Biomechanics of Breathing: Muscles and Mechanics

Breathing (ventilation) is driven by pressure gradients generated by respiratory muscles.

Inspiration (Active)

• Diaphragm: Contracts → flattens → ↑ vertical thoracic volume (75% of tidal volume at rest).
• External intercostals: Elevate ribs → ↑ anteroposterior + lateral dimensions (bucket-handle, pump-handle).
• Accessory muscles (if needed): Scalenes, SCM, pectoralis minor → ↑ upper chest expansion.
• Intrapleural pressure (Ppl) becomes more negative (−5 to −8 cmH2O) → air flows in.

Expiration (Passive at Rest)

• Diaphragm/external intercostals relax → elastic recoil of lungs/chest wall → Ppl returns toward zero → air flows out.
• Active expiration (exercise, disease): Internal intercostals, abdominal muscles (rectus abdominis, obliques) → ↓ thoracic volume forcibly.

Gas Exchange

• Alveolar-capillary membrane: O2 diffuses into blood (↑PO2), CO2 diffuses out (↓PCO2).
• Perfusion-matched ventilation (V/Q ratio) optimizes exchange.

Control: Medullary (preBötzinger complex, VRG) + pontine centers; modulated by chemoreceptors (central: CO2/pH; peripheral: O2, CO2, pH) and mechanoreceptors.