How Breathing Works: The Mechanics of Air Entry Into the Lungs

Pulmonary Ventilation: A Pressure-Gradient Process

Air moves into and out of the lungs along a pressure gradient generated by changing thoracic volume.

1. Inspiration (Active)

• Diaphragm contracts → flattens, descends ~1.5 cm → ↑ vertical thoracic dimension.
• External intercostals contract → ribs elevate (bucket-handle) and sternum lifts (pump-handle) → ↑ anteroposterior & lateral dimensions.
• Thoracic volume ↑ → Intrapulmonary pressure ↓ (to ~ -1 to -3 cmH₂O below atmospheric).
• Air flows in: Nose/mouth → pharynx → larynx → trachea → bronchi → bronchioles → alveoli (gas exchange units).

2. Alveolar Expansion & Gas Exchange

• Alveoli expand; surfactant reduces surface tension, preventing collapse.
• O₂ diffuses across alveolar-capillary membrane → binds hemoglobin.
• CO₂ diffuses blood → alveoli.

3. Expiration (Passive at Rest)

• Diaphragm & external intercostals relax.
• Lung/chest wall elastic recoil → volume ↓ → pressure ↑ (>atmospheric).
• Air flows out along same path.

Forced expiration recruits internal intercostals & abdominal muscles.

4. Cycle Repeats

~12–20 breaths/min at rest; tidal volume ~500 mL. Continuous O₂ supply/CO₂ removal for cellular respiration.

Clinical Correlates

• Obstructive disease (asthma, COPD): ↑ resistance → prolonged expiration, air trapping.
• Restrictive disease (fibrosis, neuromuscular): ↓ compliance → shallow, rapid breathing.
• Diaphragmatic paralysis: Paradoxical abdominal motion.

Breathing is a beautifully orchestrated mechanical pump sustaining life.