The air pathway, and the muscles that move the air
Air passes larynx → trachea (held open by rings of cartilage) → bronchi (one per lung) → bronchioles → alveoli, each alveolus wrapped in capillaries. Gas exchange happens only across the shared alveolus-capillary wall. Breathing is a separate, mechanical job: the diaphragm (dome of muscle forming the floor of the thorax) and the intercostal muscles (between the ribs) change the volume of the thorax. B11.1.1 expects every one of these named in a diagram or photograph.
Inspired vs expired air — three comparisons
Expired air has less oxygen (some diffused into the blood for respiration in body cells), more carbon dioxide (made by respiration and diffused out of the blood) and more water vapour (evaporated from the moist lining of the airways and lungs). The credit sits on the comparison, not on the number. Limewater is the practical test for the carbon dioxide difference: it turns cloudy/milky far faster with expired air, because its carbon dioxide content is much higher.
(Extended) Surface features, and the exercise chain
Four features make the alveoli an efficient gas exchange surface: large surface area, thin surface (alveolus and capillary walls each one cell thick), good blood supply and good ventilation with air. The first two shorten and widen the diffusion path; the last two keep the concentration gradient steep. Separately, exercise makes muscles respire faster, blood carbon dioxide rises, the brain detects the rise, and both the rate and the depth of breathing increase.
Drawn from real examiner reports.
Rate rises — so does depth
Physical activity increases both the rate of breathing (breaths per minute) and its depth (volume of air moved per breath). Depth is the element candidates leave out. "Breathing gets faster" on its own is incomplete, and it describes the change rather than explaining it — both elements must appear before the explanation marks are reachable.
Jun 2022 P11 Q8; P42 Q10d — depth of breathing often omitted alongside rate.
A vague cause instead of the CO2 chain
The explanation the mark scheme wants is a chain: muscles respire faster → carbon dioxide concentration in the blood rises → the brain detects the rise → the brain increases the rate and depth of breathing. Vague causes such as "damage to the respiratory system" earn nothing. The two load-bearing links are the rise in blood carbon dioxide and the brain detecting it.
Jun 2022 P11 Q8; P42 Q10d — vague "damage to respiratory system" answers given instead of the mechanism.
Water vapour goes missing
Three comparisons are required and the water-vapour one is the most frequently dropped. Expired air contains more water vapour than inspired air because water evaporates from the moist lining of the airways and lungs into the air passing through. An answer covering only oxygen and carbon dioxide caps itself below full marks however well the other two are argued.
Jun 2022 P31 Q7b; Nov 2022 P43 Q1biii — water-vapour difference frequently omitted.
Values quoted without the comparison
Naming a figure — "expired air is 4% carbon dioxide" — does not by itself answer a differences question. The credited wording is comparative: less oxygen, more carbon dioxide, more water vapour THAN INSPIRED AIR. Lead with the comparison for each of the three, then add a value only if the question actually asks for one.
Jun 2022 P31 Q7b; Nov 2022 P43 Q1biii — comparative wording essential across all three gases.
An incomplete list of surface features
There are exactly four: large surface area, thin surface, good blood supply, good ventilation with air. Offering two or three is the usual loss. Loose wording costs too — "big" is not "large surface area", "small" is not "thin". Where the command word is explain rather than describe, each feature must also be linked to a faster rate of diffusion.
Nov 2022 P42 Q10cii — the four gas-exchange surface features incompletely or vaguely recalled.
Gas exchange ≠ respiration (B12)
Gas exchange is physical diffusion between the alveoli and the blood. Respiration is the chemical process inside body cells that releases energy from glucose and produces the carbon dioxide in the first place. Losing carbon dioxide at the lungs is gas exchange, not respiration — and respiration is never a word for breathing.
Breathing ≠ gas exchange
Breathing (ventilation) is the mechanical movement of air in and out of the lungs, driven by the diaphragm and intercostal muscles changing thoracic volume. Gas exchange is the diffusion of oxygen and carbon dioxide across the alveolus-capillary wall. Breathing delivers fresh air to the exchange surface; it does not itself move any gas into the blood.
Run a fixed three-gas frame
Answer every composition question in the same order — oxygen, carbon dioxide, water vapour — each as more/less than inspired air plus its reason. Keeping the frame fixed is what stops the water-vapour comparison being dropped under time pressure.
Name the feature, then its mechanism
Describe wants the four names. Explain wants each name plus its effect: large surface area → more diffusion happening at once; thin surface → short diffusion distance; good blood supply and good ventilation → each keeps the concentration gradient steep.
Write the exercise answer as four links
Set it out as a chain: muscles respire faster, blood carbon dioxide rises, the brain detects it, rate and depth both increase. The marks sit on the links, so a chain of four short clauses scores where one sentence about faster breathing does not.
Convert counts before you compare
Breathing data usually arrives as breaths counted in 15 or 30 seconds. Divide by the time in minutes (0.25 or 0.5) to get breaths/minute before comparing anything. For a percentage change, always divide by the resting value, never the exercise value.
Composition of inspired vs expired air (comparative, all three needed):
| Gas | Change from inspired to expired air | Reason |
|---|---|---|
| Oxygen | Less | Oxygen diffuses out of the alveoli into the blood, for respiration in body cells |
| Carbon dioxide | More | Carbon dioxide, produced by respiration in body cells, diffuses out of the blood into the alveoli |
| Water vapour | More | Water evaporates from the moist lining of the airways and lungs into the air passing through |
(Supplement) Features of a gas exchange surface — all four needed:
| Feature | Why it speeds up diffusion |
|---|---|
| Large surface area | More room for gas molecules to diffuse across at the same time |
| Thin surface (one cell thick) | Short diffusion distance for gases to cross |
| Good blood supply | Blood constantly carries gases away/in, maintaining a steep concentration gradient |
| Good ventilation with air | Fresh air constantly replaces stale air, also maintaining a steep concentration gradient |
(Supplement) Effect of physical activity on breathing: faster respiration in muscles produces more carbon dioxide raises the carbon dioxide concentration in the blood detected by the brain the brain increases BOTH the rate (breaths per minute) and the depth (volume of air per breath) of breathing.
Define gas exchange, and state where in the breathing system it happens.
Describe an investigation, using limewater, to compare the carbon dioxide content of inspired air and expired air. State the expected result. (4 marks)
