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- [Narrator] Starting with
the beginning of the GI tract,

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we're looking at the mouth.

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And when we're in the mouth,
we're dealing with teeth.

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So I'm pretty sure, I'm
going to probably guess,

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OK, teeth are meant to chew
food, meant to bite food,

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tear food off, you get the idea.

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But the different types of
teeth, they're called incisors,

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canines, pre-molars, and molars.

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Now children, before they
lose their baby teeth,

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are going to have 20 teeth, while adults,

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that means once they have
lost the baby teeth and

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the new ones grow in, we're
talking 32 permanent teeth.

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Does that mean everyone
has 32 permanent teeth?

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No.

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That third molar, known
as the wisdom tooth,

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sometimes it comes in,
sometimes it doesn't.

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Sometimes it has to be pulled out.

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So these aren't hard and cold numbers,

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they don't have to be there.

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It' just the typical
or the average numbers.

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The tongue itself, positioning the food.

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Well, hopefully positioning the food while

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not getting chewed on itself.

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But also it's tasting the
food, because the tongue

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has several different
types of taste buds on it.

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But the tongue is more than
just moving food and tasting it,

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it's also dealing with speech.

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Have you ever tried talking
with the tongue always

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at the bottom of your mouth?

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Or hold the tongue to
the top of your mouth.

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It doesn't sound normal.

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Because the tongue is going
to help and shape the air

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coming out of your lungs.

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To make sure the air flows
in the proper direction,

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therefore helping the resonant sound

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flow in the proper direction.

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So the tongue, positioning
food, tasting food,

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but also speech is an
important part as well.

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So here you can see a picture
of your lower and upper jaws.

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What we're going to notice
is, we have a total of

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eight incisors, four on the
bottom, four on the top.

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These are your front teeth.

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They're meant really for biting into,

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for tearing things off.

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Then a total of four canines,

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you're going to have two on
the top, two on the bottom.

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These are those a little
bit more pointy sharp teeth.

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Canine, think of cat.

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Then we have eight premolars.

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The total of four on top,
four on the bottom again.

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These teeth are starting to
get a little bit flatter,

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meant more for grinding.

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But the true grinding is in the molars.

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And we're noticing the same 12 molars,

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top and bottom, so six top, six bottom.

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That's if you have the wisdom teeth.

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That third molar, the one
all the way to the back,

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sometimes it just doesn't even come in.

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So, you might have a
total of eight molars,

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you might have nine, 10,
11, 12, it all depends on

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which molars grew in.

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Genetics is what controls
which ones are to come in,

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and which ones don't.

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So here you can see a
picture of a child's mouth.

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What you're noticing is,
there's little tiny lines

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going to deciduous teeth.

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Deciduous teeth are known as baby teeth.

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The permanent teeth, which
you can see above it,

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into the gums, are starting
to push the baby teeth out.

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That means the formation
of the permanent teeth

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are actually pushing the
baby teeth out of the way

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to allow them to come in.

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So you have your deciduous
teeth or baby teeth,

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but then the permanent teeth
will start to replace them

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as you age.

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The tooth itself is going
to have two major regions,

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referred to as the crown,
which is what you can see,

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and the root.

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While the crown of the tooth,
this is the natural crown,

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not a crown put on by a
dentist, the natural crown is

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covered by this substance called enamel.

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Enamel is going to be the
hardest substance in your body.

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But even though it's
the hardest substance,

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can you still wear it down?

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Sure, you can still wear your teeth down.

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So enamel's hard, but
it must be cared for.

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Below the enamel, inside
the tooth is the dentin.

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This helps give more
structure to the tooth itself,

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below that hard enamel.

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Within the dentin, you're
going to find the pulp cavity.

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The pulp cavity has your blood
vessels, it has your nerves,

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and those blood vessels
and nerves run down through

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the root canal.

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If you've ever had a hole in the enamel,

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or somehow had something
get into the pulp cavity,

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the root canal, you'll know
how painful that is to have

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those bare nerves touch something.

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But the entire tooth itself
is held within the bone

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via this periodontal membrane.

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Little tiny bits of
lines or ligaments help

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hold the membrane and keep the
tooth anchored in the bone.

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But even with this periodontal
membrane holding the tooth

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in place, can your teeth
move just a little bit?

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Sure, the teeth can move just
a little bit here and there,

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the adult teeth.

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Maybe sometime in the
morning, you woke up and you

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kind of closed your mouth and realized,

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"Huh.

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"My teeth don't quite
align the right way."

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That means maybe you had a lot
of stress the night before,

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and you were clenching and
grinding your teeth overnight.

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You can actually move the
tooth just a little bit.

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Thankfully, the tooth
will usually go back to

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its original position, it just
takes a little bit of time.

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Well, the teeth can't work alone.

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We also have to look at the saliva.

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The saliva is produced
primarily by three main

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salivary glands.

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The parotid, submandibular,
and sublingual.

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What's in the saliva, you
start with a lot of mucin.

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Well mucin, it's like a mucus,
but it's kind of watery.

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Then we have the salivary amylase,

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breaking down carbohydrates,

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bicarbonate, it's a buffer, and lysozyme,

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meant to get rid of bacteria.

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So a lot of different
things in your saliva.

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So here you can see a view
from the side of the head,

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or a lateral view.

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The parotid gland is
located just below the ear.

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Sublingual, well lingual refers to tongue,

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so it's below the tongue,
submandibular, well the mandible

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is your lower jaw bone, so submandibular,

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below the lower jaw.

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Now once we've chewed
everything up, using our teeth

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and mixed it up with the
saliva, we need to swallow.

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Swallowing is actually
a two-phase process.

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You have the voluntary phase
and the involuntary phase.

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The voluntary phase is when
you're consciously thinking

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about using the tongue to
push the food backwards.

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What will happen is, you'll
take this bolus of food,

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this kind of chewed up mass
of food, you'll consciously

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push it backwards towards the pharynx.

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The pharynx is the throat
at the back of your mouth.

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Well, once it's in the
pharynx, then you enter

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the involuntary phase.

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This is simply a swallowing reflex.

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What happens is, your
body automatically knows

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which muscles to contract,
which muscles to relax,

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to allow the food to go down the pharynx,

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and down the esophagus.

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But food is not the only thing
traveling down the pharynx.

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Air, what you breathe, can
travel through the pharynx.

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It doesn't matter if you
breathe through your nose

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or your mouth, both the
nose and mouth connect to

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the pharynx in the back.

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So we have air and food passing through.

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Well, somehow we need to
figure out how to sort out

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air from food and water.

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The epiglottis is a structure
that will help to sort out

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and make sure that the
food and water goes down

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the esophagus and stomach, and the air

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heads down towards the lungs.

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What ends up happening is, the
epiglottis is always opened

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when you're breathing, but
once you start to swallow

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food or water, the food or
water will actually push

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or close the epiglottis.

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That way, it prevents
any of that food or water

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from going down the trachea to the lungs.

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So it's the actual action
of swallowing that will

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close the epiglottis,
allowing the food and water

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to head down to the stomach,

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and not into the lungs, thankfully.

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But once you've swallowed, and
you're past the epiglottis,

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you're down in the esophagus.

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The esophagus will have these
peristaltic contractions,

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these peristaltic
contractions are rhythmic.

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You'll contract over and over,

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about once every twelve seconds or so,

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you'll go through this
rhythmic contraction.

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Food passes then, once
it leaves the esophagus,

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down through the lower
esophageal sphincter.

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Now I'm willing to bet you
guys can probably figure out

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where that is.

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If you've ever swallowed
something, and it was a little bit

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too much, and you've felt
that kind of pressure

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in your chest as it
slowly moved its way down?

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A lot of times it happens when
you're eating peanut butter

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for instance, or anything
else very sticky and tacky.

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Well, slowly moves its way down, that's

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the peristaltic contractions
moving the bolus of food

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down the esophagus.

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Then, all of a sudden, as
it moves down, down, down,

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you feel it right about
where your ribcage is ending,

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all of a sudden the pressure disappears.

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That's because the bolus
of food went through

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the lower esophageal sphincter,

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and it entered the stomach.

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The stomach is larger,
there's not as much pressure

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on the walls when the food enters.

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So you have that lower
esophageal sphincter.

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It's right between the
esophagus and the stomach.

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Its main function, prevents backflow.

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The lower esophageal
sphincter is preventing

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the stomach acid and the stomach contents

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from coming up into the esophagus.

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Because, if this sphincter's not working,

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and the stomach contents
are coming back up

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into the esophagus, what is that?

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Yeah, that's acid
reflux, that's heartburn.

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So your body really wants
to keep that contents of

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the stomach in the stomach.

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So hopefully this lower
esophageal sphincter

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is working properly.

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So here's a little diagram
showing the voluntary

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versus involuntary.

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You start off with the
tongue, pushing the food back.

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Now, before swallowing,
so while you're breathing,

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the esophagus is normally closed.

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There's no food or liquids
going down there, so,

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no point having it opened.

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But then, once that food
enters the posterior portion

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of the oral cavity,
the back of your mouth,

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it's entered into the
pharynx, and it pushes

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the epiglottis closed.

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Now if you notice, the
epiglottis is that little

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kind of blue structure sticking out

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in the back of the throat.

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As that green bolus of food comes down,

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you can see how it pushed
the epiglottis down.

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By pushing the epiglottis
down, it covered the trachea,

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it covered the airway,
so that food can only

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go into the esophagus.

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That's all done, because
you pushed that food back,

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sealed off the nasal cavity,
and started to swallow.

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So the food can only go one
way, it can only go down.

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As it keeps going down, it
travels down the esophagus,

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down, down, down through
your peristaltic contraction.

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It'll pass the lower esophageal sphincter,

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and eventually enter into the stomach.

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Once it enters the stomach,
now we're going to start

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mixing and churning it
up, making sure all that

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gets mixed up with the stomach acid and

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the stomach enzymes.