Pulmonary Ventilation: How Atmospheric Pressure Drives Airflow

Air enters the lungs because atmospheric pressure (Patm) exceeds intrapulmonary pressure (Ppulm) during inspiration—a bulk flow mechanism governed by Boyle's law.

Pressure Dynamics

• At Sea Level: Patm = 760 mmHg = 101.3 kPa = 14.7 psi.
• Inspiration: Diaphragm contracts → thoracic volume ↑ → Ppulm < Patm → air flows in.
• Expiration (Passive): Elastic recoil → volume ↓ → Ppulm > Patm → air flows out.
• Forced Expiration: Abdominal/internal intercostal contraction → Ppulm >> Patm.

Pressure Gradients in Numbers

• Quiet breathing: ΔP = 1–3 mmHg.
• Exercise: ΔP = 20–50 mmHg.
• Maximal effort: ΔP up to 100+ mmHg.

Other Forces at Play

• Elastic Recoil: Lung (inward) + chest wall (outward) → FRC equilibrium.
• Surface Tension: Surfactant reduces alveolar collapse (Laplace's law).
• Airway Resistance: Poiseuille's law (radius⁴); turbulence at high flow.

Clinical Relevance

• Altitude: Patm ↓ → hypoxic drive, HAPE risk.
• Mechanical Ventilation: Positive pressure reverses gradient.
• Pneumothorax: Pleural pressure = Patm → lung collapse.

Atmospheric pressure is the sine qua non of spontaneous ventilation.