Explain inspiration through thorax volume and pressure
Trace external intercostal and diaphragm contraction to air movement into the lungs.
Section B11.1 · Gas exchange in humans
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-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.
Before you begin
- Recall intercostal layers and pressure differences.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In which direction does bulk air move under a pressure difference?
What you will learn
- Follow the muscle and rib actions
- Pressure difference determines air direction
Explain inspiration through thorax volume and pressure
Follow the muscle and rib actions
During inspiration in the required breathing model, external intercostal muscles contract and the ribs move upwards and outwards. The diaphragm contracts and becomes flatter, moving downwards. Together these movements increase the volume of the thorax, with the lungs expanding as the chest boundary changes.
The internal intercostals are relaxed in this simplified inspiration model. Do not state that all between-rib muscle layers contract identically in every breathing phase. The anatomical layer lesson establishes identification; the present sequence links the required coordinated muscle actions to the volume change.
Pressure difference determines air direction
As lung volume increases, pressure inside the lungs falls below atmospheric pressure briefly and air moves in down that pressure difference. Air does not enter because the lungs create an empty absolute vacuum or because oxygen is actively pumped through cartilage. At the end of inspiration, pressure equalises and the inward flow stops.
Use the full chain: muscle action → ribs and diaphragm movement → increased thorax volume → decreased lung pressure relative to outside → inward air flow. For supplied illustrative pressures, outside 100.0 kPa and inside 99.8 kPa give a 0.2 kPa difference favouring entry. These invented values explain direction, not a clinical pressure measurement.
Sources for this lesson
Pause and explain
Which chain describes inspiration?
Worked example
Predict flow when outside pressure is 100.0 kPa and lung pressure is 99.8 kPa.
Show the worked solution
- Outside is higher by 0.2 kPa.
- Air moves from higher to lower pressure.
Answer Air moves into the lungs while that difference exists.
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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 inspiration from muscle actions through rib and diaphragm movement.
Give me a hint
Name the muscles and their effects.
Compare my reasoning
- External intercostals contract; ribs move up and out.
- Diaphragm contracts and flattens downwards.
External intercostal and diaphragm contraction moves ribs up/out and diaphragm down/flat, increasing thorax volume.
Look for these in your work
- Include both muscles
- Include both movements
Complete the chain from increased thorax volume to inward air movement.
Give me a hint
Compare lung with atmospheric pressure.
Compare my reasoning
- Lung pressure falls below outside pressure.
- Air enters until the difference is removed.
Increased thorax/lung volume lowers lung pressure below atmospheric pressure, causing air to enter.
Look for these in your work
- State the pressure comparison
- Give correct flow direction
At the end of inspiration the inside and outside pressures are equal. Does air continue entering because the ribs are still raised?
Give me a hint
Flow needs a pressure difference.
Compare my reasoning
- The raised position does not by itself maintain a pressure difference.
- Equal pressure removes the stated driver of inward flow.
No net inward flow is driven by the stated equal pressures, even though the chest remains expanded at that instant.
Look for these in your work
- Use pressure evidence
- Separate position from flow
Common mistakes
- Reversing volume and pressure changes.
- Saying inspiration requires an absolute vacuum.
What can you explain now?
Which chain describes inspiration?
Compare with the explanation
Thorax volume rises → lung pressure falls → air enters
It follows the required movements.
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Make it stick
Rebuild the full inspiration chain without skipping pressure.
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.