Air does not flow straight into blood vessels. It enters tiny air sacs called alveoli, where a very thin barrier separates air from blood. Watch breathing, then enter an alveolus to see oxygen and carbon dioxide move in opposite directions.
How does oxygen get from a breath into the bloodstream?
The pink organs are the lungs; the branching pale tubes are the airways. Below them is the diaphragm. The enlarged alveolus shows air inside a thin wall and a capillary outside it. Gas markers are enlarged; their motion illustrates diffusion, not individually tracked real molecules.
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Ventilation moves air into and out of the lungs using pressure differences. Diffusion exchanges gases across the alveolar barrier using partial-pressure differences. Breathing maintains the gradient; circulation brings fresh blood to exchange with.
A large surface, a short distance
Millions of alveoli provide a large total surface. The barrier is extremely thin, so gases cross a short distance. Thicken the barrier in the model: gas exchange slows. This comparison isolates thickness; real lung diseases can change several factors at once.
Check your understanding · Does the pulmonary artery carry oxygen-rich blood?
No. “Artery” means away from the heart. The pulmonary artery carries oxygen-poor blood to the lungs.