IGCSE Biology: Gas Exchange in Humans Practice Questions
Gas exchange takes place in the alveoli, which provide a large, thin, moist surface with a rich blood supply. Oxygen diffuses from the alveolar air into the blood and carbon dioxide diffuses in the opposite direction, both down concentration gradients.
Topic 11 of Cambridge IGCSE Biology 0610 combines structure, mechanism and data interpretation. The two questions that appear most often are the adaptations of the alveoli and a step by step description of breathing in. Both are marked point by point, so a structured answer scores far better than a paragraph. The questions below follow that structure.
What you need to know for Gas Exchange in Humans
- Features of a gas exchange surfaceLarge surface area, thin so the diffusion distance is short, moist, and with a good blood supply and ventilation to maintain steep concentration gradients.
- AlveoliMillions of tiny air sacs at the ends of the bronchioles. Their walls are one cell thick and each is surrounded by a network of capillaries.
- InspirationThe intercostal muscles contract and pull the ribs up and out, and the diaphragm muscle contracts and flattens. The volume of the thorax increases, the pressure inside falls below atmospheric pressure, and air is pushed in.
- ExpirationThe intercostal muscles and diaphragm relax. The ribs move down and in and the diaphragm returns to its domed shape. The volume of the thorax decreases, pressure rises, and air is forced out.
- Inspired and expired airExpired air contains less oxygen, much more carbon dioxide, more water vapour and is warmer than inspired air. The nitrogen content is unchanged.
- Protecting the airwaysGoblet cells secrete mucus that traps particles and pathogens, and ciliated cells sweep the mucus up away from the lungs to be swallowed.
IGCSE Biology Gas Exchange in Humans questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Explain how the alveoli are adapted for efficient gas exchange.
Show the worked answer
- There are millions of alveoli, giving a very large total surface area, so many gas molecules can diffuse at once.
- The alveolar wall is one cell thick and the capillary wall is one cell thick, so the diffusion distance is very short.
- The alveolar surface is moist, so gases dissolve before diffusing across.
- A dense network of capillaries carries oxygen away and brings carbon dioxide, maintaining steep concentration gradients for both gases. Ventilation refreshes the air in the alveoli for the same reason.
Describe what happens in the thorax during inspiration.
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- The external intercostal muscles contract, pulling the ribs upwards and outwards.
- The diaphragm muscle contracts and flattens, moving downwards.
- These two movements increase the volume of the thorax.
- The increase in volume decreases the pressure inside the lungs below atmospheric pressure, so air moves in from outside down the pressure gradient.
State two differences between inspired and expired air and explain why each difference occurs.
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- Expired air contains less oxygen than inspired air, because oxygen has diffused from the alveoli into the blood to be used in aerobic respiration in body cells.
- Expired air contains much more carbon dioxide, because carbon dioxide is a waste product of aerobic respiration and diffuses from the blood into the alveoli.
- Expired air also contains more water vapour, because water evaporates from the moist surfaces of the alveoli and airways.
- Expired air is warmer, because it has been warmed to body temperature inside the lungs.
Explain why the rate and depth of breathing increase during vigorous exercise and stay high for a short time afterwards.
Show the worked answer
- During exercise the muscles contract more frequently and need more energy, so the rate of aerobic respiration increases.
- More oxygen is therefore required and more carbon dioxide is produced.
- The increased carbon dioxide concentration in the blood is detected, and breathing becomes faster and deeper to supply oxygen and remove carbon dioxide more quickly.
- After exercise breathing stays high because extra oxygen is needed to break down the lactic acid produced by anaerobic respiration in the muscles. This is repaying the oxygen debt.
Common mistakes in this topic
- Confusing breathing with respiration.
- Saying the diaphragm moves up during inspiration.
- Claiming expired air contains no oxygen.
- Describing alveolar adaptations without linking each to diffusion.
- Saying nitrogen is used by the body. The percentage of nitrogen is essentially unchanged between inspired and expired air.
Exam tips
- Answer breathing mechanism questions in the fixed order: muscles, volume, pressure, air movement. That order matches the mark scheme.
- For alveoli, use the four standard adaptations and add so that to each one.
- Limewater turns milky more quickly with expired air. If asked to design that experiment, name the control clearly.
- In data questions, compare percentages rather than quoting one figure alone.
- Link smoking to specific damage: tar paralyses cilia and causes a build up of mucus, carbon monoxide reduces oxygen transport, and nicotine is addictive.
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Gas Exchange in Humans FAQs
How are the alveoli adapted for gas exchange?
They provide a very large total surface area from millions of tiny sacs, their walls are one cell thick so the diffusion distance is short, their surface is moist so gases dissolve, and they are surrounded by a dense capillary network with continuous ventilation, which keeps the concentration gradients steep.
What happens to the diaphragm when you breathe in?
The diaphragm muscle contracts and flattens, moving downwards. Together with the intercostal muscles pulling the ribs up and out, this increases the volume of the thorax, lowers the pressure inside the lungs below atmospheric pressure, and air moves in.
What is the difference between inspired and expired air?
Expired air contains less oxygen, roughly 16 per cent rather than 21 per cent, and much more carbon dioxide, roughly 4 per cent rather than 0.04 per cent. It also contains more water vapour and is warmer. The proportion of nitrogen is essentially unchanged.
Why does breathing stay fast after exercise stops?
During hard exercise muscles respire anaerobically and produce lactic acid. Extra oxygen is needed afterwards to break that lactic acid down, which is called the oxygen debt. Breathing therefore stays deep and rapid until the debt has been repaid and carbon dioxide levels return to normal.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.