How many alveoli — the tiny air sacs where oxygen enters the blood — are in an average pair of adult human lungs?

Answer

480,000,000 alveoli

Nearly half a billion microscopic sacs give your lungs an internal surface area about the size of half a tennis court, all folded into your chest. Researchers got the count by painstakingly sampling and counting slices of donated lungs.

Source: Ochs et al., American Journal of Respiratory and Critical Care Medicine (2004)