Cambridge IGCSE Biology 0610: Gas exchange in humans – Study Guide

This study guide teaches Topic 11, Gas exchange in humans, of the Cambridge IGCSE Biology 0610 syllabus for examination in 2026, 2027 and 2028. The topic has one section, 11.1, with five Core learning outcomes and six Supplement outcomes. Core content is examined on Papers 1 and 3; Core plus Supplement is examined on Papers 2 and 4. Everything marked Extended only below is Supplement content, which you need only if you are entered for the Extended papers.

Useful links: the Cambridge IGCSE Biology hub, the printable Biology checklist, the revision notes for this topic and the practice questions for this topic. To find your gaps quickly, try the Core diagnostic or the Extended diagnostic.

What this topic covers

Syllabus ref	What you must be able to do	Tier
11.1.1	Describe the features of gas exchange surfaces: large surface area, thin surface, good blood supply, good ventilation with air	Core
11.1.2	Identify lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli and associated capillaries	Core
11.1.3	Investigate inspired and expired air using limewater as a test for carbon dioxide	Core
11.1.4	Describe the differences between inspired and expired air (oxygen, carbon dioxide, water vapour)	Core
11.1.5	Investigate and describe the effects of physical activity on the rate and depth of breathing	Core
11.1.6	Identify the internal and external intercostal muscles	Extended only
11.1.7	State the function of cartilage in the trachea	Extended only
11.1.8	Explain how the ribs, intercostal muscles and diaphragm change thorax volume and pressure to ventilate the lungs	Extended only
11.1.9	Explain the differences in composition between inspired and expired air	Extended only
11.1.10	Explain the link between physical activity and breathing, in terms of carbon dioxide detected by the brain	Extended only
11.1.11	Explain the role of goblet cells, mucus and ciliated cells in protecting the breathing system	Extended only

Gas exchange is the diffusion of oxygen into the blood and carbon dioxide out of it, across the walls of the alveoli. Ventilation (breathing) is the movement of air into and out of the lungs. Breathing is not respiration: respiration is the chemical process in cells that releases energy, covered in Topic 12.

Features of gas exchange surfaces (11.1.1)

The syllabus limits you to four features. Learn them as a set, and for each one be ready to say why it speeds up diffusion.

Feature	In the human lungs	Why it helps
Large surface area	Millions of tiny alveoli	More gas can diffuse at the same time
Thin surface	Alveolus wall and capillary wall are each one cell thick	Short diffusion distance
Good blood supply	A dense network of capillaries around every alveolus	Blood carries oxygen away and brings carbon dioxide, so a steep concentration gradient is kept
Good ventilation with air	Breathing replaces the air in the alveoli	Keeps oxygen concentration high and carbon dioxide concentration low in the alveoli

Blood flow keeps the gradient on the blood side; ventilation keeps it on the air side. Say “a concentration gradient is maintained”, not just “lots of blood”.

Parts of the breathing system (11.1.2, 11.1.6, 11.1.7)

Air follows one route in. Learn it in order, because questions often ask you to name structures in sequence or to label a diagram:

nose or mouth → larynx → trachea → bronchi → bronchioles → alveoli

• Larynx: the voice box at the top of the trachea.
• Trachea: the windpipe, running down the neck.
• Bronchi (singular bronchus): the two branches of the trachea, one to each lung.
• Bronchioles: many small branching tubes inside each lung.
• Alveoli (singular alveolus): tiny air sacs at the ends of the bronchioles, each covered by capillaries. This is where gas exchange happens.
• Ribs: bones around the thorax that protect the lungs and move during breathing.
• Intercostal muscles: muscles between the ribs.
• Diaphragm: a sheet of muscle and tissue under the lungs that separates the thorax from the abdomen.

Extended only. There are two layers of intercostal muscles. The external intercostal muscles are the outer layer; the internal intercostal muscles are the inner layer. On a diagram of a cut through the chest wall, the external layer is the one nearer the skin.

Extended only. The trachea is held open by rings of cartilage. The function of the cartilage is to keep the trachea open and stop it collapsing when air pressure inside it falls during breathing in. Air can then flow freely.

Ventilation: how air moves in and out (11.1.8, Extended only)

Air moves from higher pressure to lower pressure. The breathing muscles change the volume of the thorax, which changes the pressure inside it.

Breathing in (inspiration)

1. The external intercostal muscles contract and the internal intercostal muscles relax.
2. The ribs move up and out.
3. The diaphragm contracts and flattens (moves down).
4. The volume of the thorax increases.
5. The pressure in the thorax decreases below atmospheric pressure.
6. Air is pushed into the lungs by the higher pressure outside.

Breathing out (expiration)

1. The external intercostal muscles relax; the internal intercostal muscles contract (strongly when you breathe out hard).
2. The ribs move down and in.
3. The diaphragm relaxes and returns to its dome shape (moves up).
4. The volume of the thorax decreases.
5. The pressure in the thorax increases above atmospheric pressure.
6. Air is forced out of the lungs.

The chain “muscles → ribs and diaphragm → volume → pressure → air flow” is what earns the marks. Write every link, and always say “volume increases, so pressure decreases”. Never say the lungs “suck” air in.

Inspired and expired air (11.1.3, 11.1.4, 11.1.9)

What differs (Core)

The syllabus limits the comparison to three gases. Typical approximate values by volume are:

Gas	Inspired air	Expired air
Oxygen	about 21%	about 16%
Carbon dioxide	about 0.04%	about 4%
Water vapour	varies (usually lower)	higher (saturated)

In words: expired air has less oxygen, more carbon dioxide and more water vapour than inspired air.

Worked example 1. Use the table to compare the gases.

• Carbon dioxide: 4 ÷ 0.04 = 100 times more in expired air than in inspired air.
• Oxygen: 21 − 16 = 5 percentage points lower. As a fraction of the oxygen breathed in: 5 ÷ 21 × 100 = 23.8% of it is removed.

A “how many times” answer is a division, not a subtraction.

The limewater investigation (Core)

Limewater turns milky (cloudy) when carbon dioxide passes through it. A common set-up uses two boiling tubes, each with the same volume of limewater, joined to one mouthpiece by tubes arranged so that:

• when you breathe in, air from the room is drawn through tube A first;
• when you breathe out, your breath is pushed through tube B.

Breathe gently in and out and record the time for each tube to turn milky. Tube B (expired air) turns milky much sooner than tube A (inspired air). This shows that expired air contains more carbon dioxide. To make the comparison fair, keep the volume and concentration of limewater the same in both tubes, and use the same person breathing normally.

Limewater is a test for carbon dioxide only. It tells you nothing about oxygen or water vapour.

Why they differ (Extended only)

• Less oxygen in expired air: oxygen diffuses from the air in the alveoli into the blood, and is carried to cells to be used in aerobic respiration.
• More carbon dioxide in expired air: carbon dioxide is produced by respiration in cells, carried in the blood, and diffuses from the blood into the alveoli.
• More water vapour in expired air: water evaporates from the moist lining of the alveoli into the air that is then breathed out.

Each explanation needs two things: the process (diffusion, respiration or evaporation) and the direction.

Physical activity and breathing (11.1.5, 11.1.10)

Investigating the effect (Core)

You can measure the rate of breathing by counting breaths over a fixed time and converting to breaths per minute. Depth is the volume of air taken in with each breath; in class it is usually judged by watching chest movement or measured with a spirometer if one is available. A fair investigation:

1. Sit still for a few minutes, then count breaths for one minute (the resting rate).
2. Do a set exercise for a fixed time, for example step-ups for two minutes.
3. Count breaths immediately afterwards, then every minute until the rate returns to the resting value.
4. Repeat, and use the same exercise, duration and person, so only the activity level changes.

Worked example 2. At rest, a student counts 8 breaths in 30 seconds. Straight after two minutes of skipping, she counts 11 breaths in 15 seconds.

• Resting rate = 8 × (60 ÷ 30) = 16 breaths per minute.
• Rate after exercise = 11 × (60 ÷ 15) = 44 breaths per minute.
• Percentage increase = (44 − 16) ÷ 16 × 100 = 175%.

A spirometer shows that each breath at rest moves about 0.5 dm³ of air, and after exercise about 2.0 dm³.

• Volume per minute at rest = 16 × 0.5 = 8.0 dm³ per minute.
• Volume per minute after exercise = 44 × 2.0 = 88 dm³ per minute, which is 11 times the resting value.

In words, physical activity increases the rate and the depth of breathing, and both return to normal gradually after the activity stops.

Explaining the effect (Extended only)

The syllabus asks for one specific chain. Learn it exactly:

1. During activity, muscles contract more, so they carry out more aerobic respiration.
2. More carbon dioxide is produced, so the carbon dioxide concentration in the blood increases.
3. This increase is detected by the brain.
4. The brain causes an increased rate and greater depth of breathing.
5. More carbon dioxide is removed from the blood, and more oxygen is supplied for respiration.

The trigger is carbon dioxide, not a lack of oxygen. Answers that say “the body needs more oxygen, so you breathe faster” miss the syllabus point. Why breathing stays fast after exercise is part of oxygen debt, which is taught in Topic 12.

Protecting the breathing system (11.1.11, Extended only)

Air contains pathogens (such as bacteria) and particles (such as dust). The lining of the trachea and bronchi protects the lungs using two types of cell:

• Goblet cells secrete mucus. The sticky mucus traps pathogens and particles.
• Ciliated cells have cilia on their surface. The cilia beat (waft) and move the mucus, with its trapped pathogens and particles, up the airways to the throat, where it is swallowed.

This stops pathogens and particles reaching the alveoli. Ciliated cells also appear in the Core list of specialised cells in Topic 2 (“movement of mucus in the trachea and bronchi”), so the same idea can be tested at both tiers.

Common errors

• Writing “the alveoli have thin walls” without saying why that matters: add “short diffusion distance”.
• Saying the diaphragm “moves up” when breathing in. It contracts and flattens, which moves it down.
• Getting the volume and pressure link backwards. Volume up means pressure down, so air flows in.
• Calling breathing “respiration”. Breathing is ventilation; respiration happens in cells.
• Saying limewater shows there is “no carbon dioxide” in inspired air. Inspired air contains a little; limewater just takes much longer to turn milky.
• Explaining faster breathing during exercise with “lack of oxygen” instead of “increased carbon dioxide in the blood, detected by the brain”.
• Saying the cilia trap dust. The mucus traps; the cilia move the mucus.

Next steps

Condense this guide with the gas exchange revision notes, test yourself on the gas exchange practice questions, then move on to respiration. Tick off each outcome on the Biology checklist.

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.