Gas exchange occurs in the

• A. Alveoli
• B. Trachea
• C. Bronchi
• D. Pharynx

Answer: (A)

Gas exchange happens in the alveoli, the tiny air sacs at the end of the airways. Their walls are extremely thin and wrapped in a dense network of capillaries, creating a large surface area for diffusion. Oxygen moves from the alveolar air into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled. This exchange occurs across the respiratory membrane, which includes the alveolar epithelium (mostly Type I cells), the basement membranes, and the capillary endothelium. Surfactant produced by Type II cells reduces surface tension to keep alveoli open, helping maintain efficient diffusion. The trachea, bronchi, and pharynx are conducting airways that transport air but do not participate in gas exchange because their walls aren’t thin enough and they aren’t closely associated with a capillary bed.