Gas exchange in the respiratory system occurs at which structure?

• A. The heart
• B. The alveoli
• C. The trachea
• D. The diaphragm

Answer: (B)

Gas exchange happens where the air in the lungs meets the blood: the alveoli. These tiny sacs are surrounded by a dense network of capillaries and have extremely thin walls, creating a large surface area for diffusion. Oxygen diffuses from the alveolar air, where its partial pressure is higher, into the blood, while carbon dioxide diffuses from the blood, where its partial pressure is higher, into the alveolar air to be exhaled. The thin respiratory membrane and the gradient in partial pressures drive this exchange, with surfactant helping keep the alveoli open to maintain efficient diffusion. The heart pumps blood but isn’t the site of gas exchange, the trachea is just a passage for air, and the diaphragm is a breathing muscle, not a site of gas exchange.