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. Compare the same thin surface for the two gases. The sac contains air; the vessel contains blood.
The human body · Discover & try
How do gases cross between air and blood?
Reading level 2 of 5 · DevelopingChange
Word help
Alveolus
A tiny air sac in a lung.
Diffusion
Net movement down a concentration or partial-pressure difference.
Watch · Try · Explain
See how it works
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A schematic exchange surface. Dots represent gas molecules, not bubbles in blood; distances and times are illustrative.
What to look for
- Oxygen moves from air in the lung sac into blood.
- Carbon dioxide moves from blood toward the air that can be breathed out.
Explain what you noticed
Carbon dioxide arrives at the lung in blood. Which route removes it?
Keep this idea: Breathing moves air; diffusion exchanges gases between alveoli and blood.
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Source material checked 2026-10-29. Explanations and activities by Nepedia. Source links do not imply endorsement.
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