WEBVTT

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Across the whole planet,

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humans eat on average between
one and 2.7 kilograms of food a day.

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That's over 365 kilograms 
a year per person,

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and more than 28,800 kilograms
over the course of a lifetime.

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And every last scrap makes its way
through the digestive system.

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Comprised of ten organs 
covering nine meters

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and containing 
over 20 specialized cell types,

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this is one of the most diverse
and complicated systems in the human body.

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Its parts continuously work in unison
to fulfill a singular task:

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transforming the raw materials
of your food

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into the nutrients and energy
that keep you alive.

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Spanning the entire length of your torso,

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the digestive system 
has four main components.

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First, there's the gastrointestinal tract,

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a twisting channel 
that transports your food

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and has an internal surface area
of between 30 and 40 square meters,

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enough to cover half a badminton court.

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Second, there's the pancreas,

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gallbladder,

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and liver,

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a trio of organs that break down food
using an array of special juices.

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Third, the body's enzymes,

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hormones,

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nerves,

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and blood

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all work together to break down food,

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modulate the digestive process,

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and deliver its final products.

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Finally, there's the mesentery,

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a large stretch of tissue that supports

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and positions all your digestive organs 
in the abdomen,

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enabling them to do their jobs.

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The digestive process begins
before food even hits your tongue.

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Anticipating a tasty morsel,

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glands in your mouth start 
to pump out saliva.

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We produce about 1.5 liters
of this liquid each day.

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Once inside your mouth,

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chewing combines with the sloshing saliva

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to turn food into a moist lump 
called the bolus.

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Enzymes present in the saliva
break down any starch.

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Then, your food finds itself

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at the rim of a 25 centimeter-long tube 
called the esophagus,

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down which it must plunge 
to reach the stomach.

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Nerves in the surrounding 
esophageal tissue

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sense the bolus's presence 
and trigger peristalsis,

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a series of defined muscular contractions.

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That propels the food into the stomach

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where it's left at the mercy
of the muscular stomach walls,

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which bound the bolus, 
breaking it into chunks.

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Hormones secreted by cells in the lining
trigger the release of acids

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and enzyme-rich juices 
from the stomach wall

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that start to dissolve the food
and break down its proteins.

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These hormones also alert the pancreas,

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liver,

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and gallbladder

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to produce digestive juices

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and transfer bile, a yellowish-green 
liquid that digests fat,

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in preparation for the next stage.

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After three hours inside the stomach,

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the once shapely bolus is now
a frothy liquid called chyme,

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and it's ready to move into
the small intestine.

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The liver receives bile 
from the gallbladder

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and secretes it into the first portion of
the small intestine called the duodenum.

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Here, it dissolves the fats
floating in the slurry of chyme

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so they can be easily digested
by the pancreatic and intestinal juices

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that have leached onto the scene.

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These enzyme-rich juices break the fat
molecules down into fatty acids

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and glycerol for easier 
absorption into the body.

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The enzymes also carry out 
the final deconstruction

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of proteins into amino acids

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and carbohydrates into glucose.

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This happens in the 
small intestine's lower regions,

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the jejunum and ileum,

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which are coated in millions 
of tiny projections called villi.

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These create a huge surface area
to maximize molecule absorption

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and transference into the blood stream.

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The blood takes them on the final
leg of their journey

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to feed the body's organs and tissues.

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But it's not over quite yet.

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Leftover fiber,

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water,

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and dead cells 
sloughed off during digestion

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make it into the large intestine,
also known as the colon.

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The body drains out most of the remaining
fluid through the intestinal wall.

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What's left is a soft mass called stool.

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The colon squeezes this byproduct
into a pouch called the rectum,

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where nerves sense it expanding

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and tell the body when 
it's time to expel the waste.

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The byproducts of digestion
exit through the anus

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and the food's long journey,

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typically lasting between 30 and 40 hours,

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is finally complete.