Pancreas in situ seen from the anterior view.
Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the pancreas in situ. So, today, we're going to be looking at the pancreas in situ which essentially means that we're going to be exploring this image that you see now on the screen which shows you an anterior view of the abdomen. So, we stripped a lot of the muscles, the organs, vessels to then be left with the pancreas here, and a lot of the structures – or some of the main structures – that are surrounding the pancreas. And we're going to be describing them and talking a little bit about them.
As you probably know and as I'm highlighting right now the pancreas in green, this is an accessory organ of the gastrointestinal tract and the endocrine system. It is a unique organ because it serves as two glands in one – so a digestive exocrine gland and a hormone-producing endocrine gland. This organ measuring approximately 15 cm in length is retroperitoneal except for the small part of its tail and lies mostly posterior to the stomach which we just removed here from this image now in what we call the duodenal loop and partially behind the omental bursa which you don't see here on this image because we removed these structures.
Now, the pancreas is both, as I mentioned, an endocrine and an exocrine gland that can be divided anatomically into a head and neck, body and tail which we will also talk about later on on this tutorial.
But before we do so, let's first take a look at the functions associated to the pancreas. As I previously said, the pancreas serves simultaneously as an endocrine and exocrine gland. Functioning as an exocrine gland, the pancreas will be secreting enzymes to the duodenum in order to break down proteins, lipids, carbohydrates and nucleic acids during digestion. Now, when it functions as an endocrine gland, the pancreas secretes hormones such as insulin and glucagon into the blood in order to control blood sugar levels throughout the day.
Before we continue, I would like to do a quick overview on the histology associated to the pancreas. The exocrine part which accounts for 80% of this organ has closely packed serous acini and secretes an enzyme-rich alkaline fluid into the duodenum via the pancreatic duct. The endocrine part accounts for 2% of the organ containing clusters of small islands of endocrine cells known as the islets of Langerhans which contain four types of cells that release various hormones including alpha cells which produce a very important hormone known as glucagon that increases the glucose concentration in your bloodstream.
Now, about 20% of the islets of Langerhans are comprised of alpha cells. Also what is known to be as the beta cells which are also storing and producing a very important hormone known as insulin and amylin. Now, insulin effectively decreases the glucose concentration in the bloodstream and amylin is secreted from these types of cells and this hormone helps to promote feeling of fullness as well as inhibiting the secretion of glucagon. Now, beta cells make up 65-80% of the islets of Langerhans. There are also delta cells – these somatostatin-producing cells make up 10% of the islets of Langerhans. Finally, the gamma cells which make up 5% of the islets of Langerhans, they produce pancreatic polypeptide. Now, these cells are also referred to as PP cells in some texts.
Now, before we continue on highlighting the different structures, I would like to make a quick clinical point or I just add a bit of clinical significance associated to the pancreas. Now, inflammation of pancreatic tissue will be resulting in a disorder known as pancreatitis. Now, this is a serious disease that can cause serious damage to the whole pancreatic parenchyma and can even lead to death. Now, other pancreatic diseases include pancreatic cancer, diabetes mellitus type 1 (a chronic autoimmune disorder), and diabetes mellitus type 2 which is insulin resistance and impaired insulin secretion that may develop as a result of both environmental and genetic factors.
Now, let's start by highlighting a few of the structures of the pancreas in situ and we start with this one here, this large structure that you can see – a lot of cuts here made – but to show you that we are seeing now highlighted the parietal peritoneum. Now, before we continue looking at the pancreas in situ and the surrounding structures, we first look at the parietal peritoneum which is this serous membrane seen here highlighted in green that forms the lining of the abdominal cavity. And as you can see in this illustration, the parietal peritoneum covers the pancreas. For this reason, the pancreas is what we call then retroperitoneal organ – so behind the peritoneum – as well as covering the retroperitoneal viscera, or retroperitoneal organs of the abdomen, the parietal peritoneum also provides support to those organs.
We are first looking into the different parts of the pancreas starting off with this one that you see now highlighted on this image, this is known as the head of the pancreas. So, as you remember from the beginning of this tutorial, I told you that the pancreas is then divided into four parts starting with this one that you see here. This one lies within the loop of the duodenum – as you can also see here – this loop of the duodenum.
The next part of the pancreas that we can also see here – now from a zoomed in image – we're looking at the uncinate process of the pancreas. Now, this one is found at the inferior end of the pancreas and this hook-like process passes posteriorly behind the superior mesenteric artery and vein. The other part of the pancreas that you can see now highlighted is known as the body. This is the largest portion of the pancreas and this part of this organ is found between the neck and the tail of the pancreas, mainly lies in front of the vertebral column. It has an anterior, posterior, and inferior surfaces as well.
The last part of the pancreas that we can also see here highlighted is known as the tail of the pancreas. This one is located in the upper left part of the pancreas. The tail of the pancreas comes into contact with the spleen which we removed here from this image but it should be right about here near the hilum which should be also here – the hilum of the spleen – and passes between the layers of the splenorenal ligament.
And before we leave the pancreas and see the surrounding structures, let's take a look at another important part here. We're now just focused here on the duodenum and we see here the pancreas and highlighting this structure to show you then the pancreatic duct. Now, the exocrine cells of the pancreas release their enzymes into a series of progressively larger tubes called then the ducts that are eventually joined together and form the main pancreatic duct, which is the structure highlighted in green. Now, we also see here this structure now to show you that most people have just one pancreatic duct; however, some have an additional duct that you can see now highlighted in green which is known as the accessory pancreatic duct or duct of Santorini. The main pancreatic duct joins the common bile duct and they form the hepatopancreatic ampulla or the ampulla of Vater which enters the descending or second part of the duodenum at the major duodenal papilla as you see in this illustration releasing then both the bile and the pancreatic enzymes into the duodenum. When there is an accessary pancreatic duct, this duct exits at the minor duodenal papilla, as you can see here in green, which is found in the descending part of the duodenum as well.
We are now going to be looking at the different surrounding structures and now that we have looked at the parts of the pancreas, we will focus now on the viscera – another structure seen proximal to and around the pancreas. We're going to start off with this first one that you see now highlighted in green which is the root of the transverse mesocolon – notice here this structure highlighted. Now, the transverse mesocolon – as the other mesenteries – is a peritoneal fold that attaches viscera to the posterior abdominal wall. And as we see in this illustration in which we see the root of the transverse mesocolon, this leaves the posterior abdominal wall across the anterior surface of the head, the body and, partially, the tail of the pancreas, and then pass outwards surrounding then the transverse colon attaching it to the posterior abdominal wall.
We can also see here on this other image – this next highlight – which shows then the root of the mesentery which is another important peritoneal fold is the mesentery that provides attachment for the intraperitoneal small intestines. The root of this membranous tissue – the root of the mesentery – extends from the posterior wall of the abdominal cavity from the duodenojejunal flexure to the right iliac fossa.
The next structure we can see here highlighted now in green – and note here this is the structure and this green – that we can also another structure here in green which is then the gallbladder. But the structure we're highlighting is then the hepatoduodenal ligament. This is another important peritoneal fold that can be seen here, and this ligament is part of the lesser omentum and is found between the superior part of the duodenum and the porta hepatis of the liver. Inside this ligament runs then the portal triad which is comprised of the hepatic artery proper, the hepatic portal vein, and the common hepatic duct. Next on our list, we're going – we can also see here from this view, still from the anterior view – the hepatoduodenal ligament highlighted in green, and you can see now how inside we find these structures that are part of the portal triad.
We're now going to take a look at the surrounding viscera, a few organs that we can find surrounding the pancreas and we're going to start off with this part here which is the superior part of the duodenum. Now, the duodenum is part of the small intestine that virtually encapsulates the pancreas at approximately the level of the first lumbar vertebra. We see the superior part of the duodenum which is as you can see from our illustration located proximal to the head of the pancreas. This part of the duodenum is located anterior to the head of the pancreas.
We also see here another part of the duodenum known as the descending part. Now, beginning just after the superior duodenal flexure, the descending part of the duodenum extends until the inferior duodenal flexure. This part of the duodenum is also located in front of the pancreas and, as we saw previously, this part of the duodenum contains both the major and minor duodenal papillae.
The next part of the duodenum that we see here highlighted is known as the horizontal part. So, immediately after the descending part of the duodenum, we see the third part of the duodenum which is the horizontal part which begins just after the inferior duodenal flexure and this part of the duodenum passes transversely to the left below the head of the pancreas and in front of the inferior vena cava, the abdominal aorta and the lumbar vertebral column.
The next part of the duodenum that we are going to highlight is known as the ascending part of the duodenum. Also lying in front of the head of the pancreas is then this part which begins just after the horizontal part of the duodenum and extends up to the duodenojejunal flexure. Now, this part of the duodenum is the fourth and final part of the duodenum found at approximately the level of L2. If we add here a bit more of the structures, we can then see the jejunum. So this is the continuation of the duodenum and this is the middle portion of the small intestine that begins after the duodenojejunal flexure extending then to the ileum. In this illustration, we see only the first part of the jejunum that is located proximal and anterior to the pancreas.
In close relation to the pancreas, we see then these structures here, these organs which are known as the kidneys. So, on the left image, we see the left kidney and, on the right image, we see the right kidney highlighted in green. Now, the kidneys are retroperitoneal organs and the left is located posterior to the tail of the pancreas and the spleen, while the right is partially covered by the liver. Although in this illustration, we removed the liver in order to see the right kidney clearly.
Now, the next structures we're also going to be highlighting here are known as the suprarenal glands. So, on the top of both kidneys, you see the two suprarenal glands and the left one is in close relation with the tail of the pancreas as you can see here on this image while the right one is closer to the body of the pancreas.
Another structure we can see here clearly – and we see just a part here highlighted – is the diaphragm and the part of the diaphragm that we're highlighting here is the hepatic surface of the diaphragm. In our illustration, the liver has been removed revealing the inferior surface of this part of the diaphragm that is in contact with the superior surface of the liver.
Moving on to another structure here where we're going to be highlighting the splenic recess. On the left side of the diaphragm, we find this structure which is the essentially a recess of the omental bursa in which the spleen is housed and that is known as the splenic recess. Now, as you can see in our illustration, the spleen has been removed to then allow better view of this extension of the omental bursa.
Another cut here that we can see and we're highlighting which is part of a very important structure known as the esophagus. In our illustration, you can see part of the esophagus which is essentially a fibromuscular tube extending from the pharynx all the way to your stomach and as it passes through the esophageal hiatus of the diaphragm into then the abdominal cavity.
On the next slides, we're going to be talking about the different blood vessels that we can find here or near the pancreas. Now, we're going to move on and talk about the first one here which is known as the left gastric artery and notice here the highlighted structure. The left gastric artery arises from the celiac trunk – so the celiac trunk is here and the green structure highlighted is then the left gastric artery – ascending in the left gastropancreatic fold. Now, it runs along the cardiac portion of the stomach continuing along and supplying the lesser curvature of the stomach – still see here the left gastric artery highlighted in green on this image but now you can see then the stomach as well. This artery also gives of branches that supply the lower part of the esophagus as you can see here on this image where you see some of the branches highlighted.
The next artery that we're going to be highlighting here is known as the splenic artery. So, this is another important branch of the celiac trunk. The splenic artery runs along the upper margin of the pancreas to the spleen and it is the third branch of the celiac trunk which you can clearly see now on this image. It is transmitted through the splenorenal ligament to supply then the structure here, the spleen. It also gives off several small branches to the pancreas.
The next artery that we're going to be highlighting here is known as the common hepatic artery and you see a little bit here so we wanted to also highlight and talk about it on this tutorial. Now, this artery supplies the liver as well as partially supplying the stomach, duodenum and pancreas. This artery, as I previously mentioned and as you can see in this illustration where we removed the pancreas, it then arises from the celiac trunk, as you can now see clearly on this image. It passes to the right side towards the liver and it divides into the gastroduodenal artery and the hepatic artery proper.
Another structure we're going to be highlighting here now, this is then the hepatic artery proper, also known as the proper hepatic artery. And, as we saw on the previous slide, this one arises from and is one of the two terminal branches of the common hepatic artery. This artery along with the hepatic portal vein and the common bile duct form the portal triad. Now, these three vessels that form then the portal triad – as you can also see here from this image now – run inside the hepatoduodenal ligament if you remember well and you can now see here also. We highlighted this structure and talked about it on a previous slide. The proper hepatic artery then will be bifurcating to give off the left and right hepatic arteries.
Another structure we can see now from this image from a bit more zoomed in and we see the hepatic portal vein. So, in close relation with the hepatic artery proper, we see this hepatic portal vein. Now, we already mentioned the hepatic portal vein a few times in this tutorial when referring to the portal triad. And as you may recall, this vein is one of the three vessels that make up the portal triad. The hepatic portal vein as you now see here from this view of the liver – we're still looking at an anterior view but now we just retracted the liver here – to show you how the hepatic portal vein is connected to then the liver.
Now, the hepatic portal vein is not a true vein as it conducts blood to the liver and not directly to the heart. Now, blood from the gastrointestinal tract – the GI tract that contains both nutrients and toxins from food – arrives through the hepatic portal vein to the liver which is responsible for filtering the blood.
The next structure we're going to be highlighting now is the hepatic duct, which is a vessel that transports liver secretions to the intestines. It is formed by the union of the right hepatic duct and the left hepatic duct, and this vessel joins with the cystic duct of the gallbladder to form the common bile duct which is one of the vessels of the portal triad that we saw earlier.
The next structure we're going to be highlighting here is now a vein – a very important one – known as the inferior vena cava. The inferior vena cava begins at the union of the right and left common iliac veins and then ascends upward along the posterior abdominal wall to the right of the aorta and opens into the right atrium of the heart.
The next structures we're going to be highlighting happen to be tributary veins that drain into the inferior vena cava. Now, these are known as the hepatic veins so tributaries that drain into the vena cava – the inferior vena cava to be more specific. Now, these short intrahepatic veins, they drain deoxygenated blood from the liver into the inferior vena cava and, for more on the hepatic veins, please check the tutorial about the posterior view of the liver on our website.
Another structure you can see here on this image is known as the abdominal aorta. We just see a little bit of the abdominal aorta which lie posterior and inferior to the pancreas. Now, the abdominal aorta is the portion of the aorta that extends from the aortic hiatus of the diaphragm which we cannot see here on this image to then its bifurcation which you see a bit here at the level of the fourth lumbar vertebra, L4. As you can see in the illustration, the pancreas is located anterior to the abdominal aorta. The abdominal aorta is the largest artery found in the abdomen and supplies much of the abdomen giving off then several branches.
On the next image, we're going to be highlighting a very important structure, artery – to be more specific, the superior mesenteric artery. Keep in mind, we added a few more structures here – you can see now the colon, see a bit more of the small intestine, and a lot more arteries here – to show you then this one that comes out of the aorta, this is known as the superior mesenteric artery. And as I said, this arises from the abdominal aorta just below the celiac trunk which we see here a bit superiorly. This is an unpaired artery that supplies the intestines as well as the pancreas. It passes from behind the pancreas to the uncinate process and I have here an image that you can see now a cut of the superior mesenteric artery highlighted in green to show its trajectory behind the pancreas to then the uncinate process and then is transmitted through the mesentery and the mesocolon giving off several branches.
Now, just next to the superior mesenteric artery we see now the vein, the superior mesenteric vein, which joins with the splenic vein to form the hepatic portal vein. It receives tributaries from veins that drain the pancreas, stomach, small intestine, large intestine and the appendix. This blood is then transported via the superior mesenteric vein to the liver and the superior mesenteric vein is then transmitted through the mesocolon, which we can also see here on this image, the cut of the superior mesenteric vein. So, the superior mesenteric vein is transmitted through the mesocolon and passes behind the pancreas to join the splenic vein.
On other important branches of the abdominal aorta, we have here the renal arteries but now just highlighting the left one – the left renal artery. The renal arteries are paired arteries that arise from the abdominal aorta in front of the first lumbar vertebra – L1 – just below the superior mesenteric artery. They divide into several branches which supply the left and right kidneys. When we look at the pancreas in situ, we see the left renal artery – here on both these images, you see the highlight here of the left renal artery reaching then the left kidney and you also see here even though we added the pancreas but you see a bit of more of the relation with the left renal artery to the pancreas – still see the highlights here. You see that the left renal artery runs behind the body of the pancreas, the left renal vein and the splenic vein.
Now, very close to the renal arteries, we find then the renal veins and we're now highlighting the left renal vein to be more specific which follow a very similar course to the renal arteries. Now, the renal veins, they are paired veins and they drain deoxygenated blood from the left and right kidneys into the inferior vena cava as you can clearly see here on this image. This is the inferior vena cava. Notice how the left renal vein is connecting to the inferior vena cava and also the right renal vein is connecting to the inferior vena cava. Due to the position of the inferior vena cava of the right side of the abdomen, the left renal vein receives blood from the left gonadal vein – so ovarian and testicular. It also receives from the left suprarenal vein and the left inferior phrenic vein before it opens into the inferior vena cava.
The next structure we're going to be highlighting here is now an artery – now highlighted in green – which is the left colic artery. The left colic artery passes retroperitoneally to supply descending colon and it is a branch of the inferior mesenteric artery and its cell divides into the ascending and descending branch. You can also see it here on this image. And next, we're going to be highlighting then the left colic vein, which drains blood from the descending colon into the inferior mesenteric vein, and from there, blood is drained into the splenic vein which joins the superior mesenteric vein to form the hepatic portal vein.
Another structure we can see here – so before we finish this tutorial, I would like to show you the parts of the large intestine that we can see here on this image. Now, earlier on this tutorial, we looked at the transverse mesocolon which is the suspensory ligament of the transverse colon, and here we see a portion of the transverse colon with the middle section cut to show the underlying organs. The transverse colon is the horizontal section of the large intestine that extends from the right colic flexure or hepatic flexure which you see here highlighted in green on this image – to the left colic flexure or the splenic flexure which you see here as well. As you can see in our illustration, the intraperitoneal part of the large intestine is located anterior to the head of the pancreas as well as the duodenum and some convolutions of the jejunum and the ileum. It is also known as the splenic flexure due to its proximity to the spleen.
Finally, we're going to be highlighting here what is known to be as the descending colon which begins just after the left colic flexure. This part of the large intestine is located in the retroperitoneum that extends from the left colic or splenic flexure down to the sigmoid colon. It functions to store remaining nutrients and vitamins of digested food before transporting feces into the rectum.