Mechanism of Breathing

The chain examiners are checking for

muscle action → rib and diaphragm movement → thoracic volume change → pressure change → airflow. A common way to lose marks here is skipping straight from “muscles contract” to “air enters”, without ever mentioning volume or pressure. The muscles themselves don’t pull air into your lungs — they change the volume of your thorax, that change in volume changes the pressure inside your lungs relative to the atmosphere, and it’s that pressure difference that actually causes air to move.

Inspiration — breathing in

• The diaphragm contracts and flattens, moving downward.
• The external intercostal muscles contract.
• The ribs move up and out.
• The volume of the thorax increases.
• Pressure inside the lungs decreases, falling below atmospheric pressure.
• Air flows into the lungs, from the higher-pressure atmosphere towards the lower pressure inside.

Expiration — breathing out

For ordinary, quiet expiration:

• The diaphragm relaxes, becoming dome-shaped again and moving upward.
• The external intercostal muscles relax.
• The rib cage moves down and in as the thorax returns towards its resting size.
• The volume of the thorax decreases.
• Pressure inside the lungs increases, rising above atmospheric pressure.
• Air flows out of the lungs.

During active, forced expiration— breathing out harder than usual, such as during exercise — the internal intercostal muscles contract as well, pulling the ribs down and in more strongly than relaxation alone would. This is a genuinely important qualifier, because many simplified diagrams pair “internal intercostals contract” with expiration in general, when in fact ordinary quiet expiration is largely a passive return to resting volume. Both of the following are biologically valid, depending on which expiration a question describes: internal intercostals contracting during active or forced expiration, and internal intercostals staying relaxed during quiet, resting expiration.

Internal and external intercostal muscles

Both muscle groups lie between the ribs, but they aren’t interchangeable, and Cambridge specifically expects you to identify each one separately in a diagram. In a cross-section showing both layers, the external intercostal muscles form the more superficial layer and are the principal muscle acting during inspiration, while the internal intercostal muscles lie deeper and become particularly important when expiration is active or forced. Getting the two swapped — describing the internal layer as the one that lifts the ribs during a normal breath in, say — is one of the more common ways this section loses marks, precisely because the names sound so similar.

The safest way to reason through an unfamiliar question

If a question describes an unusual scenario — an animal with a different thorax shape, or an apparatus modelling the lungs — don’t try to recall a memorised answer. Instead, follow the rule directly: does the volume described increase or decrease? If it increases, pressure must fall, and air must move in. If it decreases, pressure must rise, and air must move out. That single volume-pressure rule survives almost any rewording Cambridge can put around the mechanism.

Don’t confuse

Ventilation is bulk airflow driven by pressure differences— it is not diffusion, and air does not enter the lungs because it “spreads out” into them. Diffusion only takes over once air has already reached the alveoli, which is where gas exchange actually happens. Keep the sequence in the right order in your own head: ventilation moves the air, gas exchange moves the gases within it.