During gas exchange in the lungs, which statement is correct?

• A. Oxygen moves from blood into alveoli; carbon dioxide moves from alveoli into blood
• B. Oxygen moves from alveoli into blood; carbon dioxide moves from blood into alveoli
• C. Oxygen and carbon dioxide both move from blood to alveoli
• D. Oxygen moves from blood to alveoli; carbon dioxide stays in blood

Answer: (B)

Gas exchange in the lungs relies on diffusion, moving substances from areas of higher partial pressure to lower. In the alveoli, oxygen partial pressure is higher than in the blood, so oxygen moves into the blood. At the same time, carbon dioxide partial pressure is higher in the blood returning from the body than in the alveolar air, so carbon dioxide moves from the blood into the alveoli to be exhaled. This arrangement lets oxygen enter the bloodstream for body tissues and allows carbon dioxide, a waste product, to be removed. So the statement that oxygen moves from the alveoli into the blood and carbon dioxide moves from the blood into the alveoli is the correct description of gas exchange in the lungs.