Which air sacs allow for efficient gas exchange?

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

Answer: (A)

Gas exchange happens primarily in the alveoli, the tiny air sacs at the end of the airways. They are ideal for this task because they provide a huge surface area, with each alveolus surrounded by a dense network of capillaries. The walls of the alveolus and the surrounding capillary form a very thin barrier, the alveolar-capillary membrane, which allows gases to diffuse quickly. A moist lining and the presence of surfactant keep the surface expanded and stable between breaths, ensuring maximum contact with blood for diffusion. Oxygen moves from the alveolar air into the blood because its partial pressure is higher in the alveolus, while carbon dioxide moves from the blood into the alveolar air to be exhaled because its partial pressure is higher in blood. Structures like the pharynx, trachea, and larynx are conducting airways; they transport air but do not participate in gas exchange. Understanding this explains why diseases that damage alveoli, such as pneumonia or emphysema, most directly impair oxygenation and require assessment of gas exchange status.