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- [Lecturer] I'm sure everyone's heard

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about the digestive system
and about nutrition.

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So first off, we have the digestive system

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broken into kind of two main areas.

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We can talk about it as the GI tract,

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or the gastrointestinal tract.

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In essence, it's one, long, hollow tube.

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Running all the way from mouth,

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all the way down through the anus.

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The space inside is called a lumen.

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The lumen is where all the
food will go as you digest it.

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It'll travel through this space

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as it goes through the entire GI tract.

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You have things like
your mouth, your pharynx,

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esophagus, stomach, small
intestines, large intestines,

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rectum, anus, all these make up one

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very long continuous tube.

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But that's not it.

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You have other organs, other structures

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that are going to help out.

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The salivary glands are gonna help

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by secreting obviously, saliva.

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The liver, gall bladder, pancreas,

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will all help by secreting
different enzymes

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and different molecules.

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They'll aid in the digestive process

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as you go through the GI tract.

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So let's take a look at what we have here.

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First off, mouth.

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Now I'm willing to be you can probably

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figure out what the mouth does.

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Yeah sure, it chews the food.

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But what's the tongue doing?

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It moves the food around.

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But more importantly,
it's not just moving it

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in a random order, it's moving
it so the food is positioned

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in between the teeth as you chew.

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But there's a trick here.

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The trick is to get the tongue

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out of the way for you to chew.

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Well, once you've chewed
up your food enough,

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so hopefully it's not a solid mass,

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you can swallow.

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Once you swallow, you head
down into the pharynx.

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The pharynx is the passage at the back,

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or the posterior side of the mouth.

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It allows the food to head
down towards the stomach,

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but the pharynx also allows the air

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you breathe through your nose

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to travel down towards your lungs.

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That means the pharynx is going to be

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used for both digestive and
the respiratory systems.

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Esophagus.

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Once the food passes through the pharynx,

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it heads down through the esophagus.

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The esophagus is a long, muscular tube

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that runs all the way
down into your stomach.

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Now the stomach is kind
of a holding ground,

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kind of a storage area.

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The foods you eat will be
momentarily held in the stomach.

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Won't be kept here for days,

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just kept here for a couple dozen minutes.

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Might be pushing an hour or two.

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Depends on what you ate
and how much you ate.

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But in the stomach, you
start to mix up the food.

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You're gonna mix it
with different enzymes.

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You're gonna mix it in
with different chemicals.

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It's meant to start the digestive process.

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Now that physical mixing
where the stomach's actually

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churning and mixing it up,

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that's kind of like a mechanical.

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You're squeezing, so mechanical digestion.

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While the enzymes that are being secreted

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are gonna help to start
with a chemical digestion.

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You actually start to break down

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the physical protein molecules.

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But every so often a
little bit of this mixture

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is heading to the small intestines.

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Now the small intestines is going to help

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with digesting more protein.

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It'll start the digestion of fat.

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It'll do some more
digestion of carbohydrates.

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Most of the water and
nutrients are absorbed

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in the small intestines.

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You can see how long that small intestines

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is curling and curving around for.

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So we have this slow progression

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heading down through the small intestines

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that allow for more
chemical digestion because

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there's going to be more
digestive enzymes being secreted.

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The enzymes here can come from the

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walls of the small intestine.

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They can come from the liver or

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the gall bladder, the pancreas.

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So all these different
organs, these helping

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or accessory organs
also can secrete enzymes

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and other chemicals into
the small intestine.

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Now once they finish
in the small intestine

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it's time for the large intestine.

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You may have heard the
of the large intestine

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by name of the colon.

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The colon is the same
as the large intestine.

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Now the large intestine is
meant to be a storage area.

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It stores the waste material.

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Whatever is waste, is what was not

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absorbed by the small intestines.

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It could be undigested food products

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they didn't need to use.

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It could be plant
components, things like fiber

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that you simply cannot digest.

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So large intestines is
going to store this waste,

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while absorbing as much of the water

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and nutrients that are left over.

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So that stored waste eventually

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heads down to the sigmoid
colon to be stored as feces.

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The feces must exit through the rectum.

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So the rectum is simply a passageway

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to get this waste, to get
these feces, out of the body.

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So from mouth all the way
down to rectum and anus,

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that is the complete pathway that food

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can take through your body.

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So right from mouth through anus.

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But we have some accessory organs.

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The salivary glands.

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You can see there are three lines here

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because there are three
different major salivary glands.

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Now saliva's meant to mix
with the foods you eat.

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Helps to moisten the food, but
it also helps to maintain pH.

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So if you're eating something very acidic,

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it'll partially help to neutralize it.

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If something's a little basic,

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it'll help to neutralize it.

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Tries to keep the natural
pH in a normal range.

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You're gonna have the enzyme
amylase present in the saliva.

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Amylase is an enzyme
that can digest starch.

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Well maybe you ever had
to try this experiment

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back in you know middle
school, high school,

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some other college class, where you had

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to chew on a cracker for 10 to
15 minutes, the same cracker.

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You're probably thinking
oh, that's just disgusting.

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What was interesting, the
cracker will not taste

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like a cracker after
that 15 minutes, because

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while you're chewing
on it you're constantly

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mixing in these enzymes, this amylase.

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The amylase will break down the starch

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into simple sugar.

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That cracker should become very sweet

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by the end of that 15 minutes.

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But it's not just the amylase
that's here as an enzyme.

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We also have lysosomes which
contain plenty of enzymes

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meant to destroy, or
nothing else, inhibits

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bacterial progression.

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The liver.

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Now liver sits up just
below the diaphragm,

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so think about the top
of your abdominal cavity.

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This will produce bile.

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Now, bile is going to help
with the breakdown of lipids.

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But what's interesting, is that bile

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is not actually secreted from the liver.

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The bile is sent down to the gall bladder.

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The gall bladder can
concentrate and then secrete.

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But that's not the only
thing the liver does.

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The liver does quite
a few other functions.

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Quite a few other steps and processes.

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From cleaning out some
particles that shouldn't

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be there in the blood to
detoxifying to helping out

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with a ton of other
processes from digestion.

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Even stores nutrients.

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So here's that gall bladder
I mentioned a second ago.

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The gall bladder stores
and concentrates the bile.

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The gall bladder does not make the bile.

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The liver makes it.

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Pancreas, more digestive enzymes.

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These digestive enzymes are heading

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right down into the small intestines.

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But it's not just enzymes,
it's also bicarbonate.

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Bicarbonate is gonna be a buffer.

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The buffers help keep the pH neutral.

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Well your stomach is very acidic.

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So having bicarbonates
in the small intestines,

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which is right up to the stomach,

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you can help neutralize
some of that stomach acid.

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And the appendix.

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Even though it's attached
to the large intestines,

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you'd think it'd be part
of the digestive system.

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Well unfortunately they don't know of

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any digestive functions for the appendix.

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The only thing they've
been able to figure out

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is the appendix actually has
some immune function to it.

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So appendix is part of the
digestive system, structurally,

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but doesn't really do
anything for digestion.

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Now if we start looking at the
actual walls of the GI tract,

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doesn't matter if we're
talking about esophagus,

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stomach, small intestines,
large intestines.

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Any of the actual GI tract
has a total of four layers.

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You have mucosa which
is the innermost layer.

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It's a mucus membrane.

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By being a mucus membrane, it allows

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the particles of food to
slide by with less friction.

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The mucus helps to coat the walls

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and ease the passage
of the chewed up food.

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Some mucosa, well it's just behind that.

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It's just a little bit further away

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from the mucosa in the lumen.

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It's mostly connective
tissue, blood vessels,

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lymph vessels, nerves.

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It's meant to help with the
controlling factor of mucosa.

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The muscularis layer.

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The muscularis layer has either two

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or three layers of smooth muscle.

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Pretty much it's almost always two.

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The three layers is only found

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in the stomach, if you're curious.

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But this muscularis layer
is responsible for movement.

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It's what helps to push and
propel the digesting food

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through the entire GI tract.

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So its main function is motility.

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Then the outermost layer is the serosa.

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The serosa's kind of meant
to keep everything together,

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provide a little bit of
protection on the outside,

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help anchor it in place.

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So if we go from the most internal

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to the most external, the
most internal is mucosa.

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Then moving out we go submucosa,

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muscularis, and then serosa.

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But every so often throughout the walls

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of the GI tract, you get these
things called sphincters.

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Sphincters are going
to be a muscular ring.

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They're meant to create stop points.

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Either preventing particles
from going forward,

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or preventing things from going backwards.

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For example, you have a
sphincter at the start

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and end of the stomach.

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It's meant to keep the
contents within the stomach.

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So sphincters can keep things within,

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or prevent backflow.

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Because the sphincter that's
at the top of the stomach,

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right at the bottom of the esophagus,

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it's meant to prevent backflow,

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prevent the stomach acid from going

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up into your esophagus.

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If you've ever had heartburn,

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you'll know how unpleasant that is.

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Here we're looking at a histological view

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kinda artist's rendition.

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You can see the four different layers.

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The lumen is labeled.

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That's where all the food
particles will pass through.

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You have the mucosa, which
is that pinkish layer,

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the most internal.

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Then the submucosa.

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You can see how it's
supporting, it's around mucosa.

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Then this case they're showing
two layers of smooth muscle.

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Two layers of the muscularis.

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The internal layer you can see

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is almost forming rings around it.

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That internal layer, the
one touching the submucosa,

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is what helps squeeze the tube

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propelling the particles forward.

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And the most external, the serosa

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is covering and anchoring the entire tube.

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So how do things move?

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How do we accomplish anything?

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We have first off a mechanical
process and movement.

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Chewing, mixing.

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You're mechanically grinding down the food

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to break it into smaller pieces.

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It's still the same food,

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just smaller pieces of that food.

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Secretions.

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Well various fluids can be secreted.

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We have enzymes, we have
hormones, bile, acid,

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alkali, mucus.

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There's tons of different things.

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Everything has its own purpose.

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Obviously enzymes, that
are digestive enzymes

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are meant to break down.

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Hormones help regulate the process.

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Bile from the liver and then
stored in the gall bladder

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was mentioned to help with
lipid, or fat breakdown.

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Acid, alkali, mucus.

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All helps with the
movement because it creates

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a more slippery environment.

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Digestion itself.

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With digestion, you're now breaking

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down the food in the
smallest absorbable units.

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You have what's called a chemical

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and a mechanical breakdown.

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00:15:03,814 --> 00:15:07,407
Mechanical breakdown,
or mechanical digestion

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is known as when you're chewing,

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when you're grinding the food particles.

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So it's very similar number one.

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Chewing is mechanical digestion.

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But then chemical
digestion is when you take

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00:15:21,429 --> 00:15:23,742
the actual molecule and you're

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breaking the bonds of the molecule.

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00:15:27,349 --> 00:15:30,484
You might be making it
into a smaller particle

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00:15:30,484 --> 00:15:32,901
to be easier for absorption,.

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Now the whole point is absorption.

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You must absorb the nutrients, absorb

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00:15:39,570 --> 00:15:42,070
the particles that have been digested

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through the most inert
layer called mucosa.

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00:15:46,619 --> 00:15:48,519
These absorbed particles go into

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00:15:48,519 --> 00:15:51,102
the blood or the lymph vessels.

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00:15:52,137 --> 00:15:54,002
This way they can be transported

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throughout the entire body.

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And finally elimination.

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Whatever is not utilized,
whatever is not absorbed,

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must be removed from the body.

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00:16:05,916 --> 00:16:07,812
If it's not removed, the body would

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00:16:07,812 --> 00:16:09,916
turn into a toxic waste dump

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00:16:09,916 --> 00:16:14,681
with all this undigested,
non-necessary material.

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00:16:14,681 --> 00:16:18,822
So simply passes on
through, and once it's done

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00:16:18,822 --> 00:16:20,086
there's no more digestion.

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00:16:20,086 --> 00:16:23,919
It will eliminate this
material from the body.

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00:16:26,572 --> 00:16:29,011
For the way all this food is moving,

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00:16:29,011 --> 00:16:30,234
so obviously you can think of okay

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00:16:30,234 --> 00:16:33,151
in the mouth it's meant to swallow.

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00:16:34,138 --> 00:16:38,305
Well, all this movement is
aided by smooth muscle movement.

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00:16:40,599 --> 00:16:43,605
Once you've swallowed, you
have what's called peristalsis

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00:16:43,605 --> 00:16:45,022
and segmentation.

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00:16:46,524 --> 00:16:50,955
Peristalsis is helping to
propel the food forward.

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00:16:50,955 --> 00:16:53,293
What will end up happening here is

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00:16:53,293 --> 00:16:56,504
the tube that it's in, so the esophagus,

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00:16:56,504 --> 00:16:58,654
small intestine, large intestine,

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00:16:58,654 --> 00:17:00,642
it'll squeeze at one point

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00:17:00,642 --> 00:17:04,130
and then it'll just run down the tube.

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00:17:04,130 --> 00:17:07,064
Imagine if you're squeezing
a tube of toothpaste.

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00:17:07,064 --> 00:17:09,451
You start the one end and you
kind of pinch it and squeeze

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00:17:09,451 --> 00:17:11,767
all the way down to get it all empty.

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00:17:11,767 --> 00:17:14,841
That's the idea of peristalsis.

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00:17:14,841 --> 00:17:18,571
While segmentation is
meant to mix the food.

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00:17:18,571 --> 00:17:21,716
It'll move the food
forward, move the food back,

334
00:17:21,716 --> 00:17:23,854
move the food forward,
kind of is shuffling

335
00:17:23,854 --> 00:17:26,172
back and forth, moving it,

336
00:17:26,172 --> 00:17:29,922
to mix it as it goes
down through the tubing.

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00:17:31,276 --> 00:17:33,889
So here you can see peristalsis.

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00:17:33,889 --> 00:17:35,525
The esophagus, you see how it's pinched

339
00:17:35,525 --> 00:17:38,360
just above the bulge of food?

340
00:17:38,360 --> 00:17:40,108
That pinching just squeezes

341
00:17:40,108 --> 00:17:45,091
all the way down the esophagus
until you reach the stomach.

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00:17:45,091 --> 00:17:48,332
Segmentation is showing the bottom.

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00:17:48,332 --> 00:17:52,145
You notice how there are
two boluses of food there.

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00:17:52,145 --> 00:17:53,840
But then what happens is the muscles

345
00:17:53,840 --> 00:17:57,302
are going to pinch those boluses apart,

346
00:17:57,302 --> 00:17:59,802
creating four smaller boluses.

347
00:18:00,772 --> 00:18:03,120
Those four now get pinched in half

348
00:18:03,120 --> 00:18:05,427
themselves into smaller parts.

349
00:18:05,427 --> 00:18:06,730
And you can see how the parts get squeezed

350
00:18:06,730 --> 00:18:10,897
towards each other, to cause
them to mix with each other.

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00:18:11,743 --> 00:18:13,160
So segmentation.