How you breathe
One breath, followed all the way from the muscle that starts it to the three hundred million air sacs where oxygen slips into your blood, narrated step by step.
The diaphragm pulls down
Breathing is not done by your lungs. Underneath them sits the diaphragm, a sheet of muscle about the size of a dinner plate, and when it tightens it flattens downward and drags the lungs open. The lungs have no muscle that can pull on themselves. They are entirely passive.
Your lungs are stretched open
Stretching the lungs makes the space inside them larger, so the air already in there spreads out and the pressure drops by a whisker, only a thousandth or two below the air outside. That is the whole trick. Air simply falls into the gap.
Air rushes in through the nose
Roughly half a litre of air now pours in through your nose, which is a far better door than your mouth. Its winding passages swirl the air, and about ten thousand litres a day arrive warmed to body temperature before they go any deeper.
Mucus and cilia clean it
The lining is sticky with mucus and covered in microscopic hairs called cilia that beat around twelve times a second. They trap dust, pollen and bacteria, then sweep the whole cargo backwards to your throat, where you quietly swallow it without noticing.
Past the voice box
The air slips past the larynx, your voice box, where two folds of tissue can close and buzz. In ordinary speech they open and shut about a hundred and ten times a second for a man, and twice that for a woman. Words are just outgoing air chopped into sound.
Down the windpipe
Now down the trachea, a tube about as wide as your thumb. It is propped permanently open by around twenty rings of cartilage, each shaped like the letter C, so the soft gap at the back can bulge when swallowed food goes by.
It forks into two bronchi
Behind your breastbone the windpipe forks into two bronchi, one per lung. The right branch is wider and steeper, which is why anything accidentally inhaled, a peanut or a knocked out tooth, almost always ends up in the right lung.
Branching twenty three times
From there the tubes branch again and again, twenty three times in all, narrowing as they go until the smallest are about half a millimetre wide, thinner than the lead in a pencil. It is exactly the branching pattern of a tree, grown inwards instead of outwards.
Into the alveoli
Each of the smallest tubes ends in a bunch of air sacs called alveoli, arranged like a cluster of grapes. You have around three hundred million of them, and spread out flat they would cover about seventy square metres, the floor of a small flat.
Oxygen crosses into the blood
Wrapped around every sac is a blood vessel so narrow that red cells must squeeze through in single file. The wall between air and blood is under a thousandth of a millimetre thick, and oxygen crosses it in about a quarter of a second.
Haemoglobin locks it away
Inside each red cell sit roughly two hundred and fifty million molecules of haemoglobin, and every one can carry four molecules of oxygen. That is why blood flashes bright scarlet the moment it is loaded, and turns dark again once your muscles take their share.
Carbon dioxide crosses back
Traffic runs both ways. The blood arriving from your body is loaded with carbon dioxide, the exhaust from burning your breakfast, and it leaks out into the air sac. You breathe out about a kilogram of carbon dioxide every single day.
The diaphragm lets go
Breathing out is usually free. The diaphragm just lets go, and your stretched lungs and rib cage spring back like a released elastic band, pushing the air out. You only spend muscle on it when you blow, cough, sing or shout.
You breathe out
The air leaving you is warm, damp and still about sixteen parts in a hundred oxygen, which is exactly why rescue breathing saves lives. You will do all of this around twenty thousand times before tomorrow, and notice almost none of them.
Watch it explain itself
Every step above is narrated aloud. Play it, or open it in the editor and make it yours — no account needed.