Lesson 2 of 5 / Respiration in humans
Exchange gases at an alveolus
How do very small air spaces provide fast gas exchange?
I can link alveolar structure and blood supply to efficient diffusion.
I can identify named airways, alveoli and associated capillaries and their functions.Syllabus
Syllabus K327 / K328, Topic 6(a). Identify named airways, alveoli and associated capillaries and their functions.
I can explain alveolar adaptations for gas exchange.Syllabus
Syllabus K327 / K328, Topic 6(b). Explain alveolar adaptations for gas exchange.
About 5 min
Make a guess. You are not marked.
Oxygen passes from alveolar air into your blood. What moves it across?
Show the answer
Diffusion down a concentration gradient.
Alveolar air has more oxygen than the blood arriving. Oxygen diffuses across the thin walls; no pump is needed.
The key idea
A large total surface area, thin moist walls and maintained gradients make alveoli effective exchange surfaces.
The two gases exchange at the same time in opposite net directions. The diagram isolates one arrow; it does not imply one-way motion of every molecule. Walls and gaps are enlarged for visibility.
Explanation
Each alveolus has a thin wall, and the neighbouring capillary wall is also thin. Gases cross only a short distance between air and blood. A moist lining allows gases to dissolve before crossing the exchange surface.
The many alveoli together provide a large surface area. This permits a large amount of gas to cross at the same time. A single alveolus is small, but the combined exchange area is large.
Incoming blood has a lower oxygen concentration than alveolar air, so oxygen diffuses into the blood and enters red cells, where much binds to haemoglobin. The blood has a higher carbon dioxide concentration than alveolar air, so carbon dioxide diffuses in the opposite net direction.
Ventilation replenishes oxygen in the alveoli and removes carbon dioxide. Blood flow brings more oxygen-poor blood and takes oxygen-rich blood away. Together they maintain steep gradients for continued diffusion.
In precise gas physiology, these differences can be expressed as partial-pressure gradients. At this level, use the syllabus concentration-gradient explanation consistently. Neither gas is pumped across the alveolar wall by active transport.
Step by step
- 1
State the structural feature
Choose large surface area, thin walls, moist lining or close capillary supply.
- 2
Explain its effect
Link the feature to diffusion area, distance, gas dissolution or a maintained gradient.
- 3
Name the gas and direction
Oxygen and carbon dioxide have opposite net directions here.
Worked example
Damage to alveolar walls
Some walls separating neighbouring alveoli are destroyed. Explain why gas exchange can become less effective.
One way to explain it
Destroying walls reduces the total surface area available for diffusion. Less oxygen can cross into blood in a given time under otherwise comparable conditions, reducing oxygen delivery to body cells.
Why this answer works
- Fewer walls means less exchange area, even if the remaining air spaces are larger.
- State the effect on the rate of diffusion.
- Link the change to oxygen delivery rather than simply saying the lung is weaker.
Watch out for this
A student says: "The capillary wall must be thick to keep air from leaking into blood." What is wrong with this?
Show the answer
Thin alveolar and capillary walls permit rapid diffusion of dissolved gases. Blood and alveolar air remain in separate spaces.
Check your understanding
Why does a continuous capillary blood supply help alveolar gas exchange?
Show the answer
It brings oxygen-poor blood and removes oxygen-rich blood, maintaining a gradient.
Maintaining the difference between alveolar air and incoming blood supports continued net oxygen diffusion.
Quick recall
How do very small air spaces provide fast gas exchange?