Breathing Mechanics: Muscles, Airway, Gas Exchange & Regulation

Breathing (ventilation) is the mechanical process of moving air in/out; respiration includes gas exchange at alveoli and tissues.

1. Respiratory Muscles

• Diaphragm: Primary inspiratory muscle (75% tidal volume); contraction → flattens → ↑ thoracic volume
• External intercostals: Elevate ribs → ↑ anteroposterior & lateral diameter
• Accessory (inspiration): Scalenes, SCM, pectoralis minor (recruited in distress)
• Expiration (passive at rest): Elastic recoil of lungs/chest wall
• Forced expiration: Internal intercostals, abdominals (rectus, obliques, transversus)

2. Airway Patency

• Nasal cavity (warming, filtering, humidifying) → pharynx → larynx (protection) → trachea → bronchi → bronchioles → alveoli
• Obstruction at any level → increased work of breathing

3. Alveolar Gas Exchange

• 300M alveoli → 70–100 m² surface area
• O₂ diffuses down gradient: alveolar air (100 mmHg) → capillary blood (40 mmHg)
• CO₂ diffuses opposite: blood (45 mmHg) → alveoli (40 mmHg)
• Perfusion-ventilation matching (V/Q) critical

4. Gas Transport

• O₂: 98% bound to Hb (1.34 mL O₂/g Hb); 2% dissolved
• CO₂: 70% as bicarbonate (HCO₃⁻), 23% carbamino-Hb, 7% dissolved
• Bohr effect: ↓pH/↑CO₂ → right-shift O₂ dissociation curve → O₂ release
• Haldane effect: O₂ binding promotes CO₂ release

5. Neural & Chemical Control

• Medullary centers (DRG/VRG): automatic rhythm
• Pons (pneumotaxic, apneustic): pattern modulation
• Central chemoreceptors (medulla): respond to CSF pH (CO₂/H⁺)
• Peripheral chemoreceptors (carotid/aortic bodies): respond to PaO₂, PaCO₂, pH
• Lung stretch receptors (Hering-Breuer reflex)

Breathing integrates mechanical, chemical, and neural systems—disruption at any level causes respiratory failure.