Which gas is primarily exchanged in the alveoli?

• A. Oxygen
• B. Carbon Monoxide
• C. Nitrogen
• D. Hydrogen

Answer: (A)

Gas exchange in the alveoli happens by diffusion driven by partial pressure differences. Fresh air in the alveoli has a higher partial pressure of oxygen than the blood arriving in the pulmonary capillaries, so oxygen moves into the blood to be carried to tissues. At the same time, blood returning from the tissues carries more carbon dioxide, so it diffuses into the alveolar air to be exhaled. Among the options, oxygen is the key gas taken into the bloodstream for metabolism, making it the primary gas exchanged in the alveoli. Nitrogen is largely inert in this process, hydrogen isn’t a relevant respiratory gas in normal exchange, and carbon monoxide isn’t present in typical conditions.