A single alveolus wrapped by a blood capillary — watch the gases diffuse across the thin wall

thin wall · one cell thick O₂ CO₂ from bronchiole → Alveolus (air sac) air · high O₂ · low CO₂ Capillary low O₂ high CO₂ red blood cells
Oxygen (O₂) Carbon dioxide (CO₂)
O₂ moved into blood
0
CO₂ moved into alveolus
0
Diffusion: high → low concentration · no energy needed

O₂ in, CO₂ out — both move by diffusion, across a wall just one cell thick.