Final answer:
The alveoli increase gas exchange efficiency due to their large number, the dense network of capillaries that surround them, and their thin walls that facilitate rapid diffusion of oxygen and carbon dioxide.
Step-by-step explanation:
The structure of alveoli maximizes gas exchange by increasing the available surface area for diffusion of oxygen and carbon dioxide. Alveoli are small sac-like structures, each approximately 200 μm in diameter, surrounded by a dense network of capillaries. This intimate contact between alveoli and blood vessels, along with the large number of alveoli (about 300 million per lung), creates a massive surface area, roughly the size of a tennis court, for gas exchange. This surface area, combined with the thin-walled nature of the alveolar and capillary walls, allows gases to rapidly diffuse across them.
During inhalation, oxygen-rich air fills the alveoli and oxygen molecules diffuse through the alveolar walls into the capillaries where they are taken up by red blood cells. Conversely, carbon dioxide, a waste product of metabolism in the body, diffuses from the blood into the alveoli to be exhaled. This gas exchange process is highly efficient because of the large surface area and the short diffusion distance provided by the thin alveolar and capillary walls.