly.
• Moist lining — the inner surface is covered by a film of moisture, so the gases dissolve before diffusing through the wall.
• Rich blood supply — each alveolus is surrounded by a dense capillary network. Blood flowing past removes oxygen and brings carbon dioxide, maintaining a steep concentration gradient.

Because of these adaptations, oxygen diffuses from the air in the alveolus into the blood, while carbon dioxide diffuses from the blood into the alveolus to be breathed out — quickly and continuously enough to meet the body's needs.

Efficient gas exchange depends on the alveoli being adapted for diffusion. When the walls between alveoli break down, several of these adaptations are lost:

Loss of surface area:

• Joining many small alveoli into a few large spaces greatly reduces the total surface area.
• A smaller surface area means less diffusion can take place at once, so less oxygen diffuses into the blood.

Loss of close capillary contact:

• The capillary network is reduced and the air spaces are larger, so on average gases must travel a longer diffusion distance to reach the blood.
• A longer pathway slows the rate of diffusion.

Effect on the gradient:

• With fewer capillaries, blood is not carried past as effectively, so the concentration gradient is not maintained as steeply, further slowing diffusion.

Overall consequence:

• Because the rate of diffusion depends on surface area, diffusion distance and the steepness of the concentration gradient, all of these changes mean less oxygen diffuses into the blood per second.
• The person may become breathless and tire easily, as their tissues receive less oxygen for respiration.