Answer:
Alveoli are in direct contact with capillaries (one-cell thick) of the circulatory system. ... This large surface area, combined with the thin-walled nature of the alveolar parenchymal cells, allows gases to easily diffuse across the cells.
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Alveoli is specially designed for maximising the surface area for diffusion process. Whereas the carbon dioxide which has higher concentration in the blood capillaries diffuses into Alveoli which have lower concentration of carbon dioxide. In this way, exchange of gases takes place in the alveoli
