How do capillaries surrounding the alveoli function?

• A. They store oxygen
• B. They allow for the exchange of carbon dioxide for oxygen between alveoli and blood
• C. They contain mucus to trap pathogens
• D. They filter out nitrogen from inhaled air

Answer: (B)

Capillaries surrounding the alveoli play a crucial role in the respiratory process by facilitating gas exchange. The correct response highlights that these capillaries allow for the exchange of carbon dioxide for oxygen between the alveoli and the bloodstream.

When oxygen enters the alveoli during inhalation, it diffuses across the thin walls of the alveoli and into the capillary blood vessels. Simultaneously, carbon dioxide, which is a waste product from cellular metabolism in the bloodstream, moves from the blood into the alveoli to be exhaled. This efficient exchange is possible due to the large surface area of the alveoli and the very thin barrier formed by the alveolar and capillary walls, making it easy for gases to diffuse in opposite directions.

The other options do not accurately describe the function of the capillaries associated with the alveoli. They do not serve as storage sites for oxygen; rather, they facilitate its immediate transport. Mucus production and the trapping of pathogens are functions of other structures in the respiratory system, such as the bronchi rather than the alveolar capillaries. Lastly, while nitrogen is a major component of inhaled air, the capillaries do not have a filtering role in relation to nitrogen.