In the sequence of steps for oxygenation, which step directly follows obtaining oxygen from the air?

• A. Transport air through the respiratory tract to alveoli.
• B. Obtain oxygen from the air.
• C. Diffuse oxygen into blood to be carried to all cells of the body.
• D. Remove carbon dioxide from tissues.

Answer: (A)

After obtaining oxygen from the air, the next step is to move that oxygen-rich air down the respiratory tract to the alveoli. This delivery to the alveolar region is essential because gas exchange occurs there; only when oxygen reaches the alveoli can it diffuse into the blood. Diffusion into blood and transport to tissues happen after the air has reached the alveoli, and removing carbon dioxide from tissues occurs later as a part of the overall gas exchange cycle. So the direct follow-up is transporting air through the respiratory tract to the alveoli.