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Breathing in and out (ventilation)

Gas exchange in humans · Gas exchange in humans · note 3 of 6

Spec 11.1.8
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
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Breathing in and out (ventilation)Spec 11.1.8

In short

Ventilation 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.

Ventilation moves air into and out of the lungs. The lungs cannot inflate themselves. Air moves because the ribs, the intercostal muscles and the diaphragm change the volume and pressure in the thorax.

Breathing in (inhalation)

  1. The external intercostal muscles contract and the internal intercostal muscles relax.
  2. The ribs move upwards and outwards.
  3. The diaphragm muscles contract, so the diaphragm moves down and flattens.
  4. The volume of the thorax increases.
  5. The pressure in the thorax decreases, so it is lower than the air pressure outside.
  6. Air moves into the lungs from the higher pressure outside.

Breathing out (exhalation)

  1. The external intercostal muscles relax and the internal intercostal muscles contract (they contract most strongly when you breathe out forcefully).
  2. The ribs move downwards and inwards.
  3. The diaphragm muscles relax, so the diaphragm moves up into a dome shape.
  4. The volume of the thorax decreases.
  5. The pressure in the thorax increases, so it is higher than the air pressure outside.
  6. Air is pushed out of the lungs to the lower pressure outside.
Summary of ventilation
Breathing inBreathing out
External intercostal musclesContractRelax
Internal intercostal musclesRelaxContract
RibsMove up and outMove down and in
DiaphragmContracts and flattensRelaxes and domes upwards
Volume of thoraxIncreasesDecreases
Pressure in thoraxDecreasesIncreases
Two front views of the thorax. Breathing in: external intercostal muscles contract, ribs move up and out, the diaphragm contracts and flattens, volume increases and pressure decreases, and air moves in. Breathing out: internal intercostal muscles contract, ribs move down and in, the diaphragm relaxes and domes upwards, volume decreases and pressure increases, and air moves out. (opens full size in a new tab)
Muscles change the volume of the thorax, which changes the pressure, so air moves in or out.
Common mistake:

Do not say that air is sucked in because the lungs expand. The thorax volume increases first, the pressure falls, and then air moves in because the pressure outside is higher.

Exam tip:

Give the answer in order: muscle action, rib or diaphragm movement, volume change, pressure change, air movement. Marks are usually awarded for each link.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

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.

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