Cambridge IGCSE Biology 0610: Gas exchange in humans – Revision Notes

For full explanations and worked examples, use the gas exchange study guide. These notes condense Topic 11, Gas exchange in humans (section 11.1), of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. Core outcomes 11.1.1 to 11.1.5 are for everyone. Supplement outcomes 11.1.6 to 11.1.11 are marked Extended only, for candidates taking Papers 2 and 4.

Links: Cambridge IGCSE Biology hub · Biology checklist · gas exchange practice questions · Core diagnostic · Extended diagnostic.

Key terms

Term	Meaning
Gas exchange	Diffusion of oxygen into the blood and carbon dioxide out of the blood, across the alveolus and capillary walls
Ventilation (breathing)	Movement of air into and out of the lungs
Alveoli	Tiny air sacs at the ends of the bronchioles; the gas exchange surface
Inspired air	Air breathed in
Expired air	Air breathed out
Rate of breathing	Number of breaths per minute
Depth of breathing	Volume of air taken in with each breath
Limewater	Test for carbon dioxide: turns milky (cloudy)

Breathing is not respiration. Respiration releases energy in cells; see the respiration study guide.

11.1.1 The four features of a gas exchange surface

The syllabus limits you to exactly these four. Pair each with its reason.

Feature	Reason
Large surface area	More diffusion at once
Thin surface	Short diffusion distance
Good blood supply	Maintains a steep concentration gradient (blood side)
Good ventilation with air	Maintains a steep concentration gradient (air side)

11.1.2 The breathing system

Route of air: nose/mouth → larynx → trachea → bronchi → bronchioles → alveoli

Also know: lungs, ribs, intercostal muscles (between the ribs), diaphragm (sheet of muscle under the lungs), capillaries around each alveolus.

Extended only

• 11.1.6: external intercostal muscles = outer layer; internal intercostal muscles = inner layer.
• 11.1.7: cartilage in the trachea keeps it open so it does not collapse when pressure falls during breathing in.

11.1.8 Ventilation (Extended only)

Method in steps: always write the full chain.

Muscles → ribs and diaphragm → volume → pressure → air flow

	Breathing in	Breathing out
External intercostals	Contract	Relax
Internal intercostals	Relax	Contract (especially forced breathing out)
Ribs	Up and out	Down and in
Diaphragm	Contracts, flattens (moves down)	Relaxes, domes (moves up)
Thorax volume	Increases	Decreases
Thorax pressure	Decreases (below atmospheric)	Increases (above atmospheric)
Air	Flows in	Flows out

Memory hook: in = external contract, diaphragm down, volume up, pressure down.

11.1.3 and 11.1.4 Inspired v expired air (Core)

Gas	Inspired	Expired
Oxygen	more (about 21%)	less (about 16%)
Carbon dioxide	less (about 0.04%)	more (about 4%)
Water vapour	less (varies)	more (saturated)

Limewater investigation, method in steps

1. Two tubes with equal volumes of the same limewater.
2. Breathe in through one arrangement so room air bubbles through tube A; breathe out so your breath bubbles through tube B.
3. Time how long each tube takes to turn milky.
4. Expired-air tube turns milky first → expired air has more carbon dioxide.

Limewater tests for carbon dioxide only.

11.1.9 Why the composition changes (Extended only)

Difference	Explanation
Less oxygen breathed out	Oxygen diffuses from the alveoli into the blood, then is used in aerobic respiration in cells
More carbon dioxide breathed out	Carbon dioxide made by respiration in cells diffuses from the blood into the alveoli
More water vapour breathed out	Water evaporates from the moist alveolar lining

11.1.5 and 11.1.10 Physical activity and breathing

Core: activity increases rate and depth of breathing. Both fall back to resting values gradually afterwards.

Measuring rate, method in steps

1. Count breaths at rest for a fixed time; convert to breaths per minute.
2. Do a fixed exercise for a fixed time.
3. Count again immediately, then every minute until back to resting.
4. Keep the person, exercise and duration the same; repeat.

Worked reminder: 13 breaths in 30 s = 13 × 2 = 26 breaths per minute.

Worked reminder: rate rises from 18 to 27 breaths per minute. Percentage increase = (27 − 18) ÷ 18 × 100 = 50%.

Extended only, the syllabus chain: more muscle contraction → more aerobic respiration → more carbon dioxide in the blood → detected by the brain → increased rate and greater depth of breathing → more carbon dioxide removed and more oxygen supplied.

11.1.11 Protecting the airways (Extended only)

• Goblet cells secrete mucus.
• Mucus traps pathogens and particles.
• Ciliated cells: cilia beat and move mucus up to the throat, where it is swallowed.

The mucus traps; the cilia move. Cilia do not trap anything.

Must-know distinctions

• Breathing v respiration: air movement in the lungs v energy release in cells.
• Gas exchange v ventilation: diffusion across alveoli v air moving in and out.
• Rate v depth: breaths per minute v volume per breath.
• Bronchi v bronchioles: two large tubes from the trachea v many small tubes inside the lungs.
• External v internal intercostals: contract to breathe in v contract to breathe out.
• Goblet cells v ciliated cells: make mucus v move mucus.

Phrases that earn marks

Instead of this	Write this
“The alveoli are thin”	“The alveolus wall is one cell thick, giving a short diffusion distance”
“Lots of blood”	“A good blood supply maintains a steep concentration gradient”
“The lungs suck air in”	“Thorax volume increases, so pressure decreases and air flows in”
“Breath has CO₂ in it”	“Expired air contains more carbon dioxide than inspired air”
“You need more oxygen”	“Carbon dioxide concentration in the blood rises and is detected by the brain”
“Cilia catch dirt”	“Mucus traps particles; cilia move the mucus up to the throat”

Reading a breathing-rate graph

Questions often show breathing rate plotted against time, with a shaded bar for the period of exercise.

• Before the bar: a flat line at the resting rate.
• During the bar: the line rises, then may level off while the activity continues.
• After the bar: the line falls gradually, not instantly, back to the resting rate.
• To describe, quote values with units: “rate rose from 15 to 36 breaths per minute”.
• To find recovery time, read the time when the line first returns to the resting value and subtract the time the exercise stopped.

Links to other topics

• Diffusion (Topic 3): gas exchange is diffusion down a concentration gradient.
• Ciliated cells (Topic 2, Core): “movement of mucus in the trachea and bronchi”.
• Respiration (Topic 12): uses the oxygen and produces the carbon dioxide; oxygen debt explains why breathing stays fast after exercise.

Quick self-test

1. State the four features of gas exchange surfaces listed in the syllabus.
2. Name, in order, the structures air passes through from the larynx to the alveoli.
3. State the test for carbon dioxide and its positive result.
4. State one difference in water vapour between inspired and expired air.
5. A student counts 13 breaths in 30 seconds. Calculate the breathing rate in breaths per minute.
6. A breathing rate rises from 18 to 27 breaths per minute. Calculate the percentage increase.
7. Each breath moves 0.6 dm³ of air and the rate is 15 breaths per minute. Calculate the volume of air breathed in per minute.
8. (Extended) State the function of cartilage in the trachea.
9. (Extended) What happens to the diaphragm when you breathe in?
10. (Extended) Explain why volume increase in the thorax makes air enter the lungs.
11. (Extended) What does the brain detect that leads to faster, deeper breathing during exercise?
12. (Extended) State the role of goblet cells.

Answers

1. Large surface area; thin surface; good blood supply; good ventilation with air.
2. Trachea → bronchi → bronchioles → alveoli.
3. Limewater; turns milky (cloudy).
4. Expired air contains more water vapour.
5. 13 × 2 = 26 breaths per minute.
6. (27 − 18) ÷ 18 × 100 = 50%.
7. 0.6 × 15 = 9.0 dm³ per minute.
8. Keeps the trachea open, so it does not collapse.
9. It contracts and flattens (moves down).
10. Larger volume means lower pressure inside the thorax than outside, so air flows in from higher to lower pressure.
11. An increased carbon dioxide concentration in the blood.
12. They secrete mucus, which traps pathogens and particles.

Where marks are usually lost

• Naming a feature of the alveoli without its reason; “thin” needs “short diffusion distance”.
• Listing features that are not on the syllabus list instead of the four that are.
• Writing that the diaphragm moves up during breathing in; it flattens and moves down.
• Jumping from “muscles contract” straight to “air goes in” without volume and pressure.
• Saying “pressure increases, so air goes in”; the direction is reversed.
• Saying inspired air has “no carbon dioxide”; it has a small amount.
• Explaining faster breathing by “lack of oxygen” instead of carbon dioxide detected by the brain.
• Writing that cilia trap dust or kill bacteria; cilia move mucus.
• Forgetting to convert counts to breaths per minute when counting over 15, 20 or 30 seconds.
• Calling breathing “respiration” in an answer about ventilation.

Next steps

Try the gas exchange practice questions, then go on to the respiration revision notes.

Official syllabus

Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028, published by Cambridge Assessment International Education (Cambridge University Press & Assessment). Topic 11, Gas exchange in humans, section 11.1.