The human lungs contain approximately 500 million alveoli (range: 200–600 million), providing a total gas exchange surface area of ~70–100 m² — roughly the size of a tennis court. Each alveolus is a tiny air sac (~200–300 μm diameter) surrounded by capillaries. This enormous surface area, combined with the very thin alveolar-capillary membrane (~0.5 μm), enables efficient O₂/CO₂ exchange between air and blood.