Which sequence correctly lists the steps of oxygenation from start to finish?

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

Answer: (A)

The essential idea is the order of events that lead to oxygen entering the blood. You must first obtain oxygen from the air, since without taking in air there is no oxygen to work with. Next, that air has to be moved through the respiratory tract all the way to the alveoli, the tiny sacs where gas exchange occurs. Only after oxygen is in the alveoli can it diffuse into the blood across the alveolar–capillary membrane, where it binds to hemoglobin and is transported to tissues throughout the body. So the correct sequence is obtaining oxygen from the air, transporting air through the respiratory tract to the alveoli, then diffusing oxygen into the blood. The other sequences mix up diffusion and transport steps, or place diffusion before the air actually reaches the alveoli, which isn’t possible.