Alveoli: The Site of Gas Exchange in the Lungs
Alveoli are tiny, thin‑walled air sacs (~200–300 µm) at the terminal ends of bronchioles. Each lung contains ~300–500 million alveoli, providing ~70 m² surface area. Walls are a single layer of type I pneumocytes; capillaries envelop them. Oxygen diffuses from alveolar air (PO₂~100 mmHg) into capillary blood (PO₂~40 mmHg); CO₂ diffuses opposite. Surfactant (from type II cells) reduces surface tension, preventing collapse.
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