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Structure of the duodenum, including the mucosa and muscular layers.
Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the duodenum. So, essentially what we we’re going to be discussing here on this tutorial is the first part of – or the first segment – of the small intestine which you see here on this image of the anterior view of the abdomen where we removed a few structures including the stomach so you can see then see clearly here this segment that we're going to be exploring. We're also going to be talking about a few structures that you see here around or nearby the duodenum so you can understand what's happening.
Now, the first thing I want to say is that – as I mentioned – this is the first of the three segments of the small intestine and this initial portion of the small intestine is located between the stomach and the jejunum – the jejunum being then the other part or the continuation of the small intestine. This part measures about 20 to 25 centimeters in length and, in terms of functions, the main functions of the duodenum include then neutralization of acidic gastric juices, also mechanical processing and digestion of chyme, absorption of water and electrolyte and also nutrients, and mixing of bile and pancreatic enzymes to aid then in digestive – in the digestive – process.
Just to remind you here on this image, this is the image that we're going to mainly focus on this tutorial where you can then see this segment, the duodenum. We're going to be looking at the different structures of the duodenum. You can also see that it's cut here so we can see what is happening here on the mucosa. You also see here then a few layers of muscles that we're going to be looking at and a few other structures here that we're going to be highlighting including an organ here – the pancreas – that you see a part of.
Now, in this tutorial, we will be then looking at the parts and segments of the duodenum including the mucosa, the muscle layers as we were talking about and then also the related arteries, veins and surrounding viscera or organs. But first let's begin at the point where the duodenum begins here at this highlight, this is known as the pylorus. Now, the pylorus is the distal end of the stomach as you can also see here on this view – on this image – where you can see here the stomach and the pylorus highlighted and here is where then you'll find the duodenum. So, as I said, this is the distal end of the stomach as you can see here also on the illustration and it connects to the stomach to the duodenum. This part contains a thickened ring of gastric circular muscle known as the pyloric sphincter which you can you also see clearly here on this image. So, on this image here on the bottom, you can see not only a cut of the stomach and a bit of the duodenum here but we can show you clearly here highlighted, the pyloric sphincter, which also marks the entrance to the superior part of the duodenum.
Well, now, we're going to be talking about this highlight here going back to the image of the anterior view of the abdomen to show you then the superior part of the duodenum. Now, this one is the first part of the duodenum beginning just after the pylorus as we mentioned before and extending up to the superior duodenal flexure, which is a structure we're going to be highlighting and talking about next. Now, in this first part of the duodenum, there is a functional dilation called the ampulla which you can also see here clearly highlighted in green and also known as the duodenal cap. Now, this is the region where most duodenal ulcers occur. The superior part of the duodenum is located intraperitoneally and is attached to the liver via by the hepatoduodenal ligament which you see here highlighted in green – notice here, the hepatoduodenal ligament. This other green structure is then the gallbladder but you can see how the duodenum – part here of the duodenum – is then attached to the liver – this organ here – by or through this hepatoduodenal ligament.
The next structure we're going to be talking about I mentioned before, this is known as the superior duodenal flexure. This one marks the end of the superior part of the duodenum, and this flexure is located between the superior part of the duodenum and also the descending part which you see here on the image as well – this is the descending part – and this is found just medial to the gallbladder – so the superior duodenal flexure is found medial to the gallbladder.
Now, just speaking of the next structure that I just mentioned before, this is then the descending part of the duodenum which begins immediately after the superior duodenal flexure, and it is known as the descending part of the duodenum. Now, this part of the duodenum is located between the superior duodenal flexure which we have just looked at in the previous slides and also the inferior duodenal flexure which we will highlight on the next slides. You can also see here the descending part of the duodenum highlighted here on this image of the anterior view of the abdomen. Now, a reminder here that on the inside of the descending part of the duodenum, there are two papillae or mucosal elevations which you also see here on this image – notice here. Now, these are known as the minor duodenal papilla and the major duodenal papilla which we will also be highlighting – you see here the minor and the major one.
And speaking of which, the next structure we're going to be highlighting here is known as the minor duodenal papilla. This is an opening for the accessory pancreatic duct which we see here highlighted in green in this image. Now, this small opening is seen as an elevation in the mucosa of the descending part of the duodenum and is located superior to this one here, the major duodenal papilla – so the minor and the major duodenal papillae.
I'd like to focus a little bit more on the next structure here highlighted that I just mentioned before, this one being the major duodenal papilla just located inferiorly to the minor duodenal papilla. Now, this one is the opening for the pancreatic duct into the lumen of the descending part of the duodenum. It is the place where the main pancreatic duct and the bile duct join together to form the hepatopancreatic ampulla, also known as the ampulla of Vater. Now, pancreatic enzymes and bile enter the duodenum from the ampulla of Vater through this opening in order to then help digestion. Note that the ampulla of Vater is surrounded by the sphincter of Oddi. And, on this image here, we have then the pancreatic duct highlighted in green.
Going back to the parts of the duodenum, the next structure we see is then the inferior duodenal flexure which is a flexure or turn of – it's like a turn that you see here – of the duodenum that is located between the descending part of the duodenum and also the horizontal part which is this part here that we're going to be talking about next.
So, the next part that I'm going to be highlighting here is then the horizontal part of the duodenum which is the third segment of the duodenum that begins just after the inferior duodenal flexure as I mentioned before and extends to the ascending part of the duodenum which is this part here. It is the longest section of the duodenum and it passes transversely to the left in front of the inferior vena cava, also in front of the abdominal aorta and the vertebral column, and inferior to the head of the pancreas which you see a bit here on this image.
Now anteriorly, it is crossed by the superior mesenteric artery and vein as you can also see here on this illustration but these two vessels are cut but you can see that it's crossed by the superior mesenteric artery and also vein – as you can see here. You can also see here on this image how it is located in front of the inferior vena cava – this is the inferior vena cava – and the abdominal aorta is a bit hidden here on this image but you could see more or less located about here and the vertebral column is, of course, all the way in the back.
The next part of the duodenum that we're going to be highlighting – one that I mentioned before – the ascending part of the duodenum. This is the fourth and final segment of the duodenum and this part of the duodenum passes upward to the left of the aorta extending from the horizontal part of the duodenum up to the duodenojejunal flexure which is this part here. We can also see here the ascending part of the duodenum highlighted in green from the anterior view of the abdomen.
The next structure we're going to be highlighting – one that I mentioned before – this is then the duodenojejunal flexure. And, as the name suggests, this structure is a flexure located at the junction between the duodenum and also the jejunum that you can see a little bit here, and the jejunum is also a part – or continuation – of the small intestine. It is surrounded by a fold of peritoneum containing muscle fibers called the suspensory muscle or ligament of the duodenum. It is also known as the ligament of Treitz.
The next structure we're going to be highlighting that I just mentioned before, this is the jejunum, which is the second portion of the small intestine and begins after the duodenojejunal flexure. Now being a part of the small intestine and is located proximal to the duodenum, we mentioned this structure only in reference to the duodenum in this tutorial. We will look at the jejunum and the rest of the small intestine in greater detail on a separate tutorial.
Let's continue on with our discussion into the duodenum and moving on into the different layers that compose the wall of the duodenum known as the circular folds. Now, the structure of the wall of the duodenum is similar to that of other hollow organs of the GI tract. Now, it is composed of a mucosa, a submucosa, and a muscular layer. When we look at the internal surface of the duodenum, we see these folds – these circular folds – which you can see here then highlighted in green. It will also remind you of like ridges. Now, these folds of mucosa and submucosa are also known as the valves of Kerckring? (11:51). They are covered in small fingerlike projections called villi and the circular folds together with the villi increase then the surface area for then absorption in the duodenum, up to 1500 times.
The next layer that we see here highlighted in green immediately beneath the mucosa is the submucosa. Now, this structure is known as the tela submucosa, to be more precise. It is a dense, irregular layer of connective tissue that supports the mucosa and this layer consists mainly of collagenous connective tissue containing nerves and blood vessels. Of course in our illustration, you see a macroscopic view of the submucosa, rather than a microscopic view, so you cannot see the cells or the fine structure of the submucosa here but we will explore on a separate tutorial later on when we start dealing with histology.
Now, the muscular layer which is the third and final layer of the duodenal wall consists of a circular layer and also an outer longitudinal layer. The circular muscle layer is the first or innermost layer of muscle fibers found in the muscularis externa, the layer of then the gastrointestinal tract. Now, these circularly running muscle fibers are comprised of smooth muscle fibers.
The next structure – the next part that we're going to be highlighting – is then the longitudinal layer of the muscular coat of the duodenum. Now, seeing as we have just mentioned that the circular muscle layer is the inner layer of the muscularis externa, it follows that the longitudinal layer of the muscle fibers is then the outer layer of muscle fibers. Now, these longitudinal running smooth muscle fibers that are found in then muscularis externa layer of the gastrointestinal tract may vary in thickness.
So far we have covered the parts of the duodenum as well as the histological layers that comprise the wall of the duodenum. Now, let's move on to look at the related structures starting with the very prominent and very important blood vessel known as the abdominal aorta. Now, this blood vessel is found in and also supplies a large portion of the abdomen. As you know, the abdominal aorta is a continuation of the thoracic aorta that begins at its entrance into the abdomen through the abdominal hiatus of the diaphragm and extends to its bifurcation at the level of L4. And here on this image, we see a small portion of the abdominal aorta seen highlighted and is coursing down along the posterior wall of the abdominal cavity with the duodenum and the pancreas located anterior to it.
Another vessel that we can see here highlighted now in green is known as the common hepatic artery, which arises from the celiac trunk of the abdominal aorta and this artery supplies oxygenated blood to the liver as well as to the pylorus, pancreas and the duodenum. From the hepatic artery, we see then this one arising known as the hepatic artery proper – also an important artery – and this one is coursing along with the hepatic portal vein and the common bile duct forming then what is known as the portal triad. This artery gives off branches that supply the liver and the gallbladder, and for more on the hepatic portal vein, please look at other tutorials on the liver where we're going to be exploring it in a little bit more detail.
Now, moving on, we're going to be talking another – about another important – artery that arises from the abdominal aorta, and this time, the superior mesenteric artery as seen here highlighted in green. As I just mentioned, it arises from the abdominal aorta immediately below the celiac trunk is then this one, the superior mesenteric artery. We can see here on this image how this artery is then coursing alongside its counterpart, the vein – the superior mesenteric vein – then it will cross – as I mentioned before – in front of the horizontal part of the duodenum.
On the next image here still on the superior mesenteric artery seen still here highlighted in green, you can see a few other structures I added here including the small intestine and also a bit of the large intestine, the abdominal aorta, the duodenum as we're talking about to show you that this structure will be supplying the intestines from the inferior to major duodenal papilla up to the junction between the proximal 2/3 and distal 1/3 of the transverse colon but also be supplying the pancreas.
Next, we're going to go back to this image to show you this structure that we already mentioned, this is then the superior mesenteric vein. It courses alongside the superior mesenteric artery as I mentioned before and this vein that drains blood from the small intestine, large intestine, appendix, stomach and also the pancreas joins with the splenic vein to form then the hepatic portal vein, which happens to be the next structure that we see here and we're going to be highlighting now, the hepatic portal vein.
This one forms then the portal triad – is part of the portal triad – and is the final common pathway for the transportation of venous blood from the spleen, pancreas, gallbladder and the abdominal part of the gastrointestinal tract to the liver. It is formed by the union of the splenic vein and the superior mesenteric vein just posterior to the neck of the pancreas. Please note that this vein does not drain into the inferior vena cava.
The next structure we're going to be highlighting now, we see here is then the common bile duct. This is the third member of the portal triad. This vessel which is formed by the union of the common hepatic duct and the cystic duct functions to conduct or transport bile stored in the gallbladder to the duodenum. Now, here on this image, you can see the common bile duct in a little bit more and you also see here a cut of the gallbladder and how all these structures are connected all the way to the liver that is seen here retracted. Now, the common bile duct is joined by the pancreatic duct to form then the ampulla of Vater.
Going back to this image here, we can see another highlighted structure which is the pancreatic duct – also known as the duct of Wirsung – supplies the pancreatic juices from the pancreas to the duodenum. And as we saw earlier, this duct joins with the common bile duct to form the ampulla of Vater and perforating the descending part of the duodenum at the major duodenal papilla as you can see here on this image. Now, notice here the major duodenal papilla.
The next structure we're going to be highlighting is known as the accessory pancreatic duct which we saw already on the previous slides. This one connects the pancreas to the descending part of the duodenum opening into the minor duodenal papilla which you see here. This duct is also known as the duct of Santorini and may also not be present.
The next structure that we clearly see here – and I mentioned also before – this is the pancreas. So, before we come to the end of this tutorial, I'd like to just briefly talk about the pancreas. Now this organ measures approximately 15 centimeters in length, is found retroperitoneally except for a small part of its tail. Now, it lies most posterior to the stomach in the duodenal loop and partially behind the omental bursa. The pancreas is both an endocrine and an exocrine gland that can be divided anatomically into a head, neck, body and tail. On this image, we mostly see here the tail a bit hidden but you can see a bit of the head, neck and then the body. Functioning as an exocrine gland, the pancreas will be then secreting enzymes into the duodenum in order to breakdown proteins, lipids, carbohydrates, and nucleic acids in food during digestion. When it functions as an endocrine gland, the pancreas will be secreting insulin and glucagon into the bloodstream in order to control blood sugar levels throughout your day.
Now, for the purpose of this tutorial, I will not go into much depth about the pancreas so we're going to be focusing on it on a separate tutorial where we're going to then talk about the pancreas in greater detail. But, before I do so, I would like to just highlight here one of the structures that I mentioned before, the head of the pancreas. So, as we're looking at the relation of the pancreas to the duodenum, the final structure we will look at in this tutorial is the head of the pancreas. Now, as you can see from our illustration here, the head of the pancreas is lodged in the curve of the duodenum coming into contact with the superior, descending, horizontal, and ascending parts of the duodenum.
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Now, good luck everyone, and I will see you next time.