2.6.3Inquiry Process
Mechanism of Ventilation — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 65 of 160
Section 1 of 4Inhalation
Inhalation is an active process that needs muscle contraction.
1
The external intercostal muscles contract, lifting the ribs up and outwards.
2
The diaphragm contracts and flattens, moving downwards.
3
Together these movements increase the volume of the thorax and the lungs.
4
According to the relationship between pressure and volume, the greater volume lowers the air pressure inside the lungs below atmospheric pressure.
5
Air flows down the pressure gradient from the atmosphere into the lungs until the pressures equalise.
Section 2 of 4Exhalation
1
The external intercostal muscles and the diaphragm relax during quiet breathing.
2
The ribs move down and inwards under gravity and the diaphragm returns to its dome shape.
3
Elastic recoil of stretched alveolar tissue also shrinks the lungs.
4
Thoracic volume falls, so pressure inside the lungs rises above atmospheric pressure and air is pushed out.
5
During forced exhalation, the internal intercostals contract to pull the ribs down and the abdominal muscles contract to push the diaphragm up.
Section 3 of 4Antagonistic Muscles and Comparison
Antagonistic pairs
- External and internal intercostal muscles work in opposition,
- so while one set contracts the other relaxes,
- producing opposite rib movements.
Pleural fluid
- The thin fluid between pleural membranes lets the lungs follow the chest wall smoothly and keeps them held against it.
Quiet breathing
- At rest, inhalation is active but exhalation is largely passive, relying on relaxation and elastic recoil.
Exercise
- Forced breathing uses extra muscles for both phases, which moves a larger volume of air per breath and per minute.
| Feature | Inhalation | Exhalation (quiet) |
|---|---|---|
| Diaphragm | Contracts, flattens | Relaxes, domed |
| Ribs | Up and out | Down and in |
| Thorax volume | Increases | Decreases |
| Lung pressure | Below atmospheric | Above atmospheric |
Section 4 of 4Exam Checklist & Tips
2.6.3 – Key Points for the Exam
- Ventilation The movement of air into and out of the lungs, which maintains concentration gradients at the alveoli.
- Diaphragm A dome-shaped sheet of muscle separating the thorax from the abdomen.
- Intercostal muscles Muscles between the ribs that move the rib cage during breathing.
- Antagonistic muscles Muscles that produce opposite movements, so one relaxes while the other contracts.
- Elastic recoil The tendency of stretched elastic tissue to return to its original size.
Common Mistakes
- Saying that air is 'sucked in' by the lungs; muscles change volume, and pressure differences move the air.
- Mixing up which way the diaphragm moves: it flattens (moves down) during inhalation.
- Describing exhalation at rest as needing active muscle contraction; it is mainly passive.
- Confusing ventilation with gas exchange or with cell respiration.
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