WEBVTT

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Voiceover: The next accessory organ

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we're going to talk about in
the GI tract is the liver.

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The liver, and that's this
big fatty guy right here.

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Now, the liver has to be one of the

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coolest organs in the body.

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It's the only one that
you can take a piece of,

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this small chunk right here,

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and transplant into a
recipient that has cirrhosis

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or liver failure, and
it can grow to become

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a much larger organ that
can sustain the patient.

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How wild is that?

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And so to better understand
the liver, we'll start by

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discussing the main functions
it's responsible for.

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The first thing we need to think about is

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that it's responsible for metabolism,

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metabolism, and that involves
the breakdown of nutrients

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or catabolism, and I'll just
write cat right here, catabolism,

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as well as the buildup
of larger macromolecules,

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and that's through anabolism.

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So, catabolism and anabolism
are achieved in the liver.

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The other function the
liver is responsible for

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is storage of these nutrients.

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\When we're not breaking them
down or building them up,

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we need to have a place
for them to hang out,

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and so, they're
responsible for the storage

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of our carbohydrates, so I'll
just write carbs right here,

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and also mainly our
fats, so these two guys,

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and they're stored in
three different ways.

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First, carbs or fats can
be stored as glycogen.

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Glycogen is just a
polysaccharide, so a linkage

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of a whole bunch of
monosaccharides or simple sugars.

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The other thing that we can
store our carbs or our fats as

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are lipoproteins, lipo meaning fatty

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and proteins meaning, well,
protein, and then lastly,

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we can also store these
guys as triglycerides,

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triglycerides, which is
one of the forms of fat

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that we ingest in our food.

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Now, these are two of
the major macromolecules

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we use for energy.

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The other major macromolecule
class that the liver sees

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are proteins, but these aren't
really stored in the liver.

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Instead, they're processed
into molecules like albumin

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and then cast off into the bloodstream,

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where they'll float around
and carry out other functions,

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until they need to be
retrieved by the liver

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to be broken down or
used for other molecules.

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The other major process the
liver is responsible for

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is called detoxification, detoxification,

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and this is one of those
awesome scientific terms

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where the function is in the name.

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This is where we take
toxins and modify them

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so they can't harm our body.

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This is achieved mainly by
cytochrome P450 enzymes,

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cytochrome P450, and these
are kind of funky enzymes,

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because unlike the other
ones you've probably

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heard about before, these
don't bind a specific

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single substrate or molecule.

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If you remember an enzyme like

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phenylalanine hydroxylase,
that's something that will

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add a hydroxyl group to phenylalanine

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and only phenylalanine,
but cytochrome P450's

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will take a whole bunch
of different substrates

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and react with them.

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That's something that's pretty unique

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to this class of enzymes.

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Now, because of these
cytochrome P450 enzymes,

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we have a problem when
we take medications.

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In fact, we have a
decrease in drug efficacy

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because of this detoxification process,

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and what we mean by drug efficacy

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is the amount of an effect
a drug is able to have,

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and so, if the liver or these enzymes

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perceive medications that we ingest to be

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foreign or like toxins,
they're going to try and break

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them down so they don't
cause an effect to our body.

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So, doctors knows this,
and because of that,

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a specific dose must be given

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to account for the amount of a drug

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that's going to be
detoxified by the liver.

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And finally, as we'll talk
about in a separate video

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in better detail, the last thing

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that the liver is responsible
for is bile production,

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bile production, and as you might recall

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from our discussion on small bowel,

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bile is needed for the
absorption of fats from our food.

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Okay, so now that we
understand what are the

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main functions of the liver,

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how does it get all the
nutrients that we ingest

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and absorb from our intestinal
tract to be metabolized

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or stored or detoxified in the liver?

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Well, let's take a look at
the blood supply to the liver.

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So, one of the things that
makes the liver so unique

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is that it has two
separate blood supplies.

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The first, that comes in
from the intestinal tract,

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is from the portal vein.

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The portal venous system
is another term for it,

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and the portal vein
supplies the liver with

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nutrient-rich blood, nutrient-rich.

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So, where do these nutrients come from?

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Well, food that's absorbed
in the intestinal tract

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will then go through the
circulation and end up

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in the portal vein, to
be delivered to the liver

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for metabolism, and so we
consider this blood to be

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nutrient-rich, but where does
the liver get its oxygen from?

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Well, that's a separate blood source,

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and that's actually through what's called

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the proper hepatic artery,
the proper hepatic artery,

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and as the name suggests,
this is arterial blood

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that will be supplying
oxygen-rich red blood cells.

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So great, these are the two different

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sources of blood that go to the liver.

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Well, what about blood
that leaves the liver?

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There's one main vessel that carries blood

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out of the liver, and that's
called the hepatic vein.

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The hepatic vein is the exact
opposite as the two other

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types of vessels we just talked about.

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It's nutrient- and oxygen-poor.

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Blood that leaves through the hepatic vein

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from the liver will
circulate back to the heart

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to receive oxygen, flow
past the intestines

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to receive nutrients, and
then return to the liver,

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either through the portal vein

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or through the proper hepatic artery.

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The other output of the liver,

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that I guess I should mention right now,

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we'll talk about in more
detail in a separate video,

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is bile, and bile mainly leaves the liver

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through what's called
the common hepatic duct,

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the common hepatic duct,

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common hepatic duct that will take bile.