Breathing and gas exchange · Model
Alveoli: built for exchange
Oxygen molecules cross from your alveoli into your blood, and at the same moment other oxygen molecules cross the other way. You survive on the difference between two numbers.
Start here
Replace 500 million alveoli with one smooth bag of the same volume.
Same total air, same six litres of chest, same clear airway, same blood supply arriving. The only change is that the sponge has been replaced by a single smooth-walled cavity.
How long would you survive?
Minutes. The bag holds the same air and offers roughly a twelfth of a square metre of exchange surface instead of about seventy. Volume is not what keeps you alive — surface is, and a bag is the worst possible shape for it.
At the bench · count the crossings
Both directions, all the time
both flows running
Oxygen molecules cross the alveolus wall in both directions every second. Switch breathing and blood flow on and off and watch what happens to the two counts — and to the difference between them.
Oxygen in the alveolus
more oxygen here
Oxygen in the blood
less oxygen here
Net oxygen absorbed
720 per second
Partial pressures in this state — alveolus 13.3 kPa, blood 5.3 kPa.
Both flows running. Crossings happen in both directions and you gain the difference. Neither number is zero, and the outward one never stops.
Blood flow stopped. Oxygen keeps crossing inwards until the blood side has almost caught up, then the two counts nearly match and net absorption collapses. The surface, the thin wall and the fresh air are all still there.
Breathing stopped. The blood keeps draining oxygen away, so the alveolar level falls until the difference is almost gone. Note that stopping either side has the same effect for the same reason.
Both stopped. The two sides have equalised in the middle. Molecules are still crossing in both directions at exactly the same rate — nothing has finished, nothing has settled, the imbalance has simply gone.
Same four features, second organ
A villus and an alveolus solve the same problem the same way.
- Requirement 1 · Large surface areaFolds, villi and microvilli — about 30 m² in six metres of tube.About 500 million alveoli — roughly 70 m² inside a chest you can put your arms around.
- Requirement 2 · Short diffusion distanceVillus wall one cell thick, with the capillary immediately behind it.Alveolus wall one cell thick and capillary wall one cell thick — two cells between air and blood.
- Requirement 3 · Steep concentration difference maintainedBlood flows past constantly, carrying absorbed glucose away.Blood flow drains one side and breathing refreshes the other. Two flows instead of one, because gas moves both ways.
- Requirement 4 · Moist surfaceEverything is already in solution in the gut contents.A film of liquid lines every alveolus, so gases dissolve before they cross.
Key fact
Alveoli give a huge surface area, a wall one cell thick, a dense capillary network and a moist lining. Oxygen and carbon dioxide cross by diffusion, in both directions at once, and the net movement follows the difference in concentration.
Think again
“Oxygen is pumped across into the blood.”
There is no pump anywhere in the alveolus, no channel that grabs oxygen, and nothing that spends energy moving it. The whole transfer is diffusion, which you met with bromine in Diffusion, and it is worth being exact about what that means here. Oxygen molecules are moving randomly in every direction. Some happen to hit the alveolus wall from the air side and cross; some happen to hit it from the blood side and cross the other way. There are more of them on the air side, so more crossings happen inwards than outwards, and the difference is what your body gains. Nothing is aimed and nothing is pushed.
“Oxygen moves in because it wants to spread out evenly.”
This one you also met in C1, and it is worth killing twice because the biological version feels so much more reasonable. A molecule has no aim, no preference and no information about where it is. Look at the bench above with the blood flow switched off: within seconds the two sides very nearly equalise and the net absorption drops to about zero — not because the molecules have finished spreading, but because crossings inwards and crossings outwards have become equally likely. Molecules are still moving at hundreds of metres per second and still crossing constantly in both directions. What has stopped is the imbalance, and the imbalance is the only thing that was ever doing anything.
“Alveoli are where the air is stored.”
Nothing is stored. About half a litre of air moves in and out with each quiet breath, and the alveolar air is partly replaced roughly twelve times a minute — which is precisely what keeps the alveolar oxygen high and the gradient open. Switch breathing off on the bench and you can watch the alveolar figure fall as the blood takes oxygen away. The alveoli are not a tank; they are a surface being continually refreshed on one side and continually drained on the other. Both flows exist to hold the difference open.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Why so many
Why are there 500 million small alveoli rather than one large cavity of the same total volume?
Rung 2 · The one that catches people
You hold your breath. Blood keeps flowing. After a while, net oxygen absorption falls close to zero. Why?
Rung 3 · Explain the two flows
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 · Take it somewhere new
At the top of a high mountain the air still contains 21% oxygen, yet climbers become short of oxygen. Using the bench, explain why — and why breathing faster helps a little but does not solve it.
Key note
Around 500 million alveoli give a total gas exchange surface of roughly 70 m². Each has a wall one cell thick, a moist lining and a dense capillary network. Oxygen diffuses from alveolar air into the blood and carbon dioxide diffuses the other way. Breathing refreshes the air side and blood flow drains the blood side, and both are needed to keep the concentration difference open.
Going further
A wet surface that thin should stick shut, and in premature babies it sometimes does. Alveoli are lined with a film of liquid, and the surface tension of that film pulls the walls together — enough that inflating them would take more force than a newborn's muscles can produce. The solution is surfactant, a detergent-like substance made by cells in the alveolar wall that lowers the surface tension dramatically. It is produced late in pregnancy, from about week 24 onwards, which is a large part of why gestational age matters so much to a premature baby's survival. Artificial surfactant delivered directly into the lungs has been standard treatment since the 1990s and is one of the clearest examples of a physics problem solved by a chemical.
Before this lesson
- This follows on from How breathing works.
Connects to
- Exercise, asthma and smoking
- Absorption and the small intestine
Nothing has been added — the same four requirements, met by a different structure. That repetition is the point.
- 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 Mr Badmus AI
Want to check what happens to the crossings when the gradient closes?
Crossing counts on the bench are illustrative numbers chosen to make the two-way nature of diffusion visible; they are not measured rates. Partial pressures are typical resting values in kPa.
Lesson content © MrBadmusAI.