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How does diffusion move oxygen from the alveoli into the blood?

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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.

In the human respiratory system, oxygen moves from the alveoli into the blood through the process of diffusion.12

This movement is driven by a difference in the partial pressure of the gas between two locations. Because the partial pressure of oxygen is higher in the alveolar air than in the deoxygenated blood within the surrounding pulmonary capillaries, oxygen naturally flows down this pressure gradient into the blood.12

This exchange is highly efficient due to the structure of the lungs, which provide an immense surface area for gas exchange, and the extremely thin nature of the membranous barrier—approximately 0.5 μm—separating the alveoli from the capillaries. Once in the blood, the vast majority of this oxygen binds to hemoglobin within red blood cells for transport throughout the body.13