Breathing and gas exchange · Model
How breathing works
Your lungs contain no muscle at all. They cannot pull air in, and they never have. Something else moves, and the air follows.
Start here
A puncture wound between two ribs, and the lung collapses.
A narrow wound opens the space between the chest wall and the lung to the outside air. Nothing has touched the lung itself — it is undamaged, and the airway is completely clear. Within seconds that lung has emptied and will not reinflate.
Nothing is blocking it. Why can it not fill?
Because a lung has never inflated itself. It is filled by being at a lower pressure than the outside air, and that difference is produced by muscles enlarging the chest. Let air into the space around the lung and the difference vanishes — the lung is intact, the airway is clear, and it still cannot fill.
The bell-jar model · work the diaphragm
Pull the sheet down and read the pressure
not moved yet
A sealed jar, a rubber sheet across the bottom, and a balloon on a tube through the lid. Move the sheet and watch the order in which things change.
The jar
Diaphragm contracted — breathing inDiaphragm relaxed — breathing outAt rest between breaths
Readouts
- Chest volume
- 2.9 L
- Pressure inside
- -0.00 kPa
- Pressure outside
- 0.00 kPa (atmospheric)
- Air movement
- inwardsoutwardsnone
Diaphragm
contracted, flattenedrelaxed, domed upresting position
The order of events
- Diaphragm contracts and flattens; intercostals lift the ribs.
- Chest volume increases — now 2.9 litres.
- The same air fills a bigger space, so pressure falls to 0.00 kPa below atmospheric.
- Air moves in, from higher pressure outside to lower pressure inside.
- Diaphragm relaxes and domes upwards; ribs drop.
- Chest volume decreases — now 2.9 litres.
- The same air fills a smaller space, so pressure rises to 0.00 kPa above atmospheric.
- Air moves out, from higher pressure inside to lower pressure outside.
- Diaphragm at rest, domed upwards.
- Chest volume steady at 2.9 litres.
- Pressure inside equals atmospheric.
- No net air movement in either direction.
Read the list downwards. The muscles are the cause, the pressure difference is the mechanism, and the air moving is the result — never the other way round.
Read the list downwards. The muscles are the cause, the pressure difference is the mechanism, and the air moving is the result — never the other way round.
Notice which line changes first.
Where the model fails
Four things the bell jar gets wrong, and why it is still the right model.
Wrong 1 · No ribs, no intercostal muscles
The jar is rigid, so the only thing that can move is the base. In you, the ribs swing up and out at the same time, and the intercostal muscles supply roughly a third of quiet breathing. Still worth having because: It isolates one variable. Seeing what the diaphragm alone does is easier than seeing two muscle groups act at once.
Wrong 2 · Balloons are bags, not alveoli
A balloon has one smooth inner surface. A lung ends in 500 million alveoli, which is the entire reason gas exchange can happen fast enough to keep you alive. Still worth having because: This model is not about exchange — it is about ventilation. The next lesson takes the alveoli seriously.
Wrong 3 · A flat sheet, not a dome
The rubber sheet is pulled down into a cone. A real diaphragm is a dome at rest and flattens when it contracts — so it moves down by flattening, not by being pulled. Still worth having because: The direction of movement and its effect on volume are still right, which is what the model is for.
Wrong 4 · Nothing is alive
The jar has no elastic recoil to speak of, no surfactant keeping the alveoli from sticking shut, no nerves, and nothing regulating the rate. Still worth having because: A model with all of that in it would be a chest, and you could not see inside it.
A model that got everything right would be a chest. Knowing precisely where a model stops being true is what makes it usable — the same argument as in Testing the model.
Key fact
Muscles change the volume of the chest. Changing the volume changes the pressure. Air then moves from higher pressure to lower pressure. Volume first, pressure second, air last — always in that order.
Leans on · Physics. The pressure reasoning here is the same as in Gas pressure: pressure comes from particles colliding with the walls, so the same number of particles in a bigger space collide less often and the pressure falls. Physics P5 Pressure owns the full quantitative treatment; this lesson uses only the qualitative rule, and nothing here needs to be unlearnt when you meet it.
Think again
“The lungs expand and pull the air in.”
The lungs have no muscle tissue anywhere in them, so there is nothing in a lung that could pull. They are elastic bags that are stretched by the space around them growing, in the way a plastic bag pressed against the inside of a widening box is stretched by the box. The muscles that do the work are the diaphragm underneath and the intercostals between the ribs, and neither of them is part of a lung. This is not a technicality: it is the whole explanation of the collapsed lung in the hook, of why a ventilator has to push rather than persuade, and of why paralysis of the diaphragm stops breathing while leaving perfectly healthy lungs in place.
“Air rushes in, and that is what makes the chest get bigger.”
This gets the causation exactly backwards, and it is the most common wrong answer in the topic. Watch the readouts on the model: the volume changes first, the pressure changes because of it, and the air moves last, in response. Nothing about incoming air can enlarge a chest — if it could, you would inflate when the wind blew. The test that settles it is the sealed jar: clamp the tube so no air can enter, pull the sheet down, and the volume still increases and the pressure still falls. The chest moving is the cause; the air arriving is the consequence.
“Something sucks the air in.”
Sucking is not a thing that exists. There is no force that reaches out and draws air towards a low pressure; there is only air at higher pressure being pushed by its own particles into a space where fewer particles are pushing back. The atmosphere does all the work of every breath you take, and your muscles only ever make room for it. This matters for the same reason it mattered in Diffusion — describing a pressure difference as a pull invents a mechanism that is not there, and then you cannot explain why a straw stops working at the top of a very tall glass.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Get the order right
Put these in the order they happen when you breathe in.
Rung 2 · The one that catches people
You seal the tube on the bell-jar model so no air can get in or out, then pull the rubber sheet down. What happens?
Rung 3 · Explain the collapsed lung
Explain why a small wound between the ribs can collapse an undamaged lung with a clear airway. Use the words volume, pressure and diaphragm, and say why enlarging the chest no longer helps.
Rung 4 · Take it somewhere new
A snorkel 2 m long is used to breathe while lying on the bottom of a pool. Explain, using pressure, why this is far harder than snorkelling at the surface — and why the difficulty is about the chest rather than the tube.
Key note
Breathing in: the diaphragm contracts and flattens, the intercostal muscles contract and lift the ribs up and out, chest volume increases, pressure inside falls below atmospheric, and air moves in. Breathing out reverses it, mostly by the muscles relaxing and the chest recoiling elastically. Volume changes first; air moves last.
Going further
The iron lungs used during polio epidemics worked on exactly the principle this lesson describes, and worked on the outside of the patient. The machine sealed the body from the neck down and cycled the pressure in the tank: drop it, and the patient's chest expands because the outside pressure is now lower than the pressure in their airway, so air flows in through the mouth. No tube, no pushing, nothing entering the body — the ventilator enlarged the chest and let the atmosphere do the rest. Modern ventilators reverse the geometry and push air in under positive pressure through a tube, which is more practical and, unlike the iron lung, is not how a healthy person breathes at all.
Before this lesson
Connects to
- Gas pressure
Where the rule this lesson borrows is derived: the same particles in a bigger space collide with the walls less often.
- Testing the model
Where knowing exactly where a model stops being true is the point, rather than an admission.
At GCSE this becomes
- Ventilation, lung volumes and spirometer traces, with pressure and volume treated quantitatively.
Where to next
Ask Mr Badmus AI
Want to check you have the order of events the right way round?
Pressure values on the model are illustrative, in kilopascals relative to an atmosphere of about 101 kPa. Real intrapulmonary pressure swings during quiet breathing are under 1 kPa.
Lesson content © MrBadmusAI.