IGCSE Biology • Topic 11.1
Mechanics of Ventilation
How Volume & Pressure Changes Lead to Breathing
Supplement Topic: Understand how the ribs, internal and external intercostal muscles, and diaphragm work together to create volume and pressure changes in the thorax leading to ventilation of the lungs.
The Thorax Simulator
Adjust the volume slider to see how the diaphragm, ribs, and lungs work together to create pressure changes.
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The Physics Rule
Volume UP → Pressure DOWN → Air flows IN (from high pressure to low pressure)
Air always moves from areas of high pressure to areas of low pressure.
Complete Breathing Mechanism Summary
INHALATION (Inspiration)
▸Diaphragm: Contracts and flattens (moves DOWN)
▸External intercostals: Contract
▸Internal intercostals: Relax
▸Ribs: Move UP and OUT
▸Volume: Increases
▸Pressure: Decreases (below atmospheric)
▸Result: Air flows IN
EXHALATION (Expiration)
▸Diaphragm: Relaxes (domes UP)
▸External intercostals: Relax
▸Internal intercostals: Contract (forced breathing only)
▸Ribs: Move DOWN and IN
▸Volume: Decreases
▸Pressure: Increases (above atmospheric)
▸Result: Air flows OUT
⚠️ Common Examiner Traps
- !Don't confuse breathing (ventilation) with respiration (cellular energy release)
- !Normal exhalation is passive - internal intercostals only work during forced exhalation
- !Always mention volume changes BEFORE pressure changes in explanations
- !Use exact terms: "ventilation of the lungs" not just "breathing"
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