How do gases cross between air and blood?

Tiny air sacs in the lungs sit beside tiny blood vessels. Oxygen crosses the thin boundary from air into blood. Carbon dioxide crosses the other way so it can leave with breathed-out air. The moving dots represent gas molecules, not air bubbles in the blood. Air inside a lung sac is also called alveolar air; exhalation means breathing out. Alveolar ventilation and pulmonary perfusion bring air and blood near a diffusion barrier. Differences in gas partial pressure drive oxygen into capillary blood and carbon dioxide into alveolar air. The model separates these routes for clarity: ventilation, diffusion and circulation cooperate continuously, rather than operating as three isolated steps of a real breath.