Description

Anatomy of the Small Intestine

Small Intestine Overview:

• Extends from the Pylorus to the Caecum.
• Approximately 7 meters long in adults.
• Divided into three parts:
  • Duodenum
  • Jejunum
  • Ileum

Duodenum:

• Length: 20-30 cm
• Functions:
  • Iron absorption
  • Chemical digestion
  • Chemical neutralization
  • Processes chyme from the stomach

Topography of the Duodenum:

• Skeletopy: Extends from L1 to L3.
• Syntopy:
  • Posteriorly: Inferior Vena Cava, Aorta, Right Kidney
  • Superiorly: Liver, Pancreas
  • Anteriorly: Transverse Colon
• Holotopy: Epigastric and Umbilical Regions

Jejunum and Ileum:

• No clear demarcation between them.
• Jejunum: 2-3 meters long
  • Primary function: Absorption
• Ileum: 2-4 meters long
  • Absorbs remaining nutrients and vitamins (especially Vitamin B12).

Topography of the Jejunum and Ileum:

• Skeletopy: L2 to Right Iliac Fossa
• Syntopy:
  • Surrounded by Caecum, Ascending Colon, Transverse Colon, Descending Colon, and Sigmoid Colon.
  • Fixated posteriorly to the abdominal wall through the Root of the Mesentery (Radix Mesenterii).
  • Covered anteriorly by the Greater Omentum.
• Holotopy:
  • Ileum: Umbilical and Left Lateral Region
  • Jejunum: Left Lateral Region, Right Inguinal Region, Pubic Region

Anatomical Structures:

Duodenum:

• Superior Part: From Pyloric Orifice (Ostium Pyloricum) to Superior Duodenal Flexure (Flexura Duodeni Superior).
• Descending Part: From Superior Duodenal Flexure to Inferior Duodenal Flexure (Flexura Duodeni Inferior).
• Inferior Horizontal Part: From Inferior Duodenal Flexure.
• Ascending Part: Until Duodenojejunal Flexure (Flexura Duodenojejunalis) / Ligament of Treitz.

Jejunum:

• Wider and thicker than the ileum.
• More vascular.
• Darker in color.

Ileum:

• Thinner walls.
• Less vascular.
• Lighter in color.

Layers of the Small Intestinal Wall:

Tunica Mucosa:

• Simple Columnar Epithelium
• Circular Folds (Plicae Circulare) – Absent in the Duodenal Bulb
• Intestinal Villi (Villi Intestinales)
• Microvilli
• Intestinal Glands (Glandulae Intestinales)
• Solitary Lymphoid Nodules (Noduli Lymphoidei Solitarii)
• Aggregated Lymphoid Nodules (Noduli Lymphoidei Aggregati)
• Specialized Structures in Duodenum:
  • Major Duodenal Papilla (Papilla Duodeni Major)
  • Minor Duodenal Papilla (Papilla Duodeni Minor)

Tela Submucosa:

• Duodenum: Contains Brunner’s Glands.
• Jejunum: Longer villi.
• Ileum: More goblet cells and Peyer’s Patches (GALT).

Tunica Muscularis:

• Responsible for Peristaltic Movement
• Inner Layer: Circular Muscle Layer
  • Hepatopancreatic Sphincter (Sphincter of Oddi) (M. Sphincter Ampullae Hepatopancreatica)
• Outer Layer: Longitudinal Muscle Layer

Tunica Serosa:

• Intraperitoneal Organs: Covered by Peritoneum.
• Retroperitoneal Organs (e.g., Duodenum): Have Tunica Adventitia in certain areas.

Sources:

• Memorix Anatomy, 2nd Edition by Hudák Radovan, Kachlík David, and Volný Ondřej
• Biorender
• University Notes and Lectures

Transcript

Introduction
0:04
What’s up. Meditay here, and in this video, we’re gonna go through the anatomy of the
0:08
Small Intestine. So in the last video, we went through the anatomy of the Stomach.
0:12
Now the step after the Stomach is the Small Intestine, as you see here. So in this video,
0:18
we’re first going to look at the components that make up the small intestine, then we’re going
0:23
to look at their topography and go through how the small intestine is fixated to the posterior
0:28
abdominal wall. After that, we’ll look at some important anatomical structures related to the
0:34
three parts of it. And then, we’ll go through the layers that make up the small intestinal wall.
Small Intestine Overview
0:40
Alright, so here you see the anterior view of the abdominal cavity,
0:44
the whole small intestine is highlighted in blue here.
0:47
It reaches from the Stomach all the way to the large intestine, as you see here.
0:52
To be more specific, it goes from the pylorus of the Stomach to the caecum of the large intestine.
0:58
Now the length of the small intestine may vary, but in total, if you stretch it out,
1:04
it’d be approximately 7 meters long. Let’s now look at the parts of the small
1:08
intestine to get a better sense of its anatomy. We’ll do the topography along the way as well.
1:13
The first part is the Duodenum, which is the shortest part of the small intestine.
Duodenum
1:17
Not only is it the shortest part of the small intestine, but it serves different
1:21
functions as well, and I’ll show you why. The Duodenum is between 25 to 30 centimeters
1:27
long and Is mainly responsible for iron absorption, chemical digestion, and chemical
1:33
neutralization. Let’s go through that again… When you eat your food, and it’s been processed
1:38
in the Stomach. It then becomes something called Chyme. Which is essentially the food you ate
1:44
mixed with the hydrochloric acid in the Stomach. When Chyme from the Stomach enters the Duodenum,
1:50
the Duodenum itself begins to secrete a mucus that neutralizes stomach acid because the rest of the
1:57
small intestine after the Duodenum isn’t that protected against the acid as the Duodenum is,
2:02
so they can be damaged overtime fi the acid is not neutralized. So chemical neutralization,
2:10
that’s the first function. The Chyme is then mixed with pancreatic juices and bile from the liver and
2:16
gallbladder, which break down some compounds so that nutrients can more easily be extracted
2:21
as it passes through the intestines. Once this process is complete, the Chyme is pushed towards
2:27
the Jejunum to have nutrients pulled from it. Now, let’s go through the topography of the
2:33
Duodenum. Let’s start with the Skeletopy. Skeletopy means the location of an organ
2:38
in. relation to the various bones of the body. So measurements might vary depending on your
2:43
source. But essentially, the Duodenum goes from the first lumbar vertebrae
2:48
to the third lumbar vertebrae, as you see here. Next, we have the Syntopy. Meaning its relation
2:53
to other organs, I think it’s better if we switch over to my 3d program to visualize it better,
2:59
with the Duodenum here highlighted in green. So if we start with the organs you find behind the
3:05
Duodenum, you’ll see that the Aorta, vena cava inferior, and the right kidneys are behind it
3:11
Above the Duodenum, you’ll find the liver, and you’ll notice the Duodenum going around
3:16
the pancreas, meaning at some points of the Duodenum, the pancreas will be
3:21
superior to those parts because of that. And then you’ll find the transverse colon.
3:26
Anteriorly, I’ve faded it to make the Duodenum more visible here. So that’s the syntopy.
3:29
Next, the Holotopy, meaning its relation to the body as a whole. Its upper part
3:34
is in the epigastric region, while the lower level is in the umbilical region.
Jejunum and Ileum
3:39
So that’s the topography of the Duodenum. Now let’s do the next two parts of the small
3:44
intestine, which are the Jejunum and Ileum. And one thing you’ll notice when studying
3:49
the small intestine is that there is really no strict demarcation between the Jejunum and the
3:54
Ileum. But the functions vary a little bit. The Jejunum can stretch between 2-3m long,
4:01
and its primary function is absorption. The Ileum, however, is usually a little bit longer and can
4:07
be everything between 2 to 4 meters long. The Ileum absorbs any remaining nutrients
4:13
and vitamins that did not get absorbed by the Duodenum or Jejunum, particularly vitamin B12.
4:20
So the Jejunum and the Ileum go from the second lumbar vertebra, and the lower part
4:24
of the Ilium rests inside the right iliaca fossa. So approximately like this, from L2,
4:30
and the Ilium resting on iliac fossa. That’s actually the reason why it’s called Ilium because
4:36
it’s found in the iliac fossa, as you see here. Now topographically, you’ll notice the small
4:41
intestine is actually surrounded by the caecum, the ascending colon, transverse colon,
4:46
the descending colon, and the sigmoid colon. But posteriorly, it’s fixated to the posterior
4:52
abdominal wall, so let’s see how. First, we need to make a vertical cut of the small intestine,
4:57
like this. and then look at it from this perspective. You’ll be able to see
5:01
a sagittal plane of the abdominal cavity. Now the small intestine is highlighted here in orange,
5:07
and just for orientation sake, you’ll find the liver up here, along with the Stomach,
5:11
transverse colon, and sigmoid colon. The Duodenum is up here as well,
5:15
but we’ll talk about this one again in a minute. The reason why I’m showing you this is because the
5:20
whole small intestine is covered anteriorly by a structure called the greater omentum,
5:26
which is essentially just a layer of fat. And they’re fixed to the posterior abdominal wall,
5:32
through what is called the root of the mesentery, or radix mesenterii.
5:36
And you’ll notice that it not only fixates the intestines to the posterior abdominal wall, but
5:41
it also goes along and grab the small intestines and surrounds them in a layer we call mesentery.
5:47
Those are a part of what we call the peritoneum, which we will cover in detail in a separate video.
5:53
But for now, when studying the small intestine. It’s important to know this that the small
5:58
intestines don’t really flop around inside your abdomen. They’re fixed to the posterior abdominal
6:02
wall, and the blood supply to each organ in the abdominal cavity is through the mesenteric wall.
6:08
It’s a very controlled process. And keep in mind that behind the peritoneum,
6:12
you can find the so-called retroperitoneal organs, like the kidneys and the pancreas,
6:17
located veeery posteriorly inside the abdominal cavity.
6:22
The last thing for the topography before going detailed into the different parts of the small
6:26
intestine is the Holotopy, meaning its relation to the body as a whole. So if we add the lines
6:32
right here, you can see that the Jejenum lies in the umbilical and left lateral regions.
6:38
While the Ileum lies in three different places. It lies in the pubic region. It lies in the right
6:43
lateral region. It also lies in the Right inguinal region. So that was it for the topography.
Anatomical Structures of the Duodenum
6:49
So up until now, you should be able to recall what the different parts of the small intestines are,
6:55
they’re the Duodenum, Jejunum, and the Ileum, and you should have an approximate idea of
7:01
their topography and how they’re fixated to the posterior abdominal wall. Now,
7:05
we’re going to go through the specific anatomical structures of the small intestine and then briefly
7:11
look at the layers of the small intestinal wall. Let’s now start off with the Duodenum.
7:15
Here you see the Duodenum, But let’s look at a little more realistic view of the Duodenum.
7:20
The Duodenum consists of different parts. The first part is this one right here,
7:25
the Superior Part. The superior part is going to start at the pyloric orifice or ostium pyloricum.
7:32
Remember we talked about this one when we talked about the structures of the Stomach?
7:35
SO the superior part of the Duodenum extends all the way to The superior duodenal flexure,
7:42
also called Flexura Duodeni Superior. When the Duodenum bends, that’s where
7:46
the next part will start, which is the descending part. The descending
7:50
part of the duodenum ends with the inferior duodenal flexure, or Flexura Duodeni Inferior.
7:57
Then it continues horizontally, called the inferior horizontal part, and then it ascends a
8:03
little and forms the duodenojejunal flexure. It’s kind of a long word, but just put the
8:08
Duodenum and the Jejunum together. You’ll get the duodenojejunal flexure. It’s called that because,
8:14
at the point of this flexure, that’s where the Duodenum will continue as the Jejunum. Remember
8:20
earlier in this video when I went through how the peritoneum covers the Jejunum and the Ileum? Where
8:25
we talked about how the root of the mesentery comes from the posterior abdominal wall and
8:30
grabs each part of the small intestine, completely surrounding them and forming a wet serous fatty
8:36
layer around the small intestine? Well, One thing you will notice, though, as I go through the small
8:41
intestine, is that the whole small intestines are completely surrounded and protected by the
8:47
peritoneum, but not the whole Duodenum. Only the beginning of the Duodenum is completely
8:52
surrounded, but the rest of the Duodenum is held in position in a slightly different way.
8:57
The Duodenojejunal flexure is held up through a muscular ligament, which is attached to
9:02
the diaphragm. So let’s add the diaphragm. The ligament is this one. It goes from the
9:08
right cruise of the diaphragm, and then it runs behind the Stomach and the pancreas and grabs the
9:13
duodenojejunal flexure. So let’s do that again, and I’ll fade off all other structures. This is
9:18
the duodenojejunal flexure. This is the diaphragm, and it’s right cruise. This is the Duodenojejunal
9:24
flexure ligament. It’s also called the ligament of Treitz. There are some variations between
9:30
people tho. It might come from the left crus, and it might come from the right crus. You wouldn’t be
9:34
wrong either way. So that was the Duodenum. Now let’s do the Jejunum and the Ileum.
Anatomical Structures of Jejunum and Ileum
9:41
Remember I said that there’s no strict demarcation between them? Well, in reality, there are a few
9:48
ways to distinguish them. The Jejunum is thicker and wider than the Ileum. It’s more vascularized,
9:55
which makes it look a little darker. The Ileum is thinner, less vascular, and hence lighter in
10:01
color. And I found this image that visualized just that. You see the Jejunum being thicker,
10:07
and there are more blood vessels going there? It’s quite logical if you think about it.
10:11
Most of the absorption happens in the Jejunum. You need it to be wider and thicker to increase
10:17
the surface area to absorb efficiently, and you want it to be highly vascularized
10:22
in order to absorb all of the nutrients. Awesome. Now I want to talk about the layers of
Layers of the Small Intestinal Wall
10:27
the small intestinal wall. So let’s go ahead and open up a part of the small intestine walls, zoom
10:32
in, and then take a small section of the wall. You’ll notice that the whole small intestine has
10:38
more or less the same characteristics, except for some minor differences, and I’ll highlight
10:43
these differences as we go through them. Now you’ll still see the same general
10:48
layers as the rest of the digestive system. There is the Tunica Mucosa, then Tela Submucosa.
10:54
We have a Tunica Muscularis and a Tunica Serosa. And remember, the Tela Submucosa is always towards
11:02
the lumen, meaning it’s always towards the GI tube’s inner hollow opening. Now let’s go through
11:08
each of these and then look at some histology slides to really differentiate between them.
11:12
Let’s now start with the Tunica Mucosa first. The Tunica Mucosa is the innermost layer,
11:18
and it’s made up of epithelial cells necessary for absorption. So let’s highlight some important
11:23
structures. The first thing you notice here is that there are these large bumps going into the
11:28
lumen called circular folds. And if we take a look at this diagram, these circular folds
11:34
are just folds that form a circle as it folds around the intestinal tube as you see here,
11:40
and they’re really important anatomically because that’s what you’ll be able to see
11:45
when you look at a small intestinal tube grossly. And you’ll find these projections everywhere in
11:50
the small intestine, except for the Duodenal Bulb, which is the beginning of the Duodenum.
11:55
These circular folds will also be different in lengths along with various places in the small
12:00
intestine. In the Duodenum, the circular folds reach up to 8mm long. The same in the Jejunum,
12:07
they reach 8 mm long, but as we go further, they gradually start to disappear. Then they disappear
12:14
entirely at the end of the Ileum. And that’s a good thing having these circular folds in the
12:19
small intestines because they make it easier to absorb nutrients by increasing the surface area.
12:25
Now, what does that mean? Surface area is the sum of all surfaces. So this cube has
12:31
6 walls. And if you combine all the surfaces together, you’ll get the total surface area.
12:37
Now let me see if you really understood. Which of these tubes has the greatest surface area?
12:43
Well, they’re the same length, so if you stretch tube 1 out, it’ll be longer, meaning tube 1 has
12:50
the greatest surface area. That’s why the small intestines have those circular folds,
12:55
which makes it easier to absorb the food by providing it with more walls to be absorbed by.
13:00
They do get help with the absorption tho, and they get help through these small finger-like
13:05
projections you see coming out of it called the intestinal villus, or villi in plural. And
13:10
they look like this These Villi are lined with
13:13
highly specialized cells for absorption, called simple columnar epithelial cells. And each of them
13:20
has hair-like structures on the surface supporting the absorption, called microvilli, which contract
13:27
when they come in contact with nutrients. And between each Villi, we have these
13:36
glandular depression called intestinal glands, which contain cells that lubricates the lumen,
13:42
and produces some digestive enzymes. And on the inside of each villus, remember
13:47
this is still Tunica mucosa, right? We’re going to have some solitary lymph nodules, meaning small
13:53
single lymph nodules that’s going to be a part of the immunity. But in some places in the small
13:59
intestine, you’ll find lymph nodules aggregated together, forming aggregated lymph nodules.
14:06
Now when talking about the tunica mucosa anatomically, we need to mention another
14:10
structure at the Duodenum, and that’s the bile duct piercing the Tunica mucosa for secretion
14:16
of bile. Remember that one from earlier? So if we go ahead and open up the Duodenum
14:20
and zoom in a little bit, you’ll see an elevation called the major duodenal papilla. A papilla
14:27
is an elevation around an opening. SO the major duodena papilla borders the entrance
14:33
of the common bile duct and pancreatic duct into the Duodenum. This is where most parts of bile
14:40
and other enzymes that facilitate digestion are secreted out from. A little bit superior to that,
14:47
you’ll find the minor duodenal papilla, which is the opening for the accessory pancreatic duct
14:53
So if we take a quick look into this to get a general sense of how this is done, bile is
14:58
produced by the liver, and it’s sent down through the bile ducts, which later on meets up with the
15:03
pancreatic duct to become the hepatopancreatic duct. Which will then pierces through into the
15:04
Duodenum, which at this point is going to be bordered by the Major Duodenal Papilla.
15:10
The pancreas also has an accessory duct piercing through forming the minor duodenal papilla,
15:16
which you’ll find a little superior to the Major duodenal papilla.
15:20
But I’ll go through this in a little more detail when I go through the accessory organs.
15:25
So that was the Tunica mucosa. Now beneath the tunica mucosa, you’ll find the tela submucosa.
15:32
Now in the small intestine, the Duodenum, Ileum, and Jejunum display several histological
15:38
differences, and it’s easiest to differentiate them using the tela submucosa. So if we look at a
15:44
small section of the Duodenum, Duodenum is readily distinguished from other regions of the small
15:49
intestine by the presence of submucosal Brunner’s glands, which may pack the submucosa so completely
15:56
that the typical submucosal connective tissue is hard to find. These Brunner’s glands produce
16:02
a lot of alkaline mucus to neutralize the acid contents entering from the Stomach. The Jejunum
16:09
is the longest and most “typical” region of the small intestine. It doesn’t really have any
16:14
characteristics of its own, just that the villi are longer, as you see here. The Ileum, however,
16:20
is distinguished by two things. One, it has more goblet cells in the mucosa. Those are the cells
16:26
that produce mucus for lubrication. And two is that you’ll see that the Ileum also displays
16:33
an increase in the amount of mucosal lymphoid tissue, which forms clusters of lymph nodules,
16:39
called Peyer’s patches. These lymph nodules extend down to the tela submucosa, as you see here.
16:45
They have the same function as tonsils and are also called gut-associated lymphoid tissue,
16:50
or GALT. So that is the Tela Submucosa Then we have Tunica Muscularis.
16:56
Now the tunica muscularis has two parts. And you’ll notice that there’s always gonna be
17:01
two muscle fiber layers throughout the whole GI tract, except for certain parts like the Stomach.
17:07
The inner part is nearly always going to be circular muscle fibers, while the outer part
17:12
is nearly always longitudinal muscle. And they provide a process called peristalsis, which is the
17:18
continuous contraction moving food forward. Let’s look a little bit into that. First, contraction of
17:25
the circular muscle fibers will narrow the lumen, pushing food forward. Then, the longitudinal
17:31
muscle fibers will shorten the intestinal tube, and the circular muscle fibers will contract again
17:38
to push the food forward. That’s how the bolus or the food is moved within us.
17:44
But you’ll notice that the circular muscle fibers have one more function.
17:48
And that is that at some places throughout the GI tract, these circular muscle fibers
17:54
will function as a sphincter. And remember we had the major duodenal papilla here, where the
17:59
hepatopancreatic duct opens up? It’s controlled by a sphincter called the hepatopancreatic sphincter,
18:06
or also referred to as the sphincter of Oddi. And This sphincter is really important because
18:12
it controls how much bile and enzymes are going to enter the Duodenum of the small intestine.
18:17
All right, so now the last layer of the small intestine is the Tunica serosa. Remember,
18:24
I said that the peritoneum completely covers the small intestine by forming the mesentery?
18:29
The parts that are entirely covered by the peritoneum are highlighted for you right here;
18:34
we call them the intraperitoneal organs, mainly the Ilium and the Jejunum. But it also goes on
18:40
and grabs to Superior and the ascending part of the Duodenum, as you see here.
18:45
While the rest of the small intestine is highlighted for you right here. We call them
18:49
the retroperitoneal organs because the peritoneum only covers a part of them. That means that the
18:56
areas of the Duodenum that are not covered by the peritoneum have the tunica adventitia
19:01
instead of tunica serosa. Tunica adventitia is more of a stronger connective tissue covering
19:08
instead of the typical slippery fatty covering provided by the tunica serosa.
19:13
All right, so that was everything I had for the small intestine. I really hope you got a lot out
19:18
of this video. The next video is going to be about the large intestine, as you see here.