What is the third step in the process of oxygenation?

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

Answer: (B)

Oxygenation relies on a sequence where oxygen moves from inhaled air into the lungs, then into the bloodstream, and finally to tissues. The third step is the diffusion of oxygen from the alveolar air into the blood across the alveolar-capillary membrane. At the alveoli, oxygen is at a high partial pressure, while the blood arriving there has a lower partial pressure, so oxygen diffuses into the blood and binds to hemoglobin for transport to all body cells. From there, it diffuses out of the blood into cells to fuel metabolism. The other options describe earlier stages (getting air to the alveoli) or incorrect ideas (turning carbon dioxide into oxygen), so diffusion into the blood is the pivotal step that links breathing to systemic delivery.