2.38 Understand the role of diffusion in gas exchange

Diffusion is the random movement of particles from an area of high concentration to an area of low concentration. This is exactly what happens in gas exchange. 

It occurs in the alveoli, in the lungs, where oxygen is transferred into the bloodstream and carbon dioxide diffuses out of the blood and is exhaled. 
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There is a high concentration of oxygen in the alveoli and a low concentration in the deoxygenated blood coming into the lungs. That is how diffusion occurs.
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There is a high concentration of carbon dioxide in the blood coming into the lungs, which diffuses back into the alveolus and gets exhaled out of the body. 

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The gases move across the wall of the alveolus. 

Figure 1: A pulmonary alveolus