11/19/2025
Each alveolus is supplied by many tiny blood vessels called capillaries. For oxygen and carbon dioxide to be exchanged, they must cross the alveolar capillary membrane. This exchange is promoted by diffusion — the passage of gas molecules through respiratory membranes. In diffusion, oxygen passes to the blood, and carbon dioxide, a by-product of cellular metabolism, passes out of the blood and is channeled away through exhalation.
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