Which of the following best describes the function of alveoli?

• A. To distribute oxygen throughout the body
• B. To facilitate gas exchange between air and blood
• C. To filter pathogens from the air
• D. To store excess carbon dioxide

Answer: (B)

The function of alveoli is best described by their role in facilitating gas exchange between air and blood. Alveoli are tiny air sacs located at the end of the respiratory bronchioles in the lungs, and they are surrounded by a network of capillaries. When air enters the lungs, oxygen from the air diffuses across the thin walls of the alveoli into the blood within these capillaries. Simultaneously, carbon dioxide—a waste product of metabolism—is transferred from the blood into the alveoli to be exhaled. This crucial exchange of gases ensures that oxygen is brought into the bloodstream for delivery to tissues throughout the body while removing carbon dioxide, thus maintaining proper respiratory function and homeostasis.

Other options suggest functions that do not accurately represent what alveoli are primarily designed for. While the body does distribute oxygen after it has been exchanged, this is not the direct role of the alveoli themselves. The filtering of pathogens is primarily the function of the upper respiratory tract and mucous membranes, not the alveoli. Lastly, although carbon dioxide is present in the respiratory system, alveoli do not store excess amounts; instead, they facilitate the removal of carbon dioxide from the blood at the time of gas exchange.