The digestive system, in 3D

What the simulation shows

• Close on the mouth: the teeth chew the food into smaller pieces and saliva flows in (labels the teeth and saliva).
• The food is swallowed: a wave of muscle squeezes it down the food pipe (oesophagus) into the stomach, the camera riding along (labelled).
• The stomach churns the food in acid into a soupy mix; the acidity drops to about pH 2 (labels the stomach and the pylorus, its exit valve).
• Time-lapse: the food streams through the coils of the small intestine and nutrients pass into the blood, on to the liver (labelled); the clock runs to about 7 hours.
• Zooms into the small intestine's lining: a forest of swaying villi catching nutrients and passing them into their blood vessels (callout: about 30 square metres of surface).
• Time-lapse: the leftovers move round the large intestine, water is drawn back out, gut bacteria swarm (labelled); the clock runs to about 36 hours.
• The whole tract lights up from mouth to end while a counter measures it: up to about 9 metres when stretched out.

The real numbers it runs on

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

• From mouth to end the digestive tract is one long tube, up to about 9 metres when stretched out (shorter in a living body, where its muscles keep it taut), folded up inside the belly. Say "up to" or "several metres", never an exact length.
• Chewing breaks food into small pieces; saliva wets it and starts digesting starch. We make about half a litre to a litre and a half of saliva a day.
• Swallowed food is pushed down the oesophagus (the food pipe, about 25 centimetres) by waves of muscle called peristalsis, in about 8 seconds; it works even upside down.
• The stomach churns food with strong acid (about pH 1.5 to 3.5) and enzymes into a thick soup called chyme; a ring of muscle at its exit, the pylorus, lets a little out at a time. A meal stays in the stomach for about 2 to 4 hours.
• The small intestine, several metres long (about 3 to 5 in a living body, up to 6 or 7 stretched out), is where most nutrients are absorbed. Its lining is folded and covered in tiny finger-like villi, about a millimetre tall, so its inner surface is about 30 square metres.
• Nutrients pass through the villi into tiny blood vessels and go first to the liver. Getting through the stomach and small intestine takes about 6 to 8 hours.
• The large intestine, about 1.5 metres long, draws water back out of what's left; trillions of bacteria live there and help break down fibre.
• The whole journey usually takes about 24 to 72 hours; most of that time is spent in the large intestine.

The script this video was made from

“Where your lunch goes”: the narration of the video above. Each highlighted moment in the simulation happens on the word that names it.

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