Alveoli in lungs improve gas exchange by increasing the surface area of the lungs.
“Our lungs are the functional interface between us and the atmosphere. The capacity of a pair of lungs is about 6 liters, but this modest volume, divided among three hundred million alveoli, is bounded by a surface of 50-100 square meters, about the floor area of a large classroom.” (Vogel 2003:47)
Vogel S. Comparative Biomechanics: Life’s Physical World. Princeton: Princeton University Press; 2003. 580 p.