Anatomy of the Small Intestine (Nursing)

by Darren Salmi, MD, MS

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    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.

    About the Lecture

    The lecture Anatomy of the Small Intestine (Nursing) by Darren Salmi, MD, MS is from the course Anatomy of the Gastrointestinal System (Nursing).

    Included Quiz Questions

    1. Four
    2. Three
    3. Two
    4. One
    5. Five
    1. Descending part of the duodenum
    2. Ascending part of the duodenum
    3. Ascending part of the jejunum
    4. Descending part of the jejunum
    5. Pylorus
    1. Duodenum
    2. Jejunum
    3. Ileum
    4. Cecum
    5. Ascending colon
    1. The ileum is narrower.
    2. The ileum has very little mesenteric fat.
    3. The ileum is thicker.
    4. The ileum is shorter.
    5. The ileum is in the abdomen.
    1. The jejunum is longer than the ileum.
    2. The ileum has numerous arcades.
    3. The jejunum has prominent folds.
    4. The jejunum has little mesenteric fat.
    5. The jejunum is wider than the ileum.

    Author of lecture Anatomy of the Small Intestine (Nursing)

     Darren Salmi, MD, MS

    Darren Salmi, MD, MS

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