Gas Exchange in the Alveoli: How Oxygen Enters Blood and Carbon Dioxide Leaves

The alveoli are microscopic, thin-walled air sacs clustered at the ends of bronchioles. Each alveolus is surrounded by a dense network of pulmonary capillaries.

The Diffusion Process

1. Inhalation brings oxygen-rich air into alveoli.
2. Oxygen diffuses across the alveolar epithelium, interstitial space, and capillary endothelium into red blood cells, binding to hemoglobin.
3. Carbon dioxide (a metabolic waste product) diffuses from capillary blood into the alveolar space.
4. Exhalation removes CO₂-rich air.

Why It's Efficient

• Enormous surface area (~100 m² in adults).
• Extremely thin barriers (0.5–1 μm).
• Continuous blood flow and ventilation maintain steep concentration gradients.

This passive, gradient-driven exchange is the foundation of respiratory physiology and essential for cellular metabolism.