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Across the whole planet,

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

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That's over 365 kilograms [br]a year per person,

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

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

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Comprised of ten organs [br]covering nine meters,

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and containing [br]over 20 specialized cell types,

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

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

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transforming the raw materials[br]of your food

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into the nutrients and energy[br]that keep you alive.

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Spanning the entire length of your torso,

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the digestive system [br]has four main components.

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First, there's the gastrointestinal tract,

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a twisting channel [br]that transports your food

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and has an internal surface area[br]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[br]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 [br]in the abdomen,

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enabling them to do their jobs.

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The digestive process begins[br]before food even hits your tongue.

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Anticipating a tasty morsel,

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glands in your mouth start [br]to pump out saliva.

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We produce about 1.5 liters[br]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 [br]called the bolus.

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Enzymes present in the saliva[br]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 [br]called the esophagus,

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down which it must plunge [br]to reach the stomach.

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Nerves in the surrounding [br]esophageal tissue

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sense the bolus's presence [br]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[br]of the muscular stomach walls,

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which bound the bolus, [br]breaking it into chunks.

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Hormones, secreted by cells in the lining,[br]trigger the release of acids

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and enzyme-rich juices [br]from the stomach wall

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that start to dissolve the food[br]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

0:02:49.717,0:02:53.815
and transfer bile, a yellowish-green [br]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[br]a frothy liquid called chyme,

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

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The liver receives bile [br]from the gallbladder

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

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Here, it dissolves the fats[br]floating in the slurry of chyme

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so they can be easily digested[br]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[br]molecules down into fatty acids

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and glycerol for easier [br]absorption into the body.

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The enzymes also carry out [br]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 [br]small intestine's lower regions,

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the jejunum and ileum,

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which are coated in millions [br]of tiny projections called villi.

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These create a huge surface area[br]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[br]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 [br]sloughed off during digestion

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

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The body drains out most of the remaining[br]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[br]into a pouch called the rectum,

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where nerves sense it expanding

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

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The byproducts of digestion[br]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.