Breathing and gas exchange · Model
Alveoli: built for exchange
Start here
Imagine swapping all 500 million alveoli for one smooth bag of the same volume.
If you had to guess, could you stay alive with the bag instead?
No, not for long. A smooth bag has far too little surface for gas exchange.
No, not for long. A smooth bag has far too little surface for gas exchange.
Key words
One of the roughly 500 million tiny air sacs where gas crosses between the lungs and the blood.
The net movement of particles from where they are more concentrated to where they are less concentrated, caused by their random motion.
The smallest blood vessel, with a wall one cell thick, carrying blood past every alveolus.
- concentration
Key knowledge
Air in the alveoli has a high concentration of oxygen.
Blood arriving at the lungs has a low concentration of oxygen.
So oxygen diffuses from the alveoli into the blood.
Carbon dioxide diffuses the other way, from the blood into the alveoli.
How alveoli are adapted
| Row | The feature | Why it helps |
|---|---|---|
| Large surface area | ||
| Thin walls | ||
| Good blood supply | ||
| Moist lining |
Breathing brings fresh air into the alveoli.
Blood flow carries oxygen away from them.
Together they keep the concentration difference high.
If either one stops, oxygen stops entering the blood overall.
Wrong: Oxygen is pumped into the blood.
Right: Oxygen moves by diffusion. No pump is involved.
Wrong: Oxygen moves because it wants to spread out.
Right: Particles move randomly. More cross from the high concentration side.
Wrong: Gases only cross in one direction.
Right: Particles cross both ways. The overall movement follows the concentration difference.
Key fact
Alveoli have a large surface area, thin walls and a good blood supply. Oxygen diffuses into the blood and carbon dioxide diffuses out.
Practice
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1
Why are there 500 million small alveoli rather than one large cavity of the same total volume?
Rung 2
You hold your breath. Blood keeps flowing. After a while, oxygen almost stops entering the blood overall. Why?
Rung 3
Explain why gas exchange needs both breathing and blood flow, and what happens if either one stops. Use the words diffusion and concentration difference, and do not use the words pump, suck or want.
Rung 4
At the top of a high mountain the air still contains 21% oxygen, yet climbers become short of oxygen. Using what you have learnt, explain why. Then explain why breathing faster helps a little but does not solve it.
The small intestine absorbs digested food.
It also has a large surface area, thin walls and a good blood supply.
Air on a mountain is still 21% oxygen.
But the air is less dense, so each breath holds less oxygen.
The concentration difference is smaller, so less oxygen diffuses into the blood.
Breathing faster helps a little, but it cannot make the air denser.
Before this lesson
- This follows on from How breathing works.
Connects to
- Exercise, asthma and smoking
- Absorption and the small intestine
The same features of a good exchange surface, in a different organ.
- Diffusion
At GCSE this becomes
- Exchange surfaces and surface area to volume ratio, plus Fick's law relating rate to area, concentration difference and thickness.
Where to next
Ask MrBadmus
Want to check what happens to the crossings when the gradient closes?
Lesson content © MrBadmus.