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How do oxygen and carbon dioxide move between alveoli and blood capillaries?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — human respiratory system The lungs serve as the gas-exchanging organ for the process of respiration.

Oxygen and carbon dioxide move between the alveoli and the blood in pulmonary capillaries primarily through the process of diffusion.1

This movement is driven by differences in the partial pressure of each gas, with molecules naturally flowing from an area of higher partial pressure to an area of lower partial pressure. Because the lung provides an immense internal surface area and a very thin membranous barrier—approximately 0.5 μm thick—this exchange occurs efficiently.1

Specifically:
* Oxygen: The concentration (and partial pressure) of oxygen is higher in the air within the alveoli than in the deoxygenated blood arriving at the lungs. Consequently, oxygen diffuses across the thin air-blood barrier into the capillaries.12
* Carbon Dioxide: Produced by cellular metabolism, carbon dioxide has a higher partial pressure in the blood than in the alveolar air. It therefore diffuses out of the blood and into the alveoli, where it is subsequently expelled from the body during exhalation.12