Ventilation is the movement of air into and out of the lungs. To explain it, link muscle movement to chest volume, chest volume to pressure, and pressure to air flow. It appears in questions about breathing, exercise and models of the chest.
This lesson opens gas exchange and respiration and prepares you for what happens once air reaches the alveoli.
What is the key idea?
Air moves from a region of higher pressure to a region of lower pressure. The lungs have no muscle of their own, so the thorax does the work. Changing the size of the thorax changes the pressure inside it.
If the volume of a fixed amount of gas gets bigger, the gas spreads out and its pressure goes down. If the volume gets smaller, the pressure goes up.
How do you explain inhaling, step by step?
- The external intercostal muscles contract, which moves the ribs up and out.
- The diaphragm contracts and flattens, moving down.
- The volume of the thorax increases.
- The pressure in the thorax falls below the pressure of the air outside.
- Air flows in through the airways and the lungs inflate.
Exhaling reverses the chain.
The diaphragm relaxes and rises into a dome, the ribs move down and in, the volume decreases, the pressure rises above atmospheric, and air flows out. In forceful breathing out, the internal intercostal muscles contract to pull the ribs down and in. Check the Cambridge syllabus for how much of this your exam year expects.
Worked example
A model has a bell jar with a balloon inside it fixed to a tube. A rubber sheet closes the bottom of the jar, and a student pulls the sheet downwards. The balloon inflates.
Step 1, name what changed: pulling the sheet down increases the volume of the space in the jar.
Step 2, link to pressure: the same air now has more space, so the pressure in the jar falls. In this invented example, it drops from 101 kPa to 98 kPa.
Step 3, link to flow: the pressure outside the jar is still 101 kPa, which is higher than the 98 kPa inside. Air flows through the tube into the balloon.
Step 4, link to the body: the rubber sheet represents the diaphragm, the balloon represents the lungs and the jar represents the thorax.
Step 5, state a limit: the model has no ribs, so it cannot show the intercostal muscles moving.
The mistake to watch for
A plausible mistake is to write the muscle and stop, or to treat the lungs as the active part.
Mistaken answer: “The diaphragm moves down and the lungs suck in air.”
This names one muscle but skips volume and pressure, and it says the lungs act.
The correction is to use the full chain: the diaphragm contracts and moves down, so the thorax volume increases, so the pressure falls below atmospheric pressure, so air flows in. Every “so” is a mark-worthy link.
Check yourself
1. State what happens to the thorax volume and the pressure when the diaphragm relaxes.
Show answer
The diaphragm rises into a dome, so the thorax volume decreases and the pressure increases above atmospheric pressure, which pushes air out.
2. In the bell jar model, what does the balloon represent, and why is this only a model?
Show answer
It represents the lungs. It is a model because there are no ribs, no real airways and no alveoli, and the rubber sheet moves differently from a real diaphragm.
3. A student writes: “The ribs move up and out, so air is pulled in.” Rewrite it as a complete explanation.
Show answer
The external intercostal muscles contract, moving the ribs up and out, so the thorax volume increases. The pressure inside falls below atmospheric pressure, so air flows into the lungs.
Where this leads next
Once you can explain air movement, go on to alveolar structure and diffusion, then test yourself with the gas exchange and respiration practice set.
Some students can recite the steps but lose marks when a question phrases them differently. Our teachers work on that in online one-to-one Biology tuition.