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 K224 / K225, Topic 6(a). Identify named airways, alveoli and associated capillaries and their functions.
I can explain alveolar adaptations for gas exchange.Syllabus
Syllabus K224 / K225, 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
The wall of each alveolus is very thin, and so is the wall of the capillary next to it. So gases only have a short distance to travel between air and blood. The inside of the alveolus is moist, so gases dissolve before they cross.
There are millions of alveoli. Together they give a very large surface area, so a lot of gas can cross at the same time.
Blood arriving at the lungs has less oxygen than the air in the alveoli, so oxygen diffuses into the blood. It enters red blood cells and joins with haemoglobin. The blood has more carbon dioxide than the air in the alveoli, so carbon dioxide diffuses the other way, into the alveoli.
Breathing brings fresh oxygen into the alveoli and removes carbon dioxide. Blood flow brings new low-oxygen blood and takes oxygen-rich blood away. Together, they keep the concentration gradients steep, so diffusion keeps going.
Neither gas is pumped across. Both cross by diffusion, down their concentration gradients.
Step by step
- 1
Pick a feature
Choose one: a large surface area, thin walls, a moist lining or many capillaries close by.
- 2
Say what it does
Link the feature to more area, a shorter distance, gases dissolving, or a steep difference in concentration.
- 3
Name the gas and the way it goes
Oxygen goes into the blood. Carbon dioxide comes out of the blood.
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?