Mechanics of breathing: pressures and muscles !
- Mechanism
- Exam priority: high
To breathe in, muscles enlarge the chest, pressure inside the lungs drops and air flows in by itself. Quiet breathing out is passive.
Updated 2026-11-01
Also called: Respiratory mechanics, Boyle's law in breathing
See it in 3D: Diaphragm · External intercostals · Internal intercostals · Scalenes · Sternocleidomastoid · Rectus abdominis · Pleura · Phrenic nerves
What it is
Air always moves from higher to lower pressure. When the diaphragm and external intercostals contract, the chest enlarges, pleural pressure becomes more negative, the lung expands and alveolar pressure falls below atmospheric. At rest, expiration is driven by the lung's elastic recoil and costs no muscle work.
How it works, step by step
- The medullary respiratory centre fires down the phrenic and intercostal nerves.
- The diaphragm contracts and descends; the external intercostals lift the ribs up and forward.
- Thoracic volume rises and pleural pressure becomes more negative.
- The lung, coupled to the chest wall by the pleural fluid film, expands.
- By Boyle's law alveolar pressure falls by about −1 cmH₂O and air flows in until pressures equalise.
- When the muscles relax, elastic recoil and surface tension shrink the lung again.
- Alveolar pressure rises above atmospheric (≈ +1 cmH₂O) and air flows out without effort.
Lung volumes and capacities
TLC 6.0 LVC 4.8 LFRC 2.4 LRV 1.2 LFEV1/FVC 0.80
Normal adult pattern (approximate values: they vary with height, age and sex).
Tap a volume, a capacity or FEV1.
- IRV · Inspiratory reserve volume
- TV · Tidal volume
- ERV · Expiratory reserve volume
- RV · Residual volume
- Capacity (sum of volumes)
- Current forced expiration
- Normal
Reference values (resting adult)
| Parameter | Value | Note |
|---|---|---|
| Pleural pressure, end-expiration | ≈ −5 cmH₂O | |
| Pleural pressure, end-inspiration | ≈ −7.5 cmH₂O | |
| Alveolar pressure during inspiration | ≈ −1 cmH₂O | relative to atmospheric |
| Transpulmonary pressure | alveolar − pleural | the pressure that holds the lung open |
| Respiratory rate | 12–20 breaths/min |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Muscles of ventilation
| Phase | Muscles | When they work |
|---|---|---|
| Quiet inspiration | Diaphragm, external intercostals | Always |
| Forced inspiration | Scalenes, sternocleidomastoid | Exercise, dyspnea, obstruction |
| Quiet expiration | None (elastic recoil) | At rest |
| Forced expiration | Rectus and oblique abdominals, internal intercostals | Cough, exercise, asthma, COPD |
Clinical notes
- Pneumothorax: negative pleural pressure is lost; the lung collapses and the hemithorax over-expands. Classic: sudden pleuritic pain, absent breath sounds and hyperresonance.
- Flail chest: a segment of fractured ribs sinks in on inspiration (paradoxical motion) because it follows pleural pressure, not the muscles.
- Cord injury above C3 or bilateral phrenic palsy: no effective spontaneous breathing; with diaphragm paralysis the abdomen moves in on inspiration.
Test yourself
Think of the answer, then tap the question to see it.
Why does a pneumothorax collapse the lung even though the breathing muscles still work?
Air in the pleural space brings pleural pressure up to atmospheric; the transpulmonary pressure that held the lung open disappears and its elastic recoil collapses it.
What is alveolar pressure at the end of a quiet inspiration, when flow stops?
Zero (atmospheric): with no pressure difference between mouth and alveolus there is no airflow.
Why does quiet expiration cost no muscle energy?
Energy stored as elastic recoil (elastic fibres and surface tension) during inspiration is released when the muscles relax.
A patient with bilateral diaphragm paralysis lies supine. What sign appears on inspiration?
Paradoxical breathing: the abdomen moves in on inspiration because negative pleural pressure pulls the flaccid diaphragm up into the chest.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
- Diaphragm
- External intercostals
- Internal intercostals
- Scalenes
- Sternocleidomastoid
- Rectus abdominis
- Pleura
- Phrenic nerves
Related topics
- Lung and chest wall compliance
- Airway resistance
- The work of breathing
- Surface tension, surfactant and Laplace's law
- Pleural fluid: formation and drainage
- Lung volumes, capacities and spirometry
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 22.3 The Process of Breathing (OpenStax (Rice University), 2022); Anatomy, Thorax: Diaphragm (StatPearls (NCBI Bookshelf), 2024); Acute Pneumothorax Evaluation and Treatment (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 38). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.