Voiceover: After the food is swallowed,
it leaves the mouth and then goes next to the esophagus.
So, let's focus on that.
And just to make sure,
we all know kind of how far the esophagus goes,
I'm gonna draw in some lines right here,
to show where it starts up here,
and then it ends right about there.
And kind of where I've drawn the lines, as well,
there are sphincters
that sit to make sure food only flows in one way.
Up here, we have the upper esophageal sphincter.
Esophageal sphincter.
And so, just to reiterate,
a sphincter is just a circular localization of muscle,
so just a bunch of muscle that sits in a ring right here,
that makes sure that it's closed unless we tell
that muscle to relax so we can pass food along.
And so, when I say that we're going to
tell the muscle to relax,
that implies then, that this muscle here,
is under our control.
So it's composed primarily of skeletal muscle.
Skeletal muscle, so it's under voluntary control.
And down here, I'll draw a squiggly line,
we have the lower esophageal sphincter.
So this is a little tricky because
we call it a sphincter,
you know, it smells like a sphincter,
looks like a sphincter,
but it doesn't actually work like a sphincter.
So it's sort of, I guess, a scientific joke,
if you'd like to call it that.
Because we don't have a ring of muscle that sits here
that opens and closes whenever our body deems
it's an appropriate time to.
What do we have instead?
Well, we actually have a sheet
that kind of sits right here.
There's a sheet of muscle
that lines the connection between
the thoracic cavity and the abdominal cavity.
So you can imagine that you have a thoracic cavity above,
Things that would sit here
would be your lungs and your heart.
And then below, and I'll draw that right here,
you have an abdominal cavity.
So the abdominal cavity that's gonna have
most of the GI tract.
And this muscle that we have, that sits here,
that's going to be the diaphragm.
You've probably heard of this.
This is voluntary skeletal muscle
that contracts to pull downwards to help us breathe.
When it moves downward, the lungs inflate with air.
So the diaphragm sits here and it actually makes a ring
around this lower esophageal sphincter.
And the diaphragm is actually lower esophageal sphincter
because it just sits there
holding the esophagus in place.
And that's why over time,
we can have what's called a hiatal hernia.
The esophagus can move upward and downward
through this lower esophageal sphincter
and we'll have gastric acid from the stomach
reflux upward and give us heartburn,
or Gastroesophageal reflux disease, G.R.D.
So that's why we have heartburn
but rodents or horses don't.
Rodents and horses actually have their own
true sphincter that sit here.
So instead they have an actual,
dedicated ring of muscle, rather than a sheet.
Now the other thing about the esophagus,
which is pretty interesting, is that
it works as a passageway for food, it doesn't do much.
The only process that it can kind of claim to have
any stake in, is what's referred to as peristalsis.
Peristalsis, and this is going to be an important term
for the discussion of the rest of the GI tract.
Peristalsis is just the wave-like propulsion of food.
Wave-like propulsion.
I'll just write that for now.
Because, you'll have the contraction
of the esophagus at on point up here, like so.
And by doing that, it relaxes down here.
It relaxes here.
And so when it relaxes here and contracts up here,
that means that if you have a bolus of food,
like a little circle that's right there,
it's more inclined to move this way, directly ahead.
And that's peristalsis.
And the way we control that
is kind of in three parts along the esophagus.
Because the esophagus is not just skeletal muscle
all the way or not even just smooth muscle.
It's split up into thirds.
So I'll draw three parts of the esophagus here.
So the first part that we have
is actually just skeletal muscle.
So it's under our voluntary control.
Skeletal muscle.
And so that's 1/3,
I'll say the top 1/3 of the esophagus.
In the middle right here, we actually have a mix.
We have skeletal under our control,
plus smooth muscle.
And they kind of work together.
So it's sort of in our control, but also not really.
So that's going to be the middle 1/3 of the esophagus.
The middle 1/3.
And kind of, as the progression goes,
finally when we get to the end right here,
this last 1/3 of the esophagus
is going to be smooth muscle.
So that's our last 1/3.
And so that's how the esophagus
is split up into three parts.
Skeletal at the top 1/3.
Skeletal and smooth for the middle 1/3.
and then smooth entirely for the last 1/3.
And that's how our esophagus works, as well.