What do thin-walled capillaries surrounding the alveoli do?

The function of the thin-walled capillaries surrounding the alveoli primarily involves the exchange of gases. In this context, the capillaries are crucial for delivering oxygen from the alveoli to the red blood cells. This process occurs through diffusion, where oxygen passes from the high concentration in the alveoli into the lower concentration within the capillaries. Once in the capillaries, the oxygen binds to hemoglobin molecules within the red blood cells, allowing for efficient transport throughout the body.

The other options highlight different mechanisms or processes: the exchange of gases is indeed happening, but it is not focused on merely transferring carbon dioxide for oxygen. While air filtration does occur in the respiratory system, it is primarily managed by nasal cavity structures and not notably by capillaries. Lastly, the role of expanding and contracting to regulate airflow is primarily attributed to the bronchi and alveoli themselves, not the capillaries. Therefore, the selected answer accurately captures the essential function of capillaries in the respiratory system.