Half a tennis court, folded up
Why Are Lungs Spongy Instead of Hollow?
A hollow bag would only touch the air at its walls, so lungs are split into millions of tiny sacs instead.
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Step 1 of 5
Oxygen only crosses where air touches blood.
Gas moves across the wet lining between an air space and a blood vessel. Nothing happens in the middle of a lung, only at its surfaces.
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Step 2 of 5
A hollow bag would have almost no surface.
Two balloons the size of your lungs would give you the area of a couple of dinner plates — nowhere near enough to keep a body running.
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Step 3 of 5
So the airway splits, again and again.
The windpipe divides into two, then each of those divides, twenty-three times over, ending in some 300 million sacs no wider than a hair.
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Step 4 of 5
The wall between air and blood is astonishingly thin.
One cell of sac and one cell of vessel, and that is all. Oxygen has less than a thousandth of a millimetre to cross.
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Step 5 of 5
Gas moves on its own, from crowded to empty.
No pumping is needed. There is more oxygen in the sac than in the arriving blood, and more carbon dioxide in the blood than in the sac, so each drifts the other way.
Unfolded flat, the lining of an adult's lungs would cover about 70 square metres — roughly half a tennis court.
The short version
Lungs are spongy because gas only crosses where air touches blood, so they trade space for surface — around 70 square metres of it.
Try it yourself
Squeeze a kitchen sponge and a balloon of the same size. The sponge holds less air but has vastly more inside surface. That is a lung.
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