The breathing systemSpec 11.1.2, 11.1.6, 11.1.7
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.
Air enters the body through the nose or mouth and travels to the lungs along a series of tubes. The lungs sit inside the thorax (chest), protected by the ribs. The diaphragm forms the floor of the thorax.
- Air passes through the larynx (voice box) into the trachea (windpipe).
- The trachea divides into two bronchi (singular: bronchus), one to each lung.
- Each bronchus branches into many smaller tubes called bronchioles.
- The bronchioles end in tiny air sacs called alveoli (singular: alveolus).
- Each alveolus is surrounded by a network of capillaries, where gas exchange happens.
(opens full size in a new tab)| Part | Description or function |
|---|---|
| Lungs | Organs in the thorax where gas exchange takes place |
| Ribs | Bones that protect the lungs and move to change the volume of the thorax |
| Intercostal muscles | Muscles between the ribs that move the ribs |
| Diaphragm | Sheet of muscle below the lungs that separates the thorax from the abdomen |
| Larynx | Voice box at the top of the trachea |
| Trachea | Windpipe: carries air to the bronchi |
| Bronchi | Two tubes that carry air into the lungs |
| Bronchioles | Small tubes that carry air to the alveoli |
| Alveoli | Air sacs where oxygen and carbon dioxide are exchanged with the blood in the capillaries |
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanIntercostal muscles
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanThere are two sets of intercostal muscles between each pair of ribs. The external intercostal muscles are on the outside. The internal intercostal muscles are on the inside, underneath them. The two sets work against each other to move the ribs.
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanCartilage in the trachea
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanThe wall of the trachea contains C-shaped rings of cartilage. The function of the cartilage is to support the trachea and keep it open, so air can always flow through it, even when the pressure inside falls as you breathe in.
Spell bronchi (plural, two tubes) and bronchioles (the small tubes) carefully. They are often mixed up, and the mark is lost if the wrong one is written.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026