Distinguish quiet and forced expiration
Explain the volume–pressure chain while keeping passive recoil separate from active forced breathing.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recall the inspiration volume–pressure chain.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What helps ordinary quiet expiration?
What you will learn
- Quiet expiration follows relaxation and recoil
- Use the pressure comparison for both forms
Distinguish quiet and forced expiration
Quiet expiration follows relaxation and recoil
During quiet expiration, the diaphragm relaxes and returns towards its domed position, while the external intercostals relax. Elastic recoil helps the ribs move downwards and inwards and reduces lung and thorax volume. The internal intercostals are not required to contract for ordinary quiet expiration.
This distinction prevents the common mistake that every expiration is an active contraction of all internal intercostals. In the syllabus model of forced expiration, the internal intercostals contract to pull the ribs downwards and inwards; abdominal muscle action can also assist. The diaphragm is relaxed rather than contracting upwards as a pump.
Use the pressure comparison for both forms
Decreasing lung volume briefly raises lung pressure above atmospheric pressure, causing air to move out. In the invented comparison, inside 100.2 kPa exceeds outside 100.0 kPa by 0.2 kPa. At the end of expiration the pressures equalise and the outward flow stops; pressure is not permanently higher throughout the whole resting cycle.
Both quiet and forced expiration lead through reduced volume to outward air flow, but their muscle explanations differ. The required named internal-intercostal action refers to the simplified forced-expiration model, not every fibre or respiratory situation. Use the breathing phase stated in the question and include ribs, diaphragm, volume and pressure in the explanation.
Sources for this lesson
Pause and explain
Which pressure difference drives outward flow?
Worked example
Compare the muscle driver of quiet and forced expiration.
Show the worked solution
- Quiet expiration uses relaxation and recoil.
- Forced expiration includes internal intercostal contraction in this model.
Answer Quiet: relaxation/recoil. Forced: internal intercostals contract, drawing ribs down/in.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain quiet expiration using the diaphragm, external intercostals and recoil.
Give me a hint
Start with relaxation.
Compare my reasoning
- Diaphragm and external intercostals relax.
- Recoil moves ribs down/in and diaphragm towards a dome, reducing volume.
Relaxation and elastic recoil reduce thorax/lung volume as ribs move down/in and the diaphragm domes.
Look for these in your work
- Include relaxation
- Include boundary movements
Explain how internal intercostals contribute to forced expiration in the required model.
Give me a hint
Link contraction to rib movement.
Compare my reasoning
- Internal intercostals contract.
- Ribs move downwards/inwards, helping reduce thorax volume.
Internal intercostal contraction pulls ribs down/in, helping reduce volume during forced expiration.
Look for these in your work
- Name the required muscle
- Specify forced expiration
Predict air direction for lung pressure 100.2 kPa and outside 100.0 kPa, then explain what happens when they equalise.
Give me a hint
Compare the two pressures.
Compare my reasoning
- Inside is higher by 0.2 kPa, so air exits.
- Equalisation removes the driving difference.
Air moves out while inside pressure is higher; no net pressure-driven outflow remains after equalisation.
Look for these in your work
- Calculate and compare
- Describe the end condition
Common mistakes
- Calling quiet expiration compulsory internal-intercostal contraction.
- Saying the diaphragm contracts upwards during expiration.
What can you explain now?
Which pressure difference drives outward flow?
Compare with the explanation
Lung pressure above atmospheric pressure
Air flows out down that difference.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Compare quiet and forced expiration before tracing air direction.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.