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Structure and function of the hepatic portal vein.
Hello everyone! This is Nicole from Kenhub, and today we're going to talk about the hepatic portal vein.
Today in this video we are going to discuss the hepatic portal vein and to do so we'll be using this image here which is a ventral view of the portal hepatic vein with the central portion of the liver cut out so we can see the portal vein and other portal vessels. You can also see the aorta just here as well as the inferior vena cava just posterior to the portal hepatic vein. The portal venous system is an important system that has its own unique flow and we'll talk about how this works in tandem with the venous system in the coming slides.
First let's have a little bit of a chat about the function of the hepatic portal vein. The portal vein is one of the most important vessels in the body. Its main functions are to direct blood to the liver from the gastrointestinal tract and receive nutrient rich blood from the intestines. The portal hepatic vein also receives blood from the spleen, the pancreas and the gallbladder which are channels within the vessel to the liver. Once inside the liver, these blood can be filtered and processed while also being cleansed of bacteria and toxins in a process called detoxification.
Before we move on to talk about the different components of the portal vein, however, I want to have a brief chat about the portal venous system overall and, as we mentioned, the portal venous system is a unique system that involves the movement of blood from the gastrointestinal tract to the liver. The process which involves the liver as a processing station looks a little bit like the cycle below. Veins carrying nutrient-rich blood from the gastrointestinal tract such as the superior mesenteric vein and the splenic vein which then carry blood to the portal vein itself and then through the portal triad which is a triad of structures found in the porta hepatis. Once in the liver, the blood is filtered of bacteria and toxins which are eliminated through bile or urine where the filtered blood is sent back to the inferior vena cava.
As we discuss the portal vein, we'll talk about the structures associated with the portal vein as we come to it in order of this particular flow; so firstly, the portal vein structure and location including some associated structures, the formation of the portal vein and the portal vein as it connects to the liver. But, first, let's begin with the portal vein structure and location.
As you can see in this image, the hepatic portal vein – highlighted in green – can be found in the upper right quadrant of the abdomen. Although this is not visible in this image, the portal vein is valveless and generally reaches a length of 8 centimeters or 3 inches in adults. The portal vein extends obliquely to the liver behind the duodenum. As it descends, it runs within the right free border of the lesser omentum along with two other structures – the hepatic artery proper and the common bile duct – to form a structure known as the portal triad which we'll talk a little bit more about a bit later in the tutorial.
Although this structure is not visible in our main image of the portal vein, I wanted to mention the lesser omentum which in this image is highlighted in green. The lesser omentum is a double-layered band of peritoneum which extends from the liver to the lesser curvature of the stomach and the first part of the duodenum. The nutrient-rich blood of the hepatic portal vein runs with the lesser omentum as it travels towards the liver. As you can see in this image, the lesser omentum has two ligamentous parts – the hepatogastric ligament highlighted in green and the hepatoduodenal ligament again highlighted in green. You'll notice in this image, you can see that the lesser omentum has a free border. This border is a component of the hepatoduodenal ligament and houses the portal triad.
Now that we've looked at the structure and location of the portal vein, let's look at how the portal vein is formed. Like how the inferior vena cava is formed by the convergence of the right and left common iliac veins, the portal vein is also formed by several vessels. While there is some variation between individuals, the hepatic portal vein is usually formed by the convergence of the superior mesenteric vein and the splenic vein. This confluence is often referred to as the splenic-mesenteric confluence. In turn, the superior mesenteric vein which you can see highlighted in green on the right receives the pancreaticoduodenal veins and the gastroepiploic veins while the splenic vein which you can now see highlighted in green on the right receives the inferior mesenteric vein which you can see pointed out by my arrow just here.
Let's have a closer look at all of these veins over the next few slides. First, we'll start with one of the veins that form the portal vein – the superior mesenteric vein – and we can see this highlighted in green. The superior mesenteric vein ascends close to the superior mesenteric artery which I'm pointing out with my arrow running anterior to the ureter and uncinate process of the pancreas – neither of which are visible in this image – to then dive deep to the neck of the pancreas joining the splenic vein at the level of the L1 vertebra around about here. The superior mesenteric vein drains blood from several structures – the small intestine, the stomach, the pancreas, the cecum and also the ascending and transverse colons. Blood draining into this vessel is from the intestine and is nutrient-rich as food that has been broken into large molecules is passed through the small intestine and further broken down into smaller molecules. This allows for the small nutrients to be absorbed into the blood through the luminal wall of the jejunum and ileum. This blood then travels to the liver via the superior mesenteric vein towards the portal vein then to the liver.
If you remember our list from a couple of slides ago, the superior mesenteric vein receives two further veins – the first of which is the pancreaticoduodenal vein which you can see highlighted in green running along the anterior curvature of the duodenum. And as you can tell from their name, the pancreaticoduodenal vein drains venous blood from the pancreas and the duodenum.
The superior mesenteric vein also receives venous blood from the gastroepiploic veins although in this image the stomach is cut out, you can see the right gastroepiploic vein which provides most of the drainage to the superior mesenteric vein just here curving along where the greater curvature would be to create an anastomosis with the left gastroepiploic vein over here which mostly provides drainage to the splenic vein. And as you can tell from their name, the blood comes from the stomach.
The second vein to form the portal vein is, of course, the splenic vein. The splenic vein runs medially below the torturous splenic artery which is not visible in this image and posterior to the pancreas whose left portion you can see just sere. The splenic vein receives venous blood from the stomach, the pancreas, the spleen and the large intestine. The drainage of the splenic vein is important to note as obstruction to the splenic vein or hepatic portal vein leads to a reversal of venous flow and can result in splenomegaly which is enlargement of the spleen.
The inferior mesenteric vein drains into the splenic vein and terminates along its course. The inferior mesenteric vein drains venous blood from the abdominal hindgut structures such as the rectum, the sigmoid colon, the descending colon and the distal transverse colon.
Now that we've looked at the formation of the portal vein, let's look at how the portal vein connects to the liver. The portal hepatis – as you can see in green in this image – is a deep fissure found within the inferior aspect of the liver. The porta hepatis is significant for being the site where the portal triad is located. The portal triad which we spoke about at the beginning of this tutorial is a collection of three closely related structures – the hepatic portal vein, the hepatic artery proper and the common bile duct. The portal triad can also contain lymphatics and branches of the vagus nerve.
The position of the portal triad is also significant because its location in the hepatoduodenal ligament also makes it the anterior border of the epiploic foramen otherwise known as the foramen of Winslow. In this ventral image of the internal organs with the liver pulled back and the gall bladder highlighted in green, you can see the fissure of the porta hepatis over here as the portal triad within the hepatoduodenal ligament creates the foramen of Winslow as we just described. This foramen is significant because it is the entrance to the lesser sac of the foramen.
As we saw on the overview slide, once this nutrient-rich blood enters the liver which is highlighted in green in this image, the liver can then perform its job of nutrient storage and cleansing of toxins. Once done, this blood then drains directly into the inferior vena cava which as we mentioned before lies along the posterior border of the liver. The blood is then transported back to the heart. And here's another view of the inferior vena cava – a ventral view of the abdomen with parts of the large intestine and small intestine dissected away – just to reiterate its position. You can see the inferior vena cava highlighted in green and its position posterior to the liver.
So now that we've looked at the relevant clinical anatomy of the hepatic portal vein, let's discuss some clinical notes.
Like regular hypertension, portal hypertension is an increase in blood pressure, however, rather than being systematic, it's localized to the portal system. Portal hypertension is most commonly caused by liver cirrhosis which in itself can be caused by alcoholism or other liver disease. It can also be caused by blood clots in the portal vein and schistosomiasis amongst other things. This increase in blood pressure can affect areas of anastomosis between the portal vasculature which we just discussed and the caval musculature which are classified as the vessels not relating to the portal system resulting in pressure pushing larger blood volumes into these anastomotic areas. This in turn can cause the vessels to dilate and form varicose veins which can result in potentially fatal hemorrhage. Some of these important porto-caval anastomotic areas are listed below – the first vein being the portal vein and the second vein being the caval vein – the superior rectal and inferior rectal veins, the left gastric and esophageal veins, the colonic veins and the retroperitoneal veins and the para-umbilical and epigastric veins.
In severe cases, the last anastomosis mentioned between the para-umbilical veins which are the small veins that run within the round ligament of the liver and the epigastric veins which are found in the anterior abdominal wall can form large dilations. These dilations can form the clinical presentation caput medusa or the head of the medusa as the dilated veins look like the snakes of the head of the medusa or Gorgon from Greek mythology. In this image on the right, we can only see the beginnings of a presentation of the caput medusa as in a true caput medusa, the veins would be raised and enlarged.
During some trauma situations concerning the liver, a technique called the Pringle manoeuvre may be used to gain rapid control of bleeding. As you can see in the image on the right, this technique involves clamping the hepatoduodenal ligament thereby restricting movement of blood and bile through the structures of the portal triad.
That brings us to the end of another Kenhub tutorial. In our summary, let's go over what we discussed today.
So of course we talked about the portal vein's function which is mainly to transport blood from the gastrointestinal tract of the liver, its structure and location as well as the associated structures of the lesser omentum and the portal triad. The formation of the hepatic portal vein which includes the superior mesenteric vein which receives blood supply from the pancreaticoduodenal veins and the gastroepiploic veins as well as the splenic c vein which receives blood from the inferior mesenteric vein. We talked about the portal vein's connection to the liver via the porta hepatis and the ensuing flow of the flood from the liver to the IVC. And we also talked about some clinical correlations including portal hypertension which results in caput medusa and the Pringle manoeuvre.
And that's all for today! Thanks for watching.
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