15. Three characteristics of the structure of an alveolus that aids its function includes:
a) The walls of both the capillary and the alveolus are very thin so the gases only have a short distance to diffuse.
b) Many capillaries cover the outer surface of the alveolus. This allows the blood to collect a large volume of oxygen (or lose a large volume of carbon dioxide) in a short amount of time.
c) Cells in the alveoli wall secrete a fluid that keeps the wall moist so diffusion occurs more easily. The moisture also keeps the sides of the alveoli from sticking together if the alveoli momentarily collapse on a forceful exhalation.
d) The alveoli have a small diameter. Because they are so small, many can fit into a small space, thus increasing the surface area capable of gas exchange.
