Where do carbon dioxide molecules move during lung gas exchange?

• A. Bloodstream
• B. Exhaled air
• C. Alveolar sac
• D. Red blood cells

Answer: (C)

During lung gas exchange, carbon dioxide molecules primarily move from the bloodstream into the alveolar sacs. This process occurs in the lungs where the alveoli, tiny air sacs, provide a large surface area for gas exchange. As blood flows through the capillaries surrounding the alveoli, carbon dioxide, which is a waste product of cellular metabolism, diffuses from the blood, where its concentration is higher, into the alveoli, where its concentration is lower. This diffusion is driven by the principle of gas exchange, where gases move from areas of higher concentration to areas of lower concentration. Once in the alveoli, carbon dioxide can then be exhaled as part of the breathing process.

While carbon dioxide is also transported in the bloodstream and may be contained in red blood cells, the key point of movement during gas exchange specifically refers to the transfer into the alveolar sacs, where the gas is ultimately expelled from the body.