Where does the primary gas exchange occur in the respiratory system?

• A. Alveoli in the lungs
• B. Esophagus
• C. Stomach
• D. Kidneys

Answer: (A)

Gas exchange occurs primarily in the alveoli of the lungs. The alveolar walls form a very thin, moist barrier next to a dense network of capillaries, creating a large surface area for diffusion. Oxygen moves from the alveolar air into the blood because its partial pressure is higher there, while carbon dioxide moves from the blood into the alveoli because its partial pressure is higher in the blood. This diffusion across the respiratory membrane—driven by partial pressure gradients—allows efficient transfer of gases for respiration. The other listed organs do not perform gas exchange with the external air: the esophagus and stomach handle digestion and food passage, and the kidneys filter blood and regulate fluids and wastes.