Gas exchange in the lungs happens in tiny air sacs called alveoli. How many alveoli does an adult have in both lungs combined?
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Answer
About 480 million alveoli
Spread out flat, their walls would cover roughly 70 square metres, about the floor area of a small apartment.
How to estimate it
Adult lungs hold about 5 litres, of which roughly 4 litres, or 4,000 cubic centimetres, is air sac space. An alveolus is about 0.2 mm wide, so give each one a cube of 0.02 cm: 0.000008 cubic centimetres. 4,000 / 0.000008 = 500 million. Careful microscope counts give about 480 million alveoli.
Source: Ochs et al. 2004, American Journal of Respiratory and Critical Care Medicine
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