Which structure is the site where gas exchange occurs in the lungs?

• A. Perfusion
• B. Alveoli
• C. Lipids
• D. Tidal Volume

Answer: (B)

Gas exchange happens in the alveoli, the tiny air sacs in the lungs. Their walls are extremely thin and are surrounded by a dense network of capillaries, which makes the distance for diffusion very short and the surface area very large. This setup allows oxygen to move from the air inside the alveoli into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled.

Perfusion refers to the blood flow to the lungs, which supports gas exchange but is not the site itself. Lipids are fats and not involved as a site of gas exchange, and tidal volume is the amount of air moved per breath, not where exchange occurs.