In figure 33–6, what structure is labeled a? what is the role of the delicate network ofcapillaries that surround's them? by the way, a is in the box

The structure labelled as A must be alveoli.

Explanation:

Alveoli or alveolar sac are present in lungs.They are also called as air sacs. The alveoli of lungs provide a large surface area for gaseous exchange. The number of alveoli in human lungs has been estimated to be approx. 300 million. The rich network of capillaries surrounding them help in exchange of gases. Both alveolar wall and capillary wall are made of squamous epithelium.  It allows the exchange of gases according to their partial pressure differences.

it would lead to upper respiratory infections and pneumonia.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.