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How does oxygen move from the alveoli into the blood?

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Illustration showing different animals and their respiratory organs: a spider with book lungs, a chicken with lungs, a sponge with labeled inflow and outflow pores, a fish with gills, and a caterpillar with spiracles. — Respiratory system Different methods of respiration in animals.

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 oxygen between two locations. Because the concentration—and thus the partial pressure—of oxygen is higher in the air within the alveoli than it is in the blood flowing through the surrounding pulmonary capillaries, oxygen naturally flows down this concentration gradient into the blood.12

The efficiency of this exchange is facilitated by the structure of the lungs, which provide an immense internal surface area, and by the extremely thin nature of the membranous barrier between the alveoli and the capillaries, which measures only about 0.5 μm. Once in the blood, the vast majority of this oxygen binds to hemoglobin within red blood cells for transport throughout the body.13