What is the primary purpose of the alveolar sacs in gas exchange?

What is the primary purpose of the alveolar sacs in gas exchange?

• A. To filter air
• B. To facilitate diffusion of oxygen and carbon dioxide with capillaries
• C. To warm and humidify inspired air
• D. To connect bronchi to the trachea

Answer: (B)

Gas exchange happens when oxygen and carbon dioxide diffuse across the very thin alveolar-capillary membrane. The alveolar sacs provide a huge surface area and an extremely thin barrier, surrounded by a dense network of capillaries, so gases can move readily. Oxygen moves from the air in the alveoli into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled, driven by differences in partial pressures. Surfactant helps keep the alveoli open, maintaining surface area for efficient diffusion. The other functions belong to different parts of the airway system—for example, filtering and warming air occur in the upper airways, and the trachea-bronchial tree transports air—so the primary role of the alveolar sacs is diffusion of oxygen and carbon dioxide with the capillaries.

Gas exchange happens when oxygen and carbon dioxide diffuse across the very thin alveolar-capillary membrane. The alveolar sacs provide a huge surface area and an extremely thin barrier, surrounded by a dense network of capillaries, so gases can move readily. Oxygen moves from the air in the alveoli into the blood, while carbon dioxide moves from the blood into the alveolar air to be exhaled, driven by differences in partial pressures. Surfactant helps keep the alveoli open, maintaining surface area for efficient diffusion. The other functions belong to different parts of the airway system—for example, filtering and warming air occur in the upper airways, and the trachea-bronchial tree transports air—so the primary role of the alveolar sacs is diffusion of oxygen and carbon dioxide with the capillaries.