Why Human Lungs Float: The Physics of Pulmonary Buoyancy

Lungs float in water because they are less dense than water—thanks to millions of air-filled alveoli.

The Physics

• Archimedes' principle: Buoyant force = weight of displaced fluid
• Density: Inflated lung ~0.3–0.5 g/cm³; water = 1.0 g/cm³
• Alveoli: ~300–500 million; total surface area ~70–100 m²; each a tiny air sac
• Air trapping: Even after exhalation, residual volume (~1.2 L) remains

Forensic Application

• Hydrostatic test (lung float test): Used historically to distinguish live birth (lungs float) from stillbirth (lungs sink — fluid-filled, never inflated)
• Limitations: decomposition produces gas → false float; CPR/ventilation inflates lungs postmortem
• Modern: supplemented by histology, imaging, gas analysis

Clinical Relevance

• Pneumothorax: air in pleural space → lung collapses (density ↑) → sinks in fluid
• Pulmonary edema: alveoli fluid-filled → density ↑ → loss of buoyancy
• Emphysema: hyperinflation → increased buoyancy, barrel chest

Simple Demo

Submerge a fresh lung specimen (or balloon) in water—it floats. Deflate it—it sinks. The air makes the difference.