Oxygen exchange in the lungs occurs through which process?

• A. Diffusion
• B. Filtration
• C. Osmosis
• D. Active transport

Answer: (A)

Oxygen exchange in the lungs happens by diffusion, a passive movement of molecules from an area of higher partial pressure to a lower one across the thin alveolar-capillary membrane. In the alveoli, oxygen is present at a high partial pressure, while in the nearby blood it is lower, so O2 diffuses into the blood and binds to hemoglobin for transport. The process is helped by the lungs’ large surface area and the very thin barrier, making diffusion efficient. Carbon dioxide follows the same principle in reverse, moving from blood (higher partial pressure) into the alveoli to be exhaled. Other mechanisms—filtration, osmosis, or active transport—don’t drive gas exchange in the lungs, since diffusion down the partial pressure gradient is the primary method for moving oxygen into the bloodstream.