Gas exchange in humans — Cambridge IGCSE Biology
The breathing system, how the lungs are adapted for gas exchange, how ventilation works and how breathing changes with exercise.
Revision notes
6 short notes, in the order of the specification. Each one in short:
The breathing system carries air through the larynx and trachea into two bronchi, then into smaller bronchioles that end in air sacs called alveoli, where gas exchange happens. The lungs sit in the thorax, protected by the ribs. The intercostal muscles move the ribs, and the diaphragm, a sheet of muscle, forms the floor of the thorax.
A good gas exchange surface in humans has four features: a large surface area, a thin surface, a good blood supply and good ventilation with air. In the alveoli, oxygen diffuses into the blood and carbon dioxide diffuses out. Millions of alveoli give a large surface area, and walls one cell thick give a short diffusion distance.
Breathing in and out (ventilation)
SupplementVentilation is the movement of air into and out of the lungs. When breathing in, the external intercostal muscles contract, the ribs move up and out, and the diaphragm contracts and flattens. The volume of the thorax increases and its pressure falls below the air pressure outside, so air moves into the lungs. Breathing out reverses these changes.
Inspired and expired air
PracticalExpired air is the air you breathe out, and it contains less oxygen, more carbon dioxide and more water vapour than inspired air. Expired air has about 16% oxygen and 4% carbon dioxide, compared with about 21% and 0.04% in inspired air. Limewater turns cloudy much faster with expired air, showing it contains more carbon dioxide.
Physical activity and breathing
PracticalDuring physical activity, both the rate of breathing, the number of breaths per minute, and the depth of breathing, the volume of each breath, increase. This brings more oxygen into the body and removes more carbon dioxide. Muscles respire faster and produce more carbon dioxide, and the brain detects the higher concentration in the blood.
Protecting the breathing system
SupplementThe breathing system is protected by goblet cells and ciliated cells lining the trachea and bronchi. Goblet cells make and release mucus, a sticky liquid that traps pathogens and particles from the air. Ciliated cells have cilia that beat to move the mucus up and away from the lungs towards the throat, where it is swallowed.
Exam questions
8 exam-style questions (25 marks), each with its mark scheme.
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24 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of gas exchange in humans on one page, so you can see where this subtopic fits.
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Quick check questions
Name the tubes that carry air into each lung.
Show answer
Bronchi.
State the four features of a gas exchange surface in humans.
Show answer
Large surface area, thin surface, good blood supply and good ventilation with air.
Which gas turns limewater cloudy?
Show answer
Carbon dioxide.
Supplement What happens to the pressure in the thorax when the diaphragm contracts?
Show answer
It decreases, because the volume of the thorax increases.
Supplement State the function of cartilage in the trachea.
Show answer
It supports the trachea and keeps it open.
Frequently asked questions
How are alveoli adapted for gas exchange?
Alveoli are adapted for gas exchange by having a large surface area, from millions of alveoli, and thin walls one cell thick, giving a short diffusion distance. A network of capillaries around each alveolus gives a good blood supply, and ventilation brings fresh air. Both keep a steep concentration gradient, so diffusion is fast.
How does the diaphragm help you breathe in?
Supplement When you breathe in, the diaphragm muscles contract, so the diaphragm moves down and flattens. At the same time the external intercostal muscles contract and lift the ribs up and out. The volume of the thorax increases and the pressure falls below the air pressure outside, so air moves into the lungs.
Why does expired air contain more carbon dioxide than inspired air?
Supplement Expired air contains more carbon dioxide because body cells produce carbon dioxide in respiration. It is carried to the lungs in the blood and diffuses into the alveoli, so expired air has about 4% carbon dioxide compared with about 0.04% in inspired air. Expired air also contains less oxygen and more water vapour.
Why does breathing rate increase during exercise?
Supplement Breathing rate increases during exercise because the muscles respire faster and produce more carbon dioxide. The carbon dioxide concentration in the blood rises, and the brain detects this. The brain sends signals to the breathing muscles, so breaths become faster and deeper, removing more carbon dioxide and taking in more oxygen.
How do mucus and cilia protect the lungs?
Supplement Goblet cells in the lining of the trachea and bronchi make mucus, a sticky liquid that traps pathogens and particles from the air. Ciliated cells have tiny hair-like cilia that beat to move the mucus, with the trapped particles, up and away from the lungs towards the throat, where it is swallowed.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026