How the lungs work, in 3D

What the simulation shows

• Air streaming down the windpipe and through the airways, which split again and again (labels the windpipe and bronchi).
• The diaphragm (a dome of muscle under the lungs) pulling down as you breathe in, and the lungs swelling (labelled).
• Zooms 1000× into the tip of an airway: a cluster of tiny air sacs (alveoli) wrapped in blood vessels.
• Oxygen crossing from the air sacs into the blood (which turns red) while carbon dioxide crosses out.
• A callout: the air sacs spread flat would cover about 70 square metres, half a tennis court.
• Exercise: breathing speeds up to 40 breaths a minute and each breath gets four times deeper.
• Holding the breath: breathing stops, carbon dioxide builds up and the oxygen level slowly drops.
• Breathing settles back to a calm 14 breaths a minute.
• A counter rolls up to about 22,000: breaths in a day.

The real numbers it runs on

The simulation is built on these facts, so the narration and the picture always agree:

• At rest we breathe about 12 to 20 times a minute (the simulation shows 14), about half a litre each breath.
• Breathing in is mostly the diaphragm: a dome of muscle under the lungs that flattens and pulls down, so the lungs expand and air rushes in. Breathing out at rest is just relaxing.
• Air goes down the windpipe (trachea), which splits into two bronchi, then splits again and again, over 20 times, into ever smaller tubes.
• The tubes end in about 480 million tiny air sacs (alveoli); spread flat they'd cover about 70 square metres, around half a tennis court.
• Each air sac is wrapped in tiny blood vessels; its wall is thinner than a cell, so oxygen crosses into the blood and carbon dioxide crosses out.
• The right lung has three lobes, the left only two: it leaves room for the heart.
• During hard exercise breathing can reach about 40 breaths a minute, moving more than 15 times as much air.
• The urge to breathe when you hold your breath comes from carbon dioxide building up, not from running out of oxygen.
• We take about 22,000 breaths a day.

The script this video was made from

“Why you can't hold your breath”: the narration of the video above. Each highlighted moment in the simulation happens on the word that names it.

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