Each of your lungs unfolds into half a tennis court

480 million alveoli per person. Their inner surface, laid flat, would cover 70 square metres: half a tennis court per lung, five times the skin of an adult. The numbers come from Ewald Weibel, the Swiss physiologist who in 1963 disassembled human lungs and measured them alveolus by alveolus.

1. A healthy adult has around 480 million alveoli, according to the modern count by Matthias Ochs and his team, published in 2004.

2. Each alveolus is 200 to 300 micrometres across: you cannot see it, but it would fit on the tip of a hair.

3. The heart pumps seven thousand five hundred litres a day against that network; each stream of blood has eighteen seconds to come back to the ankle carrying oxygen.

4. If that surface were a single bag, you would not fit through the door: which is why it is folded like millions of paper bags.

Weibel worked it all out without a scanner. He counted the alveoli on a squared grid slide and multiplied by the volume of the lung. The number that came out then is the same one that comes out now.