Alveoli: The Microscopic Air Sacs Where Gas Exchange Happens

Alveoli are tiny, thin‑walled, grape‑like sacs at the terminal ends of the bronchial tree. Each adult lung contains ~300–500 million alveoli, providing ~70–100 m² surface area for gas exchange.

Structure & Function

• Type I pneumocytes – 95% of surface; ultra‑thin (0.1 µm) for rapid O₂/CO₂ diffusion.
• Type II pneumocytes – Secrete pulmonary surfactant (dipalmitoylphosphatidylcholine) reducing surface tension, preventing collapse.
• Capillary network – Intimate contact with alveolar wall; blood–gas barrier <0.5 µm thick.
• Alveolar macrophages – Clear inhaled particles, pathogens.

Clinical Relevance

Alveolar damage underlies ARDS, pulmonary fibrosis, emphysema (wall destruction), pneumonia (exudate filling), and neonatal RDS (surfactant deficiency).