Alveoli

seventy square metres, folded into six litres.

fall through your lung — every split doubles the surface hidden in your chest

Each branch splits into two, and the surface exposed at that depth roughly doubles with every split — a plain compounding rule repeated, not one dramatic jump. Real lungs carry that doubling out to twenty-three generations, the same trick — repeated forking multiplying surface without needing more volume — that also runs the folded surfaces of the gut and kidneys.

Concepts: anatomy, scaling.

An interactive you play with one thumb, on Wundr.