Digestive System, part 3: Crash Course A&P #35

Nachos are great...if you are among the lucky ones whose body can digest them. When digestion goes according to plan, the small intestine performs most of your chemical digestion in the duodenum, while accessory organs including the liver, gallbladder, and pancreas contribute enzymes that all but finish the job. Then your large intestine, which is actually shorter than the small intestine, tries to extract the last bit of nutrition, including the occasional attempt to turn nachos into energy, which for most humans, ends in gassy failure.

Table of Contents
The Small Intestine Performs Most of Your Chemical Digestion in the Duodenum 1:54
The Liver, Gallbladder, and Pancreas Contribute Enzymes 4:26
The Large Intestine is Actually Shorter Than the Small Intestine 6:47
The Large Intestine Extracts the Last Bit of Nutrition 7:06

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You know, we’ve been talking about a lot
of serious stuff here lately.

Heart failure. Respiratory gas exchange. People
with holes in their stomachs.

Nachos.

Some might say I’ve even been flaunting
my ability to eat, digest, and enjoy a plate

of chips and melted cheese.

And I wouldn't blame them if they did, because
sadly, nachos aren’t for everyone.

In fact, I can safely say nachos are really
only a good idea for about a third of humans.

For the rest, what may start as a party in
your mouth will surely end in gastric distress.

Such is the fate of the lactose intolerant.

Lactose is basically milk-sugar that can only
be digested with the help of a special intestinal

enzyme -- lactase -- which many adults do
not produce enough of.

In fact, way back in the day, none of us did,
until about 7500 years ago, when a particularly

handy genetic mutation popped up in central
Europe.

This so-called lactase persistence trait probably
spread as Neolithic groups trekked north and

west through Europe. Today nearly 90 percent
of adult Britons and Scandinavians can chug

all the milk they want, whereas down toward
the Mediterranean, probably less than 40 percent

have lactase persistence, and fewer than ten
percent in Africa and Asia.

Now technically, a lactose intolerant person
can still consume dairy at their own risk,

but since their own bodies can’t break down
lactose, the job is left to the three-pound

bacteria farm living in their large intestines
-- bacteria that try their hardest to make

something of those milk sugars, the results
of which are gas, and bloating, and diarrhea.

So, it turns out, nachos aren’t just a good
way to talk about how the digestive system

works, they’re also a good way to talk about
when it doesn’t.

Remember how the stomach is great at obliterating
matter, but not so hot when it comes to actually

chemically digesting stuff, or really absorbing
much of anything?

You might say the stomach lacks subtlety.

But luckily, it’s got friends in low places,
and the small intestine is more than happy

to pick up the slack and provide a cozy environment
where your food is at long last disassembled

and absorbed by your cells.

Now there’s a lot of mechanical action and
peristalsis going on here, but there’s also

a ton of chemical digesting too.

And while homebrewed intestinal juices help
digest the chyme that your stomach turns food

into, the real power actually comes from the outside
helpers -- the liver, gallbladder, and pancreas.

Now, the small intestine is called “small”
not because it’s short but because it’s

about half the diameter of the large intestine
-- the thing is actually like 6 or 7 meters long.

Not only that, but whole deal is lined with epithelial
tissue that has more folds than an origami octopus.

These folds are lined with tiny hair-like
villi and even tinier microvilli, which create

a truly impressive surface area -- large enough that,
if it were unfolded, it would cover a tennis court.

It’s this massive surface area, and the
countless capillaries just beneath it, that

make the small intestine such a champion absorber
of nutrients.

It shares the same four tissue layers seen throughout
the GI tract, and has three main subdivisions:

Straight outta the stomach and snuggled around
the pancreas, you’ve got the relatively

short and mostly immovable duodenum, which
is where most of the chemical digestion occurs.

The middle section is the jejunum, where most
of the absorption takes place.

And finally at the end, running into the large
intestine is the ileum, where important vitamins

like A, B12, E, D, and K are absorbed.

But the duodenum is what you might call the
business end of the small intestine. It receives

chyme and gastric juices from the stomach
through the pyloric sphincter, but it also

imports bile from the liver and gallbladder,
enzymes from the pancreas, and creates its

own homegrown mix of enzymes.

Some of the imported enzymes eventually pass
through your system on the wave of gooey chyme.

But other enzymes are actually bound to cell
membranes in the intestinal mucosal layer,

and they’re reusable.

Enzymes are proteins, and proteins are expensive.
So these compounds -- known as brush border

enzymes -- can just sit around and process
food as it passes by, without your body having

to make new ones.

And the lactase that so many of us don’t
have, is one of these.

Now, the duodenum communicates with the stomach
in the last phase of gastric regulation that

we talked about in the last episode -- the
intestinal phase.

This is where the duodenum lets the stomach
know, with hormones and nerve signals, when

and how much chyme to release so it doesn’t
get overwhelmed all at once.

It’s also where stuff like bicarbonate from
the pancreas gets dumped, to help neutralize

the stomach acid before it burns a hole in
your guts.

And this brings me to your crucial accessory
organs -- the things apart from the alimentary

canal that never come in contact with ingested
material, but still play an essential role

in digestion.

First up: the liver.

The liver is a massive, fatty, four-lobed,
and very important organ. It lives directly

under your diaphragm and -- fun fact -- it
can actually fully regenerate itself after

an injury or surgery, with as little 25 percent
of its original tissue.

The liver serves tons of critical metabolic
and regulatory roles that we don’t have

time to get into right now, but its main role
in the digestive system is to make bile.

Bile is the missing ingredient your body needs
to attack fatty foods, which is a tricky business.

In part, that’s because fat isn’t water
soluble, and since your insides are mostly

water, fats will clump together, becoming
hard to digest.

To keep fat from clumping, you need an emulsifier,
so bile comes in to keep big, hydrophobic

fat molecules from sticking together, which
allows lipid-hungry enzymes to move in and

break them down into fatty acids and monoglycerides
that you can then digest and absorb.

But while your liver creates the bile, it
gets stored and concentrated in the neighboring

gallbladder, the thin, green sac cozied up
to the liver.

It gets the signal when chyme slides into
the duodenum, which activates the enteroendocrine

cells to release a pair of hormones. Those
hormones in turn tell the gallbladder to contract

and squirt bile through the cystic and bile
ducts into the duodenum.

Another crucial accessory organ is the pancreas,
a gland that looks like a fistful of cottage

cheese stuffed in a plastic bag.

The pancreas also does lots of important things
for your body, especially related to your

endocrine system, but for our purposes today,
just know that it brews up a powerful enzyme

cocktail that is also triggered by those same
two hormones.

Pancreatic juice is kinda like the Neapolitan
Ice Cream of bodily secretions -- it’s like

everybody’s favorite ingredients all put
together, and when you mix them, the result

is especially powerful.

You’ve got trypsin and peptidase in there,
which break proteins down into amino acids,

and you have lipases that turn triglycerides
into fatty acids and glycerol.

Amylase, meanwhile, reduces carbs to glucose
and fructose, and nuclease busts the nucleic

acids that are in DNA and RNA into nucleotides.

Once all of those macromolecules have been
dissembled into their monomers, the small

intestine’s epithelial cells can finally
absorb and transport them through your capillaries

and into the bloodstream, where they can travel
to pretty much to any cell in your body, and

be used to build collagen, or store fat, or
replace dying cells.

The true purpose behind all the eating that
you do.

So! Once the chyme has worked through your
small intestine, it passes through the ileocecal

valve and hits the cecum, the first part of
the large intestine, where, congratulations,

your food is now officially feces!

The large intestine -- consisting of the colon,
rectum, and anus -- is relatively short, at

about one and half meters, and it provides
a nice little frame for the small intestine,

here at the end of the alimentary canal.

By now, your body has sucked up almost all
of the nutrients it can, and is basically

just pushing indigestible goo around, so the
large intestine doesn’t have a lot of hard

work to do.

Its main functions are to absorb any remaining
water so you don’t have constant diarrhea,

and to store the rest until it’s ready to
exit the body.

It also plays host to hundreds of species
and trillions of individual gut bacteria,

which digest whatever chyme your body couldn’t,
releasing essential B and K vitamins, and

some short fatty acids, which the large intestine
can still absorb.

In doing so, they also produce gases like
carbon dioxide and methane, sulfurous compounds

called mercaptans, and hydrogen sulfide, which
eventually...pass.

OK, I know what you’re thinking now, you’re
like, “Hank, what’s up with the nachos?”

“Surely you’re not just gonna to bring
up nachos at the beginning of the episode

without explaining how they can turn on you?!”

Well, I’ve never disappointed you before,
have I?

Those of us who can’t produce the enzyme
lactase in our small intestine simply let

milk and cheese pass through the organ untouched,
leaving the digestion to these bacteria in

the large intestine.

And those bacteria possess about 1000 different
kinds of enzymes of their own, including lactase.

But their digestion process produces a whole
lotta extra gas, which is why nachos may leave

me feeling cheesy and satisfied, but leave
you bloated, and crampy, and malodorous.

But enough farting around, let’s wrap up
this fantastic journey.

Fecal matter keeps moving through in a couple
of different ways. Slow, segmenting haustral

contractions keep mixing and chopping it in
the large intestine, occurring every 30 minutes

or so and lasting about a minute.

But most people also experience a few mass
peristalsis movements a day -- big, intense

contractions that clear out a large swath
of intestine at once, pushing feces into the

rectum. These often occur just after eating.

Once in the rectum, your poop stimulates stretch
receptors that tap the parasympathetic defecation

reflex, which signals the colon and rectum
to contract, and the internal anal sphincter

to relax.

This forces the poop into the anal canal,
sending more messages to the brain that allow

us to decide whether to voluntarily open the
external anal sphincter, or just hold it for

a minute while we find a bathroom.

And when that moment arrives, what was once
food says farewell to the alimentary canal

that temporarily held it, and passes back
into the light of day.

And that, my friends, is the end of your digestive
system. Pretty cool, right?

And so it’s all over, but that doesn’t
mean you should forget about what we learned

today, which is that the small intestine performs
most of your chemical digestion in the duodenum,

while accessory organs including the liver,
gallbladder, and pancreas contribute enzymes

that all but finish the job. Then your large
intestine, which is actually shorter than

the small intestine, tries to extract the
last bit of nutrition, including the occasional

attempt to turn nachos into energy, which
for most humans, ends in gassy failure.

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help make Crash Course possible, not only

for themselves, but for everyone through their
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this one, go to patreon.com/crashcourse.

This episode was filmed in the Doctor Cheryl
C. Kinney Crash Course Studio, it was written

by Kathleen Yale, edited by Blake de Pastino,
and our consultant is Dr. Brandon Jackson.

It was directed by Nicholas Jenkins, edited
by Nicole Sweeney; our sound designer is Michael

Aranda, and the Graphics team is Thought Cafe.