Exploring anatomy: the human

An advanced look at the portal system

Welcome to this video for exploring anatomy, the human abdomen. This video is going to outline the portal system. So in order to do that, we first of all need to draw out various organs of the gastrointestinal tract. So here we can see we've got the liver. And important, as it receives the hepatic portal for metabolising the ingested products from the gastrointestinal tract. We also need to draw out various components of the gastrointestinal tract, so organs within the foregut, the midgut, and the hindgut. So here, I'm just drawing out a representation of the stomach, which obviously forms part of the foregut. We can also add in the spleen. This organ is also within the foregut. And then we can start looking at organs of the midgut. So we can detail the jejunum and the ileum . The jejunum, which is then continuous with the ileum, that forms the ileocaecal junction. So we can draw the caecum with the appendix attached. And then the ascending colon, the ascending colon being continuous with the transverse colon. So we can draw these structures in place. And these are all part of the midgut. And then, finally, we can draw the components of the hindgut, so the last third of the transverse colon, the , the sigmoid colon, and also part of the . So we can draw these in here. And this is part of the hindgut.

So all of these organs are going to give rise to various that drain the foregut, the midgut, and the hindgut and form the hepatic portal vein. So first of all if we look at how blood is draining from the midgut, then blood, venous blood, from the midgut is going to drain into the superior mesenteric vein. The superior mesenteric vein is going to run within the mesentery. And it always runs to the righthand side of the superior mesenteric . So here we have the superior mesenteric vein. And this is draining the midgut. Coming from the hindgut, we're going to have numerous veins that are draining from the rectum. So the superior rectal veins, maybe some sigmoidal veins, maybe the , all draining various aspects of the hindgut. And these branches drain into the inferior mesenteric vein. The inferior mesenteric vein is then-- I'll draw it in here-- going to drain into a large vein that's coming away from the spleen. And this is the splenic vein. It runs posterior to the pancreas. And we can see here the splenic vein is receiving the inferior mesenteric vein. So as the splenic vein runs posterior to the pancreas, it receives the inferior mesenteric vein. So blood from the inferior mesenteric vein is coming up in this direction. Blood from the splenic vein is coming across in this direction. And the inferior mesenteric vein drains into the splenic vein. Or at least it does this in the majority of people. In some people, the inferior mesenteric vein actually drains into the superior mesenteric vein. The superior mesenteric vein unites with the splenic vein posterior to the neck of the pancreas. And by doing this, it forms the hepatic portal vein. So the hepatic portal vein is formed posterior to the neck of the pancreas by the union of the splenic vein and the superior mesenteric vein. So blood coming up in this direction from the midgut meets blood from the splenic vein in this direction. We then have the hepatic portal vein that is going to head towards the liver.

The hepatic portal vein may receive a few branches from the stomach. So it may receive a few branches from the stomach, so maybe the left gastric vein. And there might also be a right gastric vein draining into the hepatic portal vein as well. So we can draw these various gastric veins that drain directly into the hepatic portal vein. The hepatic portal vein then continues. And it passes to the liver. As it passes to the liver, it gives rise to two portal veins, which we can see here. And they go to supply the left and right functional lobes of the liver. Once the blood is passed through the liver, once the products that have been absorbed from the gastrointestinal tract have been metabolised, then they pass into the inferior vena cava by way of a series of hepatic veins. And we have three hepatic veins that drain into the inferior vena cava. We have the left, middle, and right hepatic veins. Do not get these confused with the hepatic portal vein. The left, middle, and right hepatic veins drain blood away from the liver into the inferior vena cava. So they're returning it to the systemic circulation. So here we can see a basic outline of the portal system. All blood that's passed from the gastrointestinal tract is going to drain into the hepatic portal vein, which then passes towards the liver. If a patient is suffering from liver disease, like cirrhosis of the liver, this can prevent the normal blood flow through the liver. This leads to portal hypertension, as the pressure within the portal system increases. When this occurs, blood can be diverted back into the systemic circulation by a series of portal systemic anastomoses. And there's a few of these I want to detail. So we have the azygos system of veins. And this sits in the posterior mediastinum. And this azygos system of veins can receive blood from oesophageal veins and the left gastric veins-- the azygos system. And this is part of the systemic circulation. So during periods of portal hypertension, instead of blood passing through these gastric veins into the hepatic portal vein, blood can actually go in the opposite direction and it can pass into these azygos veins, as part of the azygos system. So here, we have one of our portal systemic anastomoses. A further portal systemic anastomosis is the paraumblical veins that are running within the falciform ligament. And the falciform ligament, contained in the ligament antares, passes towards the anterior abdominal wall at the level of the umbilicus. So here we have the paraumbilical veins.

Here, blood can now pass away from the hepatic portal vein and enter the epigastric veins that are running throughout the anterior abdominal wall. So here we have some epigastric veins that are running on the anterior abdominal wall. Now, blood can pass into these veins and they drain back into the inferior vena cava. So once again, going back into the systemic circulation. Another portal systemic anastomoses occurs between the superior rectal vein-- which we can have here, the superior rectal veins-- and the anastomosis this forms with the middle and the . So here we can draw the middle and the inferior rectal veins. And these drain into the internal iliac, which again will run into the inferior vena cava. So here we can see another-- I forgot to add the circle in this one. I'll just do it now. And here we can see another one of those portal systemic anastomoses. Blood is now going to pass away from the hepatic portal vein, away from the inferior mesenteric vein. And it's going to run towards the internal iliac. There's a further series of portal systemic anastomoses. And this is where blood drains into a series of retroperitoneal veins. Retroperitoneal veins are lying obviously retroperitoneal. And they drain into the , which ultimately drain into the inferior vena cava. So here we have a series of portal systemic anastomoses that enables blood to re-enter the systemic system if blood flow through the liver is impeded. [end of transcript]