PE6: Mechanics of Breathing
How the diaphragm and intercostal muscles create ventilation, and how breathing changes during exercise.
🌬️ The Process of Breathing
Key Principle: Breathing (ventilation) works on the basis of pressure differences. Air always moves from an area of high pressure to an area of low pressure. By changing the volume of the thorax (chest cavity), the body changes the pressure inside the lungs, causing air to move in or out.
Boyle's Law (simplified):
When volume increases, pressure decreases → air rushes IN (inspiration)
When volume decreases, pressure increases → air rushes OUT (expiration)
When volume increases, pressure decreases → air rushes IN (inspiration)
When volume decreases, pressure increases → air rushes OUT (expiration)
💨 Inspiration (Inhalation)
Inspiration is an ACTIVE process - it requires muscle contraction.
During inspiration, the following sequence occurs:
- The diaphragm contracts and moves downwards (flattens from its dome shape)
- The external intercostal muscles contract, pulling the ribs upwards and outwards
- These actions increase the volume of the thorax (chest cavity)
- As volume increases, pressure inside the lungs decreases below atmospheric pressure
- Air rushes into the lungs from the higher atmospheric pressure outside, down the pressure gradient
| Structure | Action During Inspiration | Effect |
|---|---|---|
| Diaphragm | Contracts and flattens (moves down) | Increases thorax volume top-to-bottom |
| External intercostals | Contract, lifting ribs up and out | Increases thorax volume front-to-back and side-to-side |
| Internal intercostals | Relax | Allow ribs to move up and out |
😮💨 Expiration (Exhalation)
Quiet expiration at rest is a PASSIVE process - it does not require muscle contraction. It relies on the elastic recoil of the lungs and the relaxation of the inspiratory muscles.
During quiet expiration at rest:
- The diaphragm relaxes and returns to its dome shape (moves upwards)
- The external intercostal muscles relax, allowing the ribs to move downwards and inwards
- These actions decrease the volume of the thorax
- As volume decreases, pressure inside the lungs increases above atmospheric pressure
- Air rushes out of the lungs down the pressure gradient
Forced expiration during exercise is an ACTIVE process. The internal intercostal muscles contract to pull the ribs down and in more forcefully. The abdominal muscles also contract, pushing the diaphragm up and compressing the lungs to expel air more quickly.
| Feature | Quiet Expiration (at rest) | Forced Expiration (during exercise) |
|---|---|---|
| Process | Passive (elastic recoil) | Active (muscle contraction) |
| Diaphragm | Relaxes (returns to dome shape) | Pushed up by abdominal muscle contraction |
| External intercostals | Relax | Relax |
| Internal intercostals | Relax | Contract (pull ribs down and in) |
| Abdominal muscles | Not involved | Contract (compress abdomen, push diaphragm up) |
🏃 Breathing During Exercise
During exercise, both the rate and depth of breathing increase to meet the higher demand for oxygen and to remove more carbon dioxide.
When exercise begins, the body responds by:
- Increased rate of breathing - more breaths per minute (from ~12-15 breaths/min at rest to 40-50+ breaths/min during intense exercise)
- Increased depth of breathing - each breath takes in a larger volume of air (tidal volume increases from ~500 ml at rest to 2,000-3,000 ml during exercise)
- Both internal and external intercostal muscles work harder - inspiration and expiration both become active processes
- Anticipatory rise in breathing rate occurs before exercise starts (similar to heart rate anticipatory rise)
Breathing During a 400m Sprint
Before the race: breathing rate increases slightly due to anticipatory rise (adrenaline).
During the race: breathing rate and depth increase dramatically. Both inspiration and expiration become active processes. The internal intercostals and abdominal muscles contract forcefully during expiration. Tidal volume rises from ~500 ml to over 2,500 ml per breath.
After the race: breathing rate and depth remain elevated during recovery to repay the oxygen debt and remove lactic acid. They gradually return to resting levels over several minutes.
How is Breathing Regulated During Exercise?
- Chemoreceptors in the aorta and carotid arteries detect increased CO₂ in the blood
- They send signals to the medulla oblongata (respiratory centre in the brain)
- The medulla sends nerve impulses to the diaphragm and intercostal muscles to increase the rate and depth of breathing
- As CO₂ levels decrease during recovery, breathing rate gradually returns to normal
📊 Summary Comparison
| Feature | Rest | Exercise |
|---|---|---|
| Breathing rate | 12-15 breaths/min | 40-50+ breaths/min |
| Tidal volume | ~500 ml | 2,000-3,000 ml |
| Minute ventilation | ~6-7.5 l/min | 100-150+ l/min |
| Expiration | Passive (elastic recoil) | Active (internal intercostals + abdominals) |
| Inspiration | Active (diaphragm + external intercostals) | Active with greater force |
❓ Practice Questions
Q1: Describe the role of the diaphragm during inspiration.
Q2: Explain why quiet expiration at rest is described as a passive process.
Q3: How does forced expiration during exercise differ from quiet expiration at rest?
Q4: Explain how an increase in thorax volume causes air to enter the lungs.
Q5: How is breathing rate regulated during exercise?
Q6: Calculate minute ventilation if tidal volume is 500 ml and breathing rate is 14 breaths/min.
✅ Answers
- During inspiration, the diaphragm contracts and moves downwards (flattens from its dome shape). This increases the volume of the thorax top-to-bottom, which decreases the pressure inside the lungs, causing air to rush in.
- Quiet expiration at rest is passive because it does not require muscle contraction. The diaphragm and external intercostal muscles simply relax, and the elastic recoil of the lungs and the weight of the ribs moving downwards and inwards decreases the thorax volume, increasing pressure and forcing air out.
- Forced expiration during exercise is an active process involving the contraction of the internal intercostal muscles (pulling ribs down and in) and the abdominal muscles (pushing the diaphragm up). This expels air more quickly and forcefully than passive expiration at rest.
- When the thorax volume increases (due to diaphragm contracting down and ribs moving up and out), the pressure inside the lungs decreases below atmospheric pressure. Air moves from the higher atmospheric pressure outside into the lower pressure inside the lungs, down the pressure gradient.
- Chemoreceptors in the aorta and carotid arteries detect rising CO₂ levels in the blood during exercise. They send signals to the medulla oblongata (respiratory centre), which sends nerve impulses to the diaphragm and intercostal muscles, increasing the rate and depth of breathing.
- Minute ventilation = tidal volume × breathing rate = 500 ml × 14 = 7,000 ml/min = 7.0 l/min.
🎯 Exam Tips
- Always explain the full chain: muscle action → volume change → pressure change → air movement
- Use "contracts" and "relaxes" - don't just say the diaphragm "goes down"
- Remember: inspiration is always active; quiet expiration is passive; forced expiration is active
- Pressure and volume have an inverse relationship - learn this principle
- Be ready to explain how breathing changes during exercise (both rate AND depth increase)
- Know the role of chemoreceptors and the medulla in regulating breathing
📝 Exam Technique
PE Exam Tips — Mechanics of Breathing:
1. For Mechanics of Breathing questions, use subject-specific terminology precisely
2. Support every point with specific evidence or examples
3. Show balanced analysis — consider different perspectives before reaching a conclusion
4. Link your understanding of Mechanics of Breathing to real-world contexts where possible
5. For longer answers, plan your response to address all parts of the question
1. For Mechanics of Breathing questions, use subject-specific terminology precisely
2. Support every point with specific evidence or examples
3. Show balanced analysis — consider different perspectives before reaching a conclusion
4. Link your understanding of Mechanics of Breathing to real-world contexts where possible
5. For longer answers, plan your response to address all parts of the question
⚠️ Common Errors
Watch Out!
Students often think lactic acid causes doms. Wrong: Lactic acid causes DOMS Correct: DOMS (delayed onset muscle soreness) is caused by micro-tears in muscle fibres, not lactic acid. Lactic acid is cleared within an hour of exercise.
Students often think veins always carry deoxygenated blood. Wrong: Veins always carry deoxygenated blood Correct: Pulmonary veins carry oxygenated blood from the lungs to the heart. It is arteries and veins relative to the heart that matters, not oxygenation.
Students often think the heart beats faster during exercise just because you need more oxygen. Wrong: The heart beats faster during exercise just because you need more oxygen Correct: Heart rate increases due to anticipatory rise (before exercise), increased CO2 in blood detected by chemoreceptors, and decreased vagal tone. It is a coordinated response, not just oxygen demand.
✍️ Model Answer
Full-Mark Response
6 marks: Explain how mechanics of breathing affects sporting performance.
Mechanics of Breathing has significant effects on sporting performance. [Key concept 1]: explain the mechanism with specific detail. [Key concept 2]: how this applies in a named sporting example. [Key concept 3]: the relationship between this topic and overall performance. A grade 9 answer uses precise anatomical/physiological terminology, specific sporting examples, and evaluates the relative importance of different factors.
Mark scheme: 2 marks per explained point with specific evidence, evaluation for top marks
📊 AO Deep Dive
Assessment Objective Analysis
GCSE PE tests four AOs: AO1 (Knowledge, 30%) — recall facts about Mechanics of Breathing including definitions, classifications and specific examples; AO2 (Application, 30%) — apply knowledge to sporting contexts and training scenarios; AO3 (Analysis and Evaluation, 25%) — analyse data (graphs, tables), evaluate training methods or strategies, and justify recommendations; AO4 (Practical, 15%) — demonstrate relevant skills. For grade 9, use precise terminology, support every point with named sporting examples, and evaluate rather than just describe. The difference between grade 5 and grade 9 is the quality of application and depth of evaluation.
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🎬 Video Resources
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