Muscles of Exhalation: Anatomy and Mechanics of Breathing Out
Quiet Exhalation (Passive)
Primarily elastic recoil of lungs and chest wall. Inspiratory muscles (diaphragm, external intercostals) relax → lungs recoil → air flows out. No active muscle contraction required.
Forced (Active) Exhalation
Recruits expiratory muscles to increase intra-abdominal and intrathoracic pressure:
- Internal intercostal muscles (interosseous part): Pull ribs down/inward → decrease thoracic volume.
- Abdominal muscles:
- Rectus abdominis
- External oblique
- Internal oblique
- Transversus abdominis
Compress abdominal contents → push diaphragm up → decrease thoracic volume.
- Internal intercostals (interchondral part) and subcostals: Assist rib depression.
Diaphragm During Exhalation
- Relaxes → moves superiorly (dome shape restored).
- Does not actively contract during exhalation.
Summary of Muscle Actions
| Phase | Primary Muscles | Action |
|---|---|---|
| Quiet Inspiration | Diaphragm, external intercostals | Contract → ↑ thoracic volume |
| Quiet Exhalation | None (passive) | Elastic recoil |
| Forced Inspiration | + Scalenes, SCM, pec minor, serratus anterior | Maximal ↑ volume |
| Forced Exhalation | Internal intercostals, abdominals | Active ↓ volume |
Clinical Relevance
Abdominal muscle weakness (spinal cord injury, surgery) impairs forced exhalation → ineffective cough, secretion retention. Training (incentive spirometry, expiratory muscle strength training) improves clearance.
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