IB Biology HLGas Exchange SystemsPaper 1 & 2~11 min read
How Ventilation Works
You cannot pull air into your lungs. Nothing in your chest grabs it. What actually happens is that muscles make the space inside your chest bigger, the pressure inside drops below the pressure outside, and the air walks in on its own. Get that one idea and the whole topic falls into place.
📚 What you need to know
Ventilation replaces the used air in the lungs with fresh air, which maintains the concentration gradient of both gases between alveoli and blood.
Ventilation has two parts: inspiration (breathing in) and expiration (breathing out).
If the volume of a container goes up, the pressure inside goes down, and gases move down a pressure gradient.
Inspiration: diaphragm contracts and flattens, external intercostal muscles contract, ribcage moves up and out, chest volume rises, pressure falls below atmospheric, air rushes in.
Expiration is normally passive: muscles relax, the diaphragm domes up, the ribcage drops down and in, and elastic fibres in the alveoli recoil.
Forced expiration is active: internal intercostal muscles and abdominal muscles contract to push air out hard.
External and internal intercostal muscles are an antagonistic pair.
Why bother breathing at all?
Gas exchange at the alveolus only works while the air inside it is different from the blood outside it. If you took one breath and stopped, oxygen in that pocket of air would fall, carbon dioxide would build up, the gradients would flatten and diffusion would stop.
Ventilation keeps refreshing the air, so the alveolus stays high in oxygen and low in carbon dioxide. That is the whole job. Everything else is mechanics.
The rule behind every breath
volume up → pressure down → air moves in volume down → pressure up → air moves out
Your chest is the container. The muscles change its volume, and the pressure change does the rest.
Inspiration: the active one
Breathing in takes muscle work every single time.
The diaphragm contracts and flattens, pulling downwards. This makes the chest cavity taller.
The external intercostal muscles contract, pulling the ribcage up and out. This makes the chest cavity wider.
Chest volume increases, so the pressure inside falls below atmospheric pressure.
Air moves down the pressure gradient from outside to inside, and the lungs fill.
Notice the order of cause and effect: muscles → volume → pressure → air movement. Answers that jump straight from “the diaphragm contracts” to “air comes in” miss the two marks in the middle.
Expiration: mostly the lazy one
At rest, breathing out costs you almost nothing. The muscles that worked so hard simply stop working.
The external intercostal muscles relax, so the ribcage falls down and in.
The diaphragm relaxes and returns to its dome shape.
Elastic fibres in the alveoli walls, stretched during inspiration, recoil and shrink the lungs.
Chest volume falls, pressure rises above atmospheric, and air is forced out down the pressure gradient.
This is why normal expiration is described as a passive process. Nothing contracts to push the air out; things relax and spring back.
Sometimes you need more than that – blowing out candles, coughing, or exercising hard. Then expiration becomes active:
Internal intercostal muscles contract, pulling the ribs firmly down and in.
Abdominal muscles contract, pushing the organs up against the diaphragm and cutting chest volume further.
The result is forced exhalation, a much bigger and faster movement of air.
Two things change chest volume at once – the diaphragm changes its height, the ribcage changes its width.
Stage
Inspiration
Expiration at rest
Diaphragm
Contracts, flattens, moves down
Relaxes, returns to a dome
External intercostals
Contract
Relax
Ribcage
Moves up and out
Moves down and in
Chest volume
Increases
Decreases
Pressure in the thorax
Falls below atmospheric
Rises above atmospheric
Air movement
In, down the pressure gradient
Out, down the pressure gradient
Energy needed
Active, muscles contract
Passive, muscles relax and tissues recoil
Antagonistic muscles
Muscles can only pull, never push. So moving the ribcage in two directions needs two sets of muscles working against each other:
External intercostals contract to raise the ribs – used in inspiration.
Internal intercostals contract to lower the ribs – used in forced expiration.
When one set contracts the other relaxes. That is what antagonistic means, and it is a nice one to name in an answer.
🧠
Ex for Exit? No – the other way round
Students guess that external intercostals do expiration. They do not. External = breathing IN, internal = forced breathing out. Learn it as the exception it is.
Worked examples
WE 1
Describing inspiration
Describe how air is drawn into the lungs during inspiration. (4 marks)
Step 1: the muscles
The diaphragm contracts and flattens and the external intercostal muscles contract.
Step 2: the movement
The ribcage moves upwards and outwards.
Step 3: volume and pressure
The volume of the thorax increases, so the pressure inside falls below atmospheric pressure.
Step 4: the air
Air moves down the pressure gradient from outside into the lungs.
Muscles → volume up → pressure down → air infour marks, four steps – never skip the volume and pressure line
WE 2
Why expiration is described as passive
Explain why expiration at rest is described as a passive process, and how it differs during hard exercise. (3 marks)
Point 1: at rest
The diaphragm and external intercostal muscles simply relax, so no muscle contraction is needed to push the air out.
Point 2: what moves the air
The stretched elastic fibres in the alveoli recoil and the ribcage drops under its own weight, reducing volume and raising pressure.
Point 3: during exercise
Expiration becomes active: internal intercostal muscles pull the ribs down and in, and abdominal muscles push the organs up against the diaphragm, forcing air out faster.
At rest it is recoil and relaxation; forced out it needs musclesthe word “recoil” is worth including – it is often on the mark scheme
WE 3
Linking ventilation to diffusion
Explain how ventilation increases the rate of gas exchange in the alveoli. (3 marks)
Point 1: what ventilation does
It replaces the used air in the alveoli with fresh air from outside.
Point 2: the gradients
Fresh air keeps the oxygen concentration in the alveolus high and the carbon dioxide concentration low, compared with the blood.
Point 3: the result
Both concentration gradients stay steep, so oxygen keeps diffusing into the blood and carbon dioxide keeps diffusing out, at a fast rate.
Fresh air = steep gradients = fast diffusionmention both gases; answers that only talk about oxygen usually drop a mark
💡 Exam tips
Write the full chain every time: muscle → volume → pressure → air movement.
Say the diaphragm contracts and flattens, not “moves down” on its own.
Use the words above and below atmospheric pressure rather than “high” and “low”.
If asked to compare, set your answer out as pairs: in this one X, in that one the opposite of X.
Name the elastic fibres and their recoil for passive expiration.
Do not say lungs “expand by themselves”. They follow the chest wall.
⚠ Common mistakes
Saying the lungs suck air in. Air moves in because the pressure inside is lower, not because anything pulls it.
Swapping the intercostals. External for breathing in, internal for forced breathing out.
Writing that the diaphragm relaxes to breathe in. It contracts.
Saying pressure increases when volume increases. They move in opposite directions.
Calling all expiration passive. Forced expiration is active.
Confusing ventilation with gas exchange. Ventilation moves air in bulk; gas exchange is diffusion across the alveolar wall.
Up next: Measuring Lung Volumes – how a spirometer turns all this movement into a graph you can read tidal volume, vital capacity and ventilation rate straight off.
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