Which organ serves as the primary site for gas exchange in respiration?

• A. Esophagus
• B. Lungs
• C. Bronchi
• D. Diaphragm

Answer: (B)

Gas exchange happens where air meets blood, specifically in the tiny air sacs called alveoli inside the lungs. The walls of the alveoli are extremely thin and surrounded by a dense network of capillaries, so oxygen passes from the alveolar air into the blood and carbon dioxide moves from the blood into the alveolar air to be exhaled. This diffusion occurs because of differences in partial pressures between the air in the alveoli and the blood, and the thin respiratory membrane facilitates it. The lungs are the organs where this exchange occurs. The esophagus is part of the digestive tract, the bronchi are larger airways that transport air but don’t exchange gases themselves, and the diaphragm is a muscle that helps ventilate the lungs by changing chest volume, not by performing gas exchange.