By the end of this lesson
- Explain inhalation as a mechanical sequence.
- Explain quiet and forced exhalation.
- Relate volume, pressure and airflow.
BIG QUESTION
How does muscle contraction outside the lungs make air flow into them?
Muscles change thoracic volume; volume changes pressure; air then flows down a pressure gradient.
Inhalation is active
The diaphragm contracts and flattens. External intercostal muscles contract, moving ribs up and out.
Thoracic volume increases, lung pressure falls below atmospheric pressure and air flows in.
Quiet exhalation is mainly passive
The diaphragm and external intercostals relax. Elastic recoil moves ribs down and in and the diaphragm becomes dome-shaped.
Thoracic volume decreases, pressure rises above atmospheric pressure and air flows out.
Forced exhalation recruits muscles
During forceful breathing, internal intercostal and abdominal muscles contract, pulling ribs down and pushing the diaphragm upward.
This produces a larger, faster fall in thoracic volume and drives air out more rapidly.
Ventilation maintains exchange gradients
Bulk airflow is not diffusion. Ventilation renews alveolar air; oxygen and carbon dioxide then diffuse across alveolar walls.
Breathing deeper as well as faster can increase minute ventilationThe volume of air breathed in (or out) in one minute: breathing rate multiplied by tidal volume.: breathing rate × tidal volume.
Ventilation sequence
Answer in causal order from muscle to airflow.
| Feature | Inhalation | Quiet exhalation |
|---|---|---|
| Diaphragm | Contracts and flattens. | Relaxes and domes. |
| External intercostals | Contract. | Relax. |
| Ribs | Up and out. | Down and in. |
| Thoracic volume | Increases. | Decreases. |
| Lung pressure | Falls below atmospheric. | Rises above atmospheric. |
| Airflow | Into lungs. | Out of lungs. |
Language that earns precision
- ventilation
- Bulk movement of air into and out of lungs.
- inhalation
- Movement of air into lungs.
- exhalation
- Movement of air out of lungs.
- tidal volume
- Volume of air moved in one normal breath.
- minute ventilation
- Volume ventilated per minute: rate multiplied by tidal volume.
Common misconceptions
Claim: The diaphragm contracts upward during inhalation.
Correction: It contracts and flattens downward.
Claim: Lungs pull air in themselves.
Correction: Breathing muscles change thoracic volume and pressure drives airflow.
Claim: Gas diffuses from the atmosphere all the way into the lungs.
Correction: Ventilation moves air in bulk; diffusion acts across exchange surfaces.
Claim: Quiet and forced exhalation use exactly the same muscles.
Correction: Forced exhalation recruits internal intercostal and abdominal muscles.
Match the answer to the command word
Use muscle → rib/diaphragm movement → volume → pressure → airflow.
State whether exhalation is quiet or forced.
Minute ventilation requires both rate and tidal volume with consistent units.
Recall, apply, then justify
Explain inhalation.
Hint
Start with two muscle changes and finish with airflow.
Suggested marking points
- Diaphragm contracts and flattens.
- External intercostals contract, moving ribs up/out.
- Thoracic/lung volume increases.
- Pressure falls below atmospheric pressure.
- Air flows into lungs down the pressure gradient.
Explain forced exhalation.
Hint
Add active muscles to the pressure sequence.
Suggested marking points
- Internal intercostal/abdominal muscles contract.
- Ribs move down/in and diaphragm is pushed upward.
- Thoracic volume decreases and pressure rises.
- Air flows out more rapidly.
A person breathes 15 times per minute with tidal volume 0.50 dm³. Calculate minute ventilation.
Hint
Rate × volume per breath.
Suggested marking points
- 15 × 0.50.
- Minute ventilation = 7.5 dm³ min⁻¹.
Explain why doubling breathing rate may not double effective alveolar ventilation.
Planning hint
Not all inhaled air reaches alveoli.
What a strong design includes
- Some air remains in conducting airways as anatomical dead space.
- Each shallow breath may deliver little fresh air beyond dead space.
- Rapid breathing can reduce tidal volume.
- Minute ventilation can rise without proportional alveolar ventilation.
- Deeper breaths may renew alveolar air more effectively than equally rapid shallow breaths.