Anatomy & Organs

Portal vein – structure, function & diseases

Portal vein

The portal vein transports oxygen-poor but nutrient-rich blood from the gastrointestinal tract to the liver , where possible toxins are metabolized. Diseases of the portal vein can severely impair the detoxification capacity of the liver.

What is the portal vein?

In general, portal veins are veins that carry venous blood from one capillary system to another capillary system. Mammals have two portal vein systems, the hepatic portal vein and the pituitary portal vein.

However, the colloquial name of the portal vein refers exclusively to the Vena Portae ( hepatic portal vein ). It has the function of transporting oxygen-poor (venous) but nutrient-rich blood from the gastrointestinal tract to the liver. To do this, it collects the blood from the organs of the gastrointestinal tract, passes it on to the liver and divides it there again in a capillary system.

In the capillaries of the liver, the venous blood is mixed with arterial (oxygen-rich) blood from the hepatic artery (Arteria hepatica propria) and conducted through the liver to process the nutrients. After the toxins have been broken down, the blood returns to the general bloodstream . The portal circulation is therefore a bypass circulation, which represents a branch from the great circulation. Some substances pass through the portal circulation several times via the enterohepatic circulation .

Anatomy & Structure

The portal vein is located horizontally behind the pancreas and runs to the top right in the hepatoduodenal ligament (connection of liver and duodenum ). From there it reaches the porta hepatis. It represents an amalgamation of several veins from the gastrointestinal tract. It collects blood from the gastric veins, the vein of the pylorus (vena pylorica), the vein of the gallbladder (vena cystica), the vein network of the navel (venae paraumbilicales), the upper intestinal vein (Vena mesenterica superior), the lower intestinal vein (Vena mesenterica inferior) and from the splenic vein (Vena splenica).The confluence of the splenic vein (Vena splenica) and the upper intestinal vein (Vena mesenterica superior) is the actual beginning of the hepatic portal vein. After the portal vein passes the hepatic portal, it divides into one branch to the right and one to the left lobe of the liver. This is followed by a further division into smaller branches that form the capillary system. From the individual liver sections, the blood is then conducted via the hepatic veins into the inferior vena cava and from there into the right atrium of the heart. The latter no longer belongs to the portal vein system.

Function & Tasks

The portal circulation is not directly integrated into the large blood circulation, but represents a side branch. However, this side circulation ensures that the blood passes through the liver. As part of this system, the nutrients absorbed in the intestines reach the liver. At the same time, the liver also breaks down the toxins that have entered the body. Only after detoxification and processing of the nutrients is the blood released back into the general bloodstream.

From there, the processed nutrients are absorbed in the appropriate target organs. The portal circulation also includes the enterohepatic circulation. Some substances are not completely broken down in the liver and can get back into the intestine via the bile . From there, these substances are again transported via the portal vein to the liver and metabolized further. Some substances pass through this cycle several times until they are partially or completely broken down. For example, some drugs cannot be taken orally because they would lose their effectiveness if they were broken down in the liver.

The effectiveness of other active ingredients is reduced because they only partially reach the large bloodstream. This so-called first-pass effect (first passage of the active substance through the liver) has an impact on the dosage or even on the form of its administration. The first-pass effect is dependent on liver function and the chemical properties of the drugs and can be circumvented by parenteral administration (by infusion ).

Diseases

Known diseases of the portal vein are portal hypertension and portal vein thrombosis . Portal hypertension is an increase in blood pressure in the portal vein. This leads to drainage problems and the blood backs up. Bypassing the portal circulation, the blood now drains through the veins of the esophagus or stomach .The consequences of this are stomach inflammation (stasis gastritis), indigestion , spleen enlargement , esophageal varices (varicose veins of the esophagus) or caput medusae (belly button varicose veins). Ascites (accumulation of fluid in the installation space) can also occur . The causes of this disease are manifold. In addition to liver diseases such as liver cirrhosis , it can also be caused by occlusions in the portal vein (portal vein thrombosis). Portal vein thrombosis is a blockage of the portal vein by a blood clot .

Depending on the extent of the thrombosis, the disease can go completely unnoticed or show the typical symptoms of portal hypertension. Esophageal varices, gastric inflammation, spleen enlargement or ascites can occur. Due to the special nature of the portal circulation, however, there is no risk of pulmonary embolism . Both portal hypertension and portal vein thrombosis can be independent or secondary to other diseases. Possible causes should be considered, for example, diseases of the pancreas, liver diseases or blood clotting disorders.

Since the portal veins can also transport parasites , parasitic diseases such as fox tapeworm (alveolar echinococcosis) or liver flukes are possible. Some pathogens can even migrate to other body organs via the gallbladder or the blood. The so-called persistent ductus venosus is a birth-related disease. Here, the connection between the portal vein and the inferior vena cava remains intact, so that the toxins absorbed in the intestine can enter the large bloodstream directly. The limited life expectancy makes surgical intervention necessary.

Website | + posts

Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.