You do not suck air into your lungs. You make your chest bigger, the pressure inside drops below the pressure outside, and the air falls in on its own. Getting that order right — muscles, then volume, then pressure, then air — is the whole of this topic.
📚 What you need to know
Ventilation replaces stale air with fresh air, which maintains the concentration gradient between the alveoli and the blood.
Inspiration: diaphragm contracts and flattens; external intercostal muscles contract; ribcage moves up and out; chest volume increases; pressure falls below atmospheric; air moves in.
Expiration is mainly passive: muscles relax, the diaphragm returns to a dome, elastic fibres in the alveoli recoil, volume decreases, pressure rises, air is forced out.
Forced expiration is active: internal intercostal muscles pull the ribs down and in, and abdominal muscles push the organs up against the diaphragm.
Gases always move down a pressure gradient, from high pressure to low pressure.
Why ventilate at all?
If air sat still in your alveoli, the oxygen in it would be absorbed and the carbon dioxide would build up until the concentrations either side of the alveolar wall matched. At that point diffusion stops.
Ventilation prevents that by continually replacing the air in the lungs with fresh air from outside. Combined with continuous blood flow on the other side, it keeps the gradient steep in both directions.
The physics you are being tested on
Bigger container → particles spread out → lower pressure | Smaller container → particles compressed → higher pressure
Inspiration and expiration
The arrows show muscle-driven movement, not airflow. Air movement is a consequence of these changes, never the cause of them.
INSPIRATION — ACTIVE
Diaphragm contracts and flattens
External intercostal muscles contract
Ribcage moves up and out
Volume of the thorax increases
Pressure falls below atmospheric
Air moves down the pressure gradient into the lungs
EXPIRATION — MAINLY PASSIVE
Diaphragm relaxes and becomes dome-shaped
External intercostal muscles relax
Ribcage moves down and in
Elastic fibres in the alveoli walls recoil
Volume decreases, pressure rises
Air is forced out down the pressure gradient
“Passive” does not mean nothing happens. It means no muscle contraction is needed — the lungs were stretched during inspiration and simply spring back, like letting go of a stretched elastic band.
Forced expiration
Sometimes you need to push air out harder than recoil alone allows — blowing out a candle, coughing, exercising hard. Then expiration becomes active:
Internal intercostal muscles contract, pulling the ribs down and in.
Abdominal muscles contract, pushing the organs upwards against the diaphragm.
Chest volume falls further and faster, causing forced exhalation.
Antagonistic pair: external intercostals contract for inspiration, internal intercostals contract for forced expiration. They pull the ribcage in opposite directions, which is exactly what an antagonistic pair means.
Volume and pressure over one breath
The pressure difference is tiny — a fraction of a kilopascal — but that is all air needs. Gases flow down even the gentlest pressure gradient.
Worked examples
WORKED EXAMPLE
Describe how air is drawn into the lungs during inspiration. [4]
Step 1: the musclesThe diaphragm contracts and flattens, and the external intercostal muscles contract.Step 2: the movementThe ribcage moves upwards and outwards.Step 3: volume and pressureThe volume of the thorax increases, so the air pressure inside falls below atmospheric pressure.Step 4: the airAir moves down the pressure gradient, from the higher pressure outside into the lungsKeep the order. Muscles, volume, pressure, air — never the other way round.
WORKED EXAMPLE
Explain why expiration at rest is described as a passive process. [2]
Step 1: what happens to the musclesThe diaphragm and external intercostal muscles simply relax rather than contracting.Step 2: what drives the air outElastic fibres in the alveoli walls recoil after being stretched, reducing lung volume without muscle contractionCompare with forced expiration, where internal intercostals and abdominal muscles do contract.
WORKED EXAMPLE
Explain why ventilation is necessary for efficient gas exchange in the alveoli. [3]
Point 1: what would happen without itOxygen in the alveolar air would be used up and carbon dioxide would build up, so the concentrations either side of the wall would equalise.Point 2: what ventilation doesIt replaces stale air with fresh air, keeping alveolar oxygen high and carbon dioxide low.Point 3: the effectA steep concentration gradient is maintained, so diffusion continues at a high rate
💡 Exam tip
Write the chain in order: muscles → volume → pressure → air. Marks are usually awarded per link.
Say external intercostals for inspiration and internal intercostals for forced expiration. The word matters.
Describe the diaphragm properly: it contracts and flattens, it does not “move down” on its own.
Use “lower than atmospheric pressure”, not just “low pressure”.
Mention elastic recoil when explaining passive expiration — it is often the difference between two and three marks.
Link ventilation back to the concentration gradient whenever the question mentions gas exchange.
⚠ Common mix-up
Saying lungs “suck” air in. Lungs have no muscle of their own. The chest changes size and air follows the pressure gradient.
Getting the order backwards. Pressure changes because volume changed, not the other way round.
Saying the diaphragm relaxes to flatten. It contracts to flatten and relaxes into a dome.
Treating all expiration as passive. Forced expiration is active.
Confusing ventilation with gas exchange. Ventilation moves air; gas exchange moves gases across a surface.
Writing “the ribs move up and down”. Up and out on inspiration, down and in on expiration.
Up next: Measuring Lung Volumes — how a spirometer turns all this movement into a trace you can take numbers off.
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