The inner surface of your lungs, unfolded, would carpet a fair sized room several times over.

What it means

Air does not exchange with blood in the big tubes. It happens in hundreds of millions of tiny sacs called alveoli, each wrapped in capillaries, and their combined area runs to something in the region of seventy square meters. Packing that much surface into a chest cavity is the whole design problem a lung solves, and it is the same trick a tree uses with its leaves.

The claim comes from NHLBI.

Nearby in the collection

The facts either side of this one.

• NIDDKhard, 74 letters
• NHLBImedium, 74 letters
• MedlinePluseasy, 72 letters
• MedlinePlushard, 76 letters

About as hard as this one

Scored closest to it by the model behind our guide to solving cryptograms.

• USGShard, 66 letters
• NIDCDhard, 74 letters
• NHGRIhard, 67 letters
• NCBI Bookshelfhard, 76 letters