Gas exchange in humans
Cambridge IGCSE Biology 0610 Topic 11 revision chapter covering gas exchange in humans, written for the Core and Extended routes at once and labelled statement by statement. Subtopic 11.1 carries five Core statements and six Supplement statements, and the split is unusually consequential: the entire ventilation mechanism and the entire account of airway protection sit in the Supplement column, so a Core candidate studies neither. The chapter opens by separating four processes that students routinely merge: ventilation, the movement of air into and out of the lungs; gas exchange, the diffusion of oxygen and carbon dioxide between alveolar air and the blood in the capillaries; transport, the carriage of those gases around the body in the blood; and respiration, the chemical reactions inside living cells that release energy from nutrient molecules. Every later section is anchored to that boundary, so that respiration is never placed in the lungs and breathing is never described as releasing energy. The Core run then teaches the four features of a gas exchange surface that the syllabus names and limits itself to - a large surface area, a thin surface, a good blood supply and good ventilation with air - each by the structure-to-function method rather than as a list, and locates all four inside a real alveolus drawn at three magnifications. The moist alveolar lining is explained but marked visibly as supporting context, because it is not one of the four. A fully labelled diagram fixes the ten parts of the breathing system the syllabus asks candidates to identify - lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli and the associated capillaries - and the air route is traced from nose or mouth through larynx, trachea, bronchi and bronchioles to the alveoli. Inspired and expired air are compared for the three components the syllabus limits the comparison to, with approximate values: less oxygen, about 16 per cent against about 21 per cent; much more carbon dioxide, about 4 per cent against about 0.04 per cent; and more water vapour. Nitrogen, argon and temperature are retained as clearly marked supporting context rather than as examinable content. The comparison is evidenced by the limewater investigation the syllabus specifies, written out with apparatus, variables, controls, predicted observation, conclusion, limitation and safety, and carefully separating what is observed from what may be concluded, so that limewater turning cloudy sooner is read as evidence of a higher concentration of carbon dioxide rather than as a measurement of it. Physical activity is then described as it affects both the rate and the depth of breathing, with the two measurements defined separately, a standardised investigation set out with its variables, repeats, ethical requirements and honest limitations, and calculations worked through in full including means, anomalies, changes from rest, percentage increases, ventilation rates and recovery times. The Supplement half follows in its own clearly bordered blocks, each opening by naming the Core idea it extends. Extended candidates separate the internal from the external intercostal muscles on a diagram and state that the cartilage in the trachea holds the tube open so air can always pass; they explain ventilation as a causal chain from muscle action through rib and diaphragm movement to a change in thoracic volume, a change in pressure and finally the movement of air, given separately for inhalation and for quiet and forced exhalation; they explain each difference between inspired and expired air in terms of the concentration gradients at the alveolus, including why oxygen falls by only about five percentage points and why the water-vapour difference is evaporation rather than diffusion; and they explain the link between physical activity and breathing in the particular terms the syllabus prescribes - an increased carbon dioxide concentration in the blood, detected by the brain, leading to an increased rate and greater depth of breathing. The chapter closes on airway protection: goblet cells secrete mucus that traps dust particles and pathogens, ciliated cells beat to move that mucus toward the throat where it is swallowed and many pathogens are destroyed in the acidic conditions of the stomach, and the role of each of the three is made visible by contrasting a working lining with one whose cilia no longer clear it.Show moreShow less
Revision notes
Interactive notes with exam tips and worked examples.
Study path
Chapter overview
A summary of this Biology chapter — open a section to read it. The full notes, worked examples and practice questions are in the study modules above.
Key ideas to remember
- If you remember one sentence from this chapter, make it this one: “Ventilation is air moving; gas exchange is diffusion at the alveolus; respiration is a chemical reaction inside a cell.” Mixing those three up is the fastest way to answer the wrong question.
What you need to be able to do
- Distinguish ventilation, gas exchange, transport and respiration, and say where each one happens.
- State the four features of a gas-exchange surface in humans — large surface area, thin surface, good blood supply and good ventilation with air — and describe how each one helps. Core 11.1.1
- Describe the structure of an alveolus and its capillary, and link it to those four features.
- Identify on a diagram or an image the lungs, diaphragm, ribs, intercostal muscles, larynx, trachea, bronchi, bronchioles, alveoli and the associated capillaries. Core 11.1.2
- Trace the route of air from the nose or mouth to an alveolus.
- Investigate the difference between inspired and expired air using limewater as the test for carbon dioxide, and separate the observation from the conclusion. Core 11.1.3
- Describe the differences in composition between inspired and expired air for oxygen, carbon dioxide and water vapour. Core 11.1.4
- Investigate and describe the effect of physical activity on the rate and the depth of breathing, and say why those are two different measurements. Core 11.1.5
- Design a safe, standardised investigation into activity and breathing, and interpret breathing data: a mean, an anomaly, a change from rest and a percentage increase.
- Identify the internal and the external intercostal muscles on a diagram or an image. Supplement 11.1.6
- State the function of the cartilage in the trachea. Supplement 11.1.7
- Explain the role of the ribs, the internal and external intercostal muscles and the diaphragm in producing the volume and pressure changes in the thorax that ventilate the lungs — for inhalation and for exhalation, without skipping the pressure step. Supplement 11.1.8
- Explain the differences in composition between inspired and expired air, using the concentration gradients at the alveolus. Supplement 11.1.9
- Explain the link between physical activity and breathing in terms of an increased carbon dioxide concentration in the blood, which is detected by the brain, leading to an increased rate and a greater depth of breathing. Supplement 11.1.10
- Explain the role of goblet cells, mucus and ciliated cells in protecting the breathing system from pathogens and particles. Supplement 11.1.11
Why Gas exchange in humans matters
The chain explains the recovery period as neatly as it explains the rise. Breathing does not return to its resting rate the instant the exercise ends, because the extra carbon dioxide that built up in the blood and tissues is still there and still has to be removed. While its concentration remains above the resting level, the brain keeps the rate and depth of breathing raised; as it falls back, they fall back with it. That is why recovery is a curve rather than a step, and why a longer or harder activity takes longer to recover from. It is also why an investigation into activity and breathing should measure during recovery and not only at the peak: the shape of that falling curve is the chain running in reverse.
Common mistakes to avoid
- 1. Putting respiration in the lungs. Respiration is a chemical reaction inside living cells — muscle cells, liver cells, root hair cells. The lungs are where gases are exchanged, not where energy is released. Writing “respiration takes place in the lungs” contradicts the boundary the whole of this topic rests on, however good the rest of the answer is.
- 2. Saying expired air has no oxygen. It has about 16 %. Only about a quarter of the oxygen you breathe in is absorbed — roughly three quarters of it is breathed straight back out. This is why rescue breathing works at all.
- 3. Adding a fifth feature to the exchange surface. Statement 11.1.1 names four: large surface area, thin surface, good blood supply and good ventilation with air. The moist lining of the alveolus is real, and this chapter explains it, but it is not one of the four and it is not what this statement asks for. Learn the four, then know why the moisture is there.
- 4. Comparing five things instead of three. Statement 11.1.4 limits the comparison of inspired and expired air to oxygen, carbon dioxide and water vapour. Nitrogen and temperature are interesting and this chapter covers both, but an answer that leads with nitrogen has spent its first line outside the statement.
- 5. Skipping the pressure step. Supplement 11.1.8 “The diaphragm contracts so air rushes in” leaves out the two links that connect the muscle to the air. Volume changes, therefore pressure changes, therefore air moves.
- 6. Getting the diaphragm backwards. Supplement 11.1.8 When the diaphragm contracts it becomes flatter and moves down, which increases the volume of the thorax. A relaxed diaphragm is the dome-shaped one. Many students write the reverse because a relaxed muscle sounds like it should be flat.
- And one that costs everybody: treating rate and depth as the same word, or a conclusion as an observation. Rate is breaths per minute; depth is the volume of each breath, and a question that says “rate and depth” wants two separate statements. Likewise, “limewater went cloudy faster” is what you saw, and “expired air contains a higher concentration of carbon dioxide” is what it means. They are two different sentences and a practical question asks for them as two.
- An alveolus is not a cell. It is a small sac, and its wall is made of cells. Writing “the alveolus is one cell thick” is acceptable shorthand only if you mean its wall; writing “an alveolus is a single cell” is wrong, and it makes the rest of your answer hard to mark. Say “the alveolar wall is one cell thick”.
- Do not say the cartilage protects the trachea, and do not say it traps particles. Protection from impact is not its job, and trapping particles is the job of the mucus, which sections N and O deal with. The cartilage does one thing: it keeps the tube open.
- Do not force the column to total 100. Rounded values do not have to add up exactly, and the carbon dioxide figure is already included inside the “argon and other gases” one per cent. An answer that writes 78 + 21 + 1 + 0.04 and worries about the extra 0.04 has misread the table, not found an error.
- “Less oxygen” does not mean “no oxygen”. Expired air still contains about 16 % oxygen — roughly three quarters of what you breathed in comes straight back out. That is why one person’s expired air can still supply another during rescue breathing, and it is why an answer that says the oxygen “is used up” contradicts the data table printed beside it. Section L explains why so little of it is absorbed.
- “I counted my own breathing.” This is a real limitation, and naming it is part of evaluating the method. Being aware of your breathing alters it, usually making it slower and deeper. Having a partner count, or using equipment, removes the problem.
- The most common error here is dividing by 35 instead of 14, which gives 60 % and describes something else entirely. Ask yourself what the change is being compared with: the value it started from.
- “The lungs pull air in” is not an explanation. Lungs contain no muscle. They are elastic bags that change shape because the space around them changes shape. If you write that the lungs expand, follow it immediately with because the thorax has expanded.
- Do not invent pressure values. Write “below atmospheric pressure” and “above atmospheric pressure”, or “lower than” and “higher than” the pressure outside. A number such as “the pressure falls to 99 kPa” is not required at this level, and if you invent one you can be marked wrong for a detail that was never asked for. If a question supplies pressure data, use it.
- The trigger the syllabus names is carbon dioxide, not oxygen. It is tempting to write “the body detects that it is short of oxygen”, because a shortage of oxygen is what it feels like. Statement 11.1.10 says something different and more precise: what rises is the carbon dioxide concentration in the blood, and that is what the brain detects. Write the carbon dioxide version. If you want to mention oxygen, mention it as part of what the increased breathing supplies, not as what triggered it.
- Breathing does not release the energy. A muscle is not powered by air arriving in the chest; it is powered by respiration inside its own cells. Breathing supplies the oxygen respiration needs and removes the carbon dioxide it produces. Writing “we breathe faster to release more energy” puts the energy release in the wrong organ and usually loses the mark.
- Cilia are not filters. A common wrong answer describes cilia as tiny hairs that filter oxygen or strain the air. They do neither. The mucus traps particles; the cilia transport the mucus. Gas molecules pass straight through both.
- Do not write that “all the oxygen is used up”. About three quarters of the inhaled oxygen is breathed straight back out, and saying otherwise contradicts the data in front of you.
- Two things to avoid. Do not write that the student breathes faster “to make more energy” — breathing supplies oxygen, and respiration inside the cells releases the energy. And do not write that the brain detects a shortage of oxygen: statement 11.1.10 names the carbon dioxide concentration of the blood as what is detected.
How Gas exchange in humans is examined
- Topic 11 is unusual in that almost everything in it can be asked as either a recall question or an application question, and the two need different answers. It is also a topic where the tier boundary is visible in the question: a describe instruction usually sits on the Core side and an explain instruction usually sits on the Supplement side.
- muscle action → movement of ribs or diaphragm → change in thoracic volume → change in pressure inside the lungs → movement of air down the pressure difference
- change one variable → observe or measure → repeat → record → evaluate → control everything else → state the timing and the safety precaution
Frequently asked questions
What is the difference between breathing and respiration? Core 11.1.1
Breathing, properly called ventilation, is the movement of air into and out of the lungs; it is a physical process driven by muscles changing the volume of the chest. Respiration is a chemical process that takes place inside living cells, in which energy is released from nutrient molecules such as glucose. Breathing supplies the oxygen that respiration needs and removes the carbon dioxide it produces, but no energy is released by breathing itself.
Where in the body does gas exchange happen? Core 11.1.1
At the alveoli, the microscopic air sacs at the ends of the finest bronchioles in the lungs. Nowhere else in the gas-exchange system does any gas move between the air and the blood. The nose, larynx, trachea, bronchi and bronchioles carry air but exchange none of it.
What is in the air we breathe in? Core 11.1.4
The two figures that matter for Cambridge IGCSE Biology 0610 Topic 11 are the oxygen content, approximately 21 %, and the carbon dioxide content, approximately 0.04 %, because those are two of the three components the syllabus asks you to compare with expired air; the third, water vapour, is present in a variable amount that depends on the humidity of the surroundings. Dry atmospheric air is also approximately 78 % nitrogen with about 1 % argon and other gases, but Topic 11 contains no statement about the composition of the atmosphere, so those are supporting context rather than required content. All of these values are approximations, and any data supplied in an examination question takes priority over them.
How does expired air differ from inspired air? Core 11.1.4
Cambridge IGCSE Biology 0610 limits the comparison to three components. Expired air contains less oxygen, about 16 % rather than about 21 %, because oxygen has diffused from the alveolar air into the blood and been used in respiration in the body's cells. It contains much more carbon dioxide, about 4 % rather than about 0.04 %, because carbon dioxide produced by respiration has diffused out of the blood into the alveolar air. And it carries more water vapour, because water evaporates into it from the moist linings of the airways and the alveoli. Two further differences are true but are supporting context rather than required content: expired air is close to body temperature, and the percentage of nitrogen is approximately unchanged because nitrogen is not exchanged in the lungs. Describing the three differences is Core content; explaining why they happen is Supplement content for Extended candidates.
Does expired air contain any oxygen? Core 11.1.4
Yes, about 16 %. Only about a quarter of the oxygen breathed in is actually absorbed, and roughly three quarters of it is breathed straight back out: a large fraction of each breath never reaches an alveolus at all, and even in the alveoli the blood does not take up all the oxygen available in a single pass. This is why one person’s expired air can still supply another person during rescue breathing.
Why are the alveoli so good at gas exchange? Core 11.1.1
Cambridge IGCSE Biology 0610 names four features of a gas exchange surface in humans. A large surface area: there are very many alveoli, so their total area is very large. A thin surface: the alveolar wall and the capillary wall are each one cell thick, so the distance a gas must travel is extremely short. A good blood supply: a dense network of capillaries carries absorbed oxygen away and delivers carbon dioxide. And good ventilation with air: breathing refreshes the air on the other side of the barrier. The last two matter most, because they are what keep the concentration gradients steep. The alveolar lining is also moist, which is how a gas dissolves before it crosses the wall, but that is not one of the four named features.
Why does air move into the lungs? Supplement 11.1.8
Because the pressure inside the lungs has become lower than the pressure of the atmosphere outside. The external intercostal muscles contract and raise the ribs, and the diaphragm contracts and flattens; both increase the volume of the thorax. The same quantity of air then occupies a larger space, so its pressure falls, and air moves in down that pressure difference. The lungs themselves contain no muscle and do not pull.
What does the diaphragm do when you breathe in? Supplement 11.1.8
It contracts, which makes it flatter and moves it downward. That increases the volume of the thoracic cavity. When it relaxes it returns to its dome shape and moves upward, which decreases the volume. Many students reverse this, because a relaxed muscle sounds as though it should be the flat one.
Do the internal intercostal muscles work during every breath out? Supplement 11.1.6 Supplement 11.1.8
No. In quiet breathing, exhalation happens because the external intercostal muscles and the diaphragm relax and the tissues recoil, so the internal intercostals stay relaxed too. They contract only during forced exhalation, such as coughing, blowing hard or breathing out during vigorous exercise, when they pull the ribs downwards and inwards more strongly.
Why do you breathe faster when you exercise? Core 11.1.5 Supplement 11.1.10
Because your muscle cells are respiring faster to release the energy needed for contraction, and faster respiration releases more carbon dioxide into the blood. Cambridge IGCSE Biology 0610 asks Extended candidates to explain the link in those terms: the concentration of carbon dioxide in the blood increases, that increase is detected by the brain, and the brain raises both the rate and the depth of breathing. The greater volume of air moved each minute removes the extra carbon dioxide and supplies the extra oxygen the muscles are using. The energy itself is released in the muscle cells, not in the lungs, and the trigger for the response is the carbon dioxide concentration of the blood rather than a shortage of oxygen.
What is the difference between breathing rate and breathing depth? Core 11.1.5
Breathing rate is how many breaths are taken in a given time, usually breaths per minute. Breathing depth is the volume of air moved in a single breath. Multiplying the two gives the ventilation rate, the total volume of air moved per minute. Counting breaths measures rate only, and says nothing about depth.
What is the cartilage in the trachea for? Supplement 11.1.7
The rings of cartilage in the wall of the trachea hold the tube open, so that air can always pass through it to the lungs. In particular they stop the trachea collapsing when the pressure inside it falls below the pressure outside during inhalation. The rings are C-shaped rather than complete, which leaves the back of the trachea soft where the oesophagus lies against it. This is Supplement content for Extended candidates in Cambridge IGCSE Biology 0610.
What do goblet cells and cilia do? Supplement 11.1.11
Goblet cells in the lining of the airways secrete mucus, a sticky fluid that traps dust particles and pathogens carried in with the air. Ciliated cells beside them carry many fine cilia that beat together and move the mucus toward the throat, where it is swallowed; many of the trapped pathogens are then destroyed in the acidic conditions of the stomach. Goblet cells make the mucus and cilia move it, and neither does the other’s job.
Why does limewater go cloudy, and what does it prove? Core 11.1.3
Limewater turns cloudy when carbon dioxide is bubbled through it, and it turns cloudy faster when the concentration of carbon dioxide is higher. Comparing two identical tubes shows that expired air contains a higher concentration of carbon dioxide than inspired air. It does not measure how much: the test is qualitative, and the time taken cannot be converted into a percentage.
Syllabus reference and sources
Written against: Cambridge IGCSE Biology (0610), syllabus for 2026, 2027 and 2028 (version 2, published December 2025), Subject Content, Topic 11: Gas exchange in humans, subtopic 11.1.
Written by: Academiq Edu Instructor Panel
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