Respiratory Physiology: Inhalation & Exhalation Mechanics

Inhalation (Inspiration) – Active Process

• Diaphragm contracts → flattens → ↑ thoracic volume (vertical).
• External intercostals contract → ribs elevate → ↑ thoracic volume (anteroposterior + lateral).
• Intrapleural pressure becomes more negative (–5 to –8 cmH₂O).
• Alveolar pressure < atmospheric → air flows in.

Exhalation (Expiration) – Passive at Rest

• Diaphragm relaxes → domes upward.
• External intercostals relax; elastic recoil of lungs/chest wall ↓ volume.
• Intrapleural pressure returns toward baseline.
• Alveolar pressure > atmospheric → air flows out.

Forced Exhalation (Exercise, Disease)

• Internal intercostals + abdominal muscles (rectus abdominis, obliques) contract → further ↓ thoracic volume.
• Active process; generates positive pleural pressure.

Control of Breathing

• Medullary centers: Dorsal respiratory group (inspiratory), ventral respiratory group (expiratory).
• Pons: Pneumotaxic (limits inspiration), apneustic (prolongs inspiration).
• Chemoreceptors: Central (medullary, pH/CO₂), peripheral (carotid/aortic bodies, PaO₂, pH, PaCO₂).
• ↑ PaCO₂ / ↓ pH → ↑ ventilation; ↓ PaO₂ (<60 mmHg) → ↑ ventilation (peripheral).

Clinical Relevance

• Work of breathing increased in obstruction (asthma, COPD), restriction (fibrosis, obesity), neuromuscular disease.
• Auto-PEEP (dynamic hyperinflation) in severe airflow limitation.
• Ventilator strategies target lung-protective volumes/pressures.