Which two gases are primarily exchanged by the respiratory system?

• A. Oxygen and Nitrogen
• B. Oxygen and Carbon Dioxide
• C. Nitrogen and Helium
• D. Carbon Dioxide and Methane

Answer: (B)

Gas exchange hinges on oxygen moving into the blood to fuel metabolism and carbon dioxide being expelled as a waste product. In the alveoli, oxygen has a higher partial pressure in the air than in the blood, so it diffuses into the bloodstream and binds to hemoglobin to be carried to tissues. Blood returning from tissues has more carbon dioxide, so CO2 diffuses from blood into the alveolar air to be breathed out. Nitrogen stays largely inert in this process, so it isn’t exchanged in a meaningful way. Helium and methane aren’t part of normal respiratory gas exchange either. So the two gases primarily exchanged are oxygen and carbon dioxide.