00:01
Now let's talk about
the lower GI tract.
00:04
Whereas the upper GI tract
was mostly about breakdown,
whether it's mechanical
or through acids.
00:11
The lower GI tract is really
more about absorption of things
nutrients, for example,
and then eventually water
in the distal parts.
00:20
So we're going to start
with the small intestine,
which is going to receive
the contents of the stomach.
00:26
Initially by a very short portion of
small intestine called the duodenum.
00:31
Then after that,
we have the true junction between
the upper and lower GI tracts.
with the jejunum.
00:39
And the jejunum will
form the ileum.
00:44
And it's a very
indistinct transition
so that jejunum and
ileum look very similar
somewhere in the middle here.
00:52
But then the transition from
the ileum to the large intestine
is going to be quite pronounced.
00:59
So the small intestine is
going to be the largest portion
overall of the GI tract
several several feet
of small intestine.
01:07
And it's going to be where
the majority of nutrients
are absorbed in the GI tract.
01:13
The large intestine is
going to be a bit shorter
than the small intestine.
01:19
And it's really
going to be concerned
with reabsorbing water and salt
from the contents of the lumen.
01:27
So the duodenum is going to be
directly after the stomach.
01:33
And that's going to have
a lot of clinical significance
for certain pathologies
of duodenum.
01:39
The distal part of the stomach
was called the pylorus,
which pylorus means gatekeeper.
01:45
And it's the gatekeeper because
it's got a lot of acid
that is trying to minimize
reaching the duodenum.
01:52
That said, the duodenum
itself is going to have
its own layer of glands
that is going to secrete a
very neutralizing substance
to help buffer against the acids
that are coming in from the stomach.
02:07
And the duodenum is going to
be broken up into four parts.
02:11
The first part is also
called the superior part.
02:15
And the second part is
going to descend downward as
the descending part.
02:20
And this is going to be probably
the most important part,
because this is where the
bile and pancreatic enzymes
are going to enter the GI tract
at that major duodenal papilla.
02:31
It's then going to turn at the
third part and be horizontal
and therefore called
the horizontal part,
then the fourth part is going to
come upward as the ascending part.
02:44
And at this point,
we're going to have its junction
with the rest of the small intestine
starting with jejunum.
02:50
And so it will be the
duodenojejunal junction.
02:54
And this junction is
clinically important because
this is where we typically divide
upper from lower GI
at least clinically.
03:03
For example, when we talk
about an upper GI bleed
versus a lower GI bleed,
we're talking about one
where the bleeding originates
either proximal to this junction
or distal to this junction.
03:16
So here again,
we have the duodenum.
03:19
And we see that it's really
holding the pancreas very tightly.
03:23
And we know why because
the pancreas is essentially
empty into the duodenum
at that major duodenal papilla.
03:33
And it's draining through
the main pancreatic duct.
03:36
But here's a little rule
of thumb with anatomy
if you see something
called a major so and so
chances are there's
a minor so and so.
03:43
If you hear there's
a main so and so
there's an accessory so and so
and that's the case here as well.
03:49
We have a major duodenal papilla
where the bile and
main pancreatic ducts end
but there's also a
minor duodenal papilla.
04:00
And that's where the accessory
pancreatic duct will drain.
04:05
Now the majority of
drainage in most people
is going to be through
the main pancreatic duct.
04:11
But in some people this
accessory duct persists
and actually drains a small
amount of pancreatic fluid
into this smaller papilla or
this minor duodenal papilla.
04:22
And the fact that there's these
two ducts for the pancreas
represents its development
embryologically from
two separate buds
that actually merged.
04:32
So that's why there's
actually two separate ducts
just that in most people
they successfully merged
that most of the fluid
can drain with the bile duct
at the major duodenal papilla.
04:46
So a little bit of surface anatomy
when it comes to the majority
of the small intestine
which is made up of
jejunum and ileum.
04:55
About 2/5 of
the length is jejunum
and about 3/5
is composed of ileum.
05:03
But after the duodenum at
the duodenal jejunal flexure.
05:08
The jejunum generally starts
in the upper left quadrant
and squiggles its
way down to the ileum
until it reaches the
right lower quadrant,
where it's going to join
the large intestine.
05:21
So if we've removed a little
bit of the colon here,
in order to see the duodenum,
we'd have to do that
because the duodenum
is actually fairly posterior.
05:29
The duodenum along with the
majority of the pancreas,
is actually stuck to the
posterior abdominal wall.
05:37
It's not very movable
within the abdomen.
05:39
So it's actually something we call
mostly retroperitoneal.
05:44
But at the duodenojejunal junction,
it becomes intraperitoneal.
05:50
It said all of these like
loops and folds of intestine
have room to undergo peristalsis.
05:56
And again, that junction begins
vaguely in the upper left quadrant,
and through many,
many complicated twists and turns,
will eventually end up
in the opposite quadrant.
06:09
So again,
we start with the jejunum,
about 2/5 of the length,
and then will form the ileum
at some point.
06:18
And then that will
be the 3/5 point.
06:20
There's not an anatomical
way of pointing
where the jejunum ends
in the ileum begins.
06:26
That's because
they're very similar.
06:28
And actually,
as you get closer and closer
to the junction between the two,
they're almost indistinct
from one another.
06:35
So it's really difficult to find
an anatomic distinction
of exactly where jejunum ends,
and exactly where the ileum begins.
06:47
However, it does get very
distinct at the end of the ileum.
06:51
So the end of the ileum is
something called the terminal ileum
down in the right lower
quadrant of the abdomen.
06:58
And it's important
for a lot of reasons.
07:01
Functionally,
it has a lot more of these
immune structures
called peyer's patches.
07:07
Certain things are
absorbed preferentially
in this area of the terminal ileum.
07:13
But it's an important landmark
because it's the junction between
the small and large intestine.
07:18
That first part of which is this
blind pouch called the cecum.
07:24
So the jejunum and ileum
very similar in many ways.
07:28
So what are some of the differences
you might be able to pick up on?
Well, the length.
07:34
It's a little bit
shorter than the ileum,
because about
2/5 of the intestine is jejunum
and 3/5 of the
small intestines is ileum.
07:42
Jejunum tends to be
a little bit wider in diameter
than the ileum.
07:48
The wall tends to be a little bit
thicker, compared to the ileum.
07:54
And the fat tends to be a little
more prominent on the ileum
than it does on the jejunum.
08:02
And the vessels within the
surrounding mesenteric fat
tend to have less
what we'd say arcades
or like intense
branching then the ileum.
08:14
And there's also some differences
that you will be able to tell
if you were able to see the inside
of the lumen
of the jejunum and ileum.
08:22
For example, the folds of mucosa
tend to be more prominent
in the jejunum than
they do in the ileum.
08:30
And also microscopically,
these things have lymphoid tissue
that we'll talk about and other
sections, called peyer's patches
tend to be more prominent in
the ileum versus the jejunum.
08:42
But as you can see, there's
a pretty subtle differences.
08:44
And that's why we really
don't pick out the exact end
of one in the
beginning of the other.