Bring in oxygen rich air and send out carbon dioxide.

• A. Lips
• B. Lungs
• C. Nose
• D. Stomach

Answer: (B)

Gas exchange in the respiratory system happens in the lungs, where oxygen from inhaled air passes into the blood and carbon dioxide from the blood passes into the air to be exhaled. Air enters through the nose or mouth, travels down the airways to the lungs, and reaches tiny air sacs called alveoli; here diffusion drives the transfer of gases due to differences in partial pressures. The lungs are the site where this exchange occurs, making them the correct choice. Lips and the nose help with airflow and conditioning the air, but they’re not where gas exchange with the blood happens; the stomach handles digestion.