Which organ is primarily responsible for gas exchange with the blood?

• A. Esophagus
• B. Trachea
• C. Lungs
• D. Heart

Answer: (C)

Gas exchange with the blood occurs primarily in the lungs, where air reaches tiny air sacs called alveoli. The alveoli are wrapped in a dense network of capillaries and have walls that are extremely thin, creating a surface where oxygen from inhaled air can diffuse into the blood and carbon dioxide from the blood can diffuse into the alveolar air to be exhaled. This setup—lots of surface area, thin diffusion barriers, and close proximity to blood vessels—makes the lungs the main site for exchanging gases with the circulatory system. The esophagus handles swallowing and digestion, not gas exchange. The trachea serves as the main airway guiding air to the lungs but doesn’t participate in gas exchange itself. The heart moves blood around the body, but gas exchange with air does not occur there.