Video: Veins of the small intestine

When we think of the circulatory system, we always tend to think of the most common pattern of blood flow around our bodies. Oxygenated blood flows from the heart through arteries which terminate as capillaries – the smallest blood vessels found in our tissues and organs where nutrient and gaseous exchange takes place. The vessel networks formed within our organs and tissues are known as capillary beds. Blood drained from these capillary beds carries deoxygenated blood and drains into veins which return the blood back to the heart. Very straightforward, right? But did you know that not all blood circulation follows this pattern? Deoxygenated blood drained from certain organs is sometimes drained through what's known as the portal system into a second capillary bed before being returned to the heart. Where does this happen and why? Well, let's find out more as we explore the veins of the small intestine.

Here we're looking at the abdominal cavity from the front. The small intestine is the longest part of the alimentary canal and is responsible for digestion of food and the absorption of nutrients. The alimentary canal starts from our oral cavity and ends at the external anal sphincter. The small intestine makes up the majority of the alimentary canal and is composed of the duodenum, which is the small highlighted structure here. Distal to the duodenum is the jejunum, and after that. we have the ileum, which is where the small intestine ends.

The venous system of the small intestine will mostly follow the arterial system of the small intestine but all of the blood has to return via the liver so that the nutrients can be processed. This means the larger veins aren't paired with arteries that come from the aorta because they go straight into the liver via the portal venous system which we'll talk about first off.

So over here, we've got a schematic diagram of the portal venous system highlighted in green, and this special venous system connects capillaries of the intestines and other organs to the hepatic sinusoids. The hepatic portal system plays an important role in delivering nutrient-rich blood to the liver which will then cleanse it of toxins and remove the nutrients which have been extracted from food. This is where all of the blood from the small intestine eventually ends up. Once the liver is done with the blood, it flows straight back into the largest vein of the body, the inferior vena cava, which is this large vein here where the blood can return straight to the heart. You can also see it here highlighted in green.

Next up is the superior mesenteric vein highlighted in green in this picture. The stomach located in the upper left quadrant, the transverse and descending colon as well as the pancreas have been cut out of this picture and you can see the C-shaped duodenum here. On top of that, we have the superior mesenteric vein. Embryologically, the superior mesenteric vein forms to drain the midgut. The midgut starts at the ampulla of Vater and ends two-thirds of the way along the transverse colon. So all of the parts in between drain to the superior mesenteric vein. This means that it drains most of the small intestine as well as some parts of the large intestine.

The midgut consists of the distal half of the duodenum, the jejunum, the ileum, the cecum, the appendix, the ascending colon, and the transverse colon. Anatomically, however, the small intestine ends at the ileocecal valve so we'll only mention the veins beyond that very briefly.

You can see that the superior mesenteric vein is very closely related to the hepatic portal vein. In fact, it's essentially the same continuous structure but with a different name. After the splenic vein, which you can see here, joins the superior mesenteric vein, it's then named the hepatic portal vein. Before it becomes the hepatic portal vein, it receives blood from all of the midgut.

So now that we've covered the major veins to which the small intestine drains, we can look at the veins for each part of the small intestine in more detail. First up is a special case, the duodenum. We've zoomed in to the abdomen here to get a better look but we're still in the anterior view. Embryologically, the duodenum is made up of two parts. The first half from the stomach to the ampulla of Vater is part of the foregut, and from the ampulla of Vater onwards, we have the midgut. This means that it has a dual blood supply and, therefore, dual venous drainage.

The first and second parts, which is to say, the superior and descending part of the duodenum is from the embryological foregut structure. This part of the duodenum drains blood to the anterior superior pancreaticoduodenal vein which normally drains into the right gastroepiploic vein and the posterior superior pancreaticoduodenal vein which then drains directly into the hepatic portal vein. Because it is a foregut structure, the superior mesenteric vein isn't involved.

The second half of the duodenum, the horizontal and the ascending parts, are embryologically midgut structures. This part of the duodenum is drained by the anterior inferior pancreaticoduodenal vein and the posterior inferior pancreaticoduodenal vein. So since this is a midgut structure, can you guess which vein comes next? That's right, the superior mesenteric vein. So the blood flows from these two inferior pancreaticoduodenal veins to the superior mesenteric vein and then to the hepatic portal vein. After this, it gets a lot simpler, so don't worry, we'll move on now to the jejunum.

So once again, we're looking at the abdominal cavity from the front with the bowel moved to one side and the jejunum is here to the left and the entire jejunum is drained by the jejunal veins which you can see highlighted here in green. This vein follows the artery exactly and drains into the superior mesenteric vein before draining into the hepatic portal vein.

The ileum, which is the final part of the small intestine, is drained primarily by the ileal veins highlighted here in green. The blood then drains into the superior mesenteric vein and then to the hepatic portal vein. The last part of the ileum or the terminal ileum, approximately the last five to ten centimeters, is covered by a different vein called the ileocolic vein. This vein drains the terminal ileum as well as the first part of the large bowel.

Okay, so we've gone through all the major veins of the small intestine now so let's have a look at some clinical aspects of the portal venous system.

So there are locations in the body where the portal venous system and the veins from the rest of the body – the systemic venous system – connect. The connection between the portal venous system and the systemic venous system is called the portosystemic anastomosis and it’s a venous flow from the GI tract to the heart through the inferior vena cava which skips the liver. The main sites of portosystemic anastomoses are esophageal, rectal, retroperitoneal, and paraumbilical sites.

Portal hypertension occurs when there's an increased blood pressure in the portal vein. A major cause of this is increased blood pressure due to liver cirrhosis which is when the liver is damaged and becomes fibrotic. This increases the resistance in the organ and causes elevated pressures in the portal venous system. Alcohol is a common cause of liver cirrhosis and portal hypertension can have major consequences and symptoms such as ascites, spider nevi, and caput medusae, which are dilated veins around the belly button due to portal hypertension. Esophageal varices are dilated veins that occur in the esophagus due to back pressure in the portosystemic anastomosis. They can often bleed if present and are a major cause of morbidity in those with liver cirrhosis.

So that's it! Let's summarize what we've gone through in this tutorial.

So we discussed that the small intestine is divided into the duodenum, the jejunum, and the ileum. The major veins draining the small intestine are the superior mesenteric vein, the hepatic portal vein, and the inferior vena cava which finally enters the right atrium of the heart. The duodenum is drained by the pancreaticoduodenal veins whereas the jejunum and the ileum are drained by the jejunal and the ileal veins. And lastly, we gave a brief clinical summary of portal hypertension.

Alright, thanks for watching. See you next time and happy studying!