The portal vein transports oxygen-depleted but nutrient-rich blood from the gastrointestinal tract to the liver, where potential toxins are metabolized. Diseases of the portal vein can severely impair the liver‘s detoxification abilities.
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 portal vein refers exclusively to the vena portae (hepatic portal vein). Its function is to transport oxygen-poor (venous) but nutrient-rich blood from the gastrointestinal tract to the liver. To do this, it collects blood from the organs of the gastrointestinal tract, passes it on to the liver and divides it again in a capillary system. In the capillaries of the liver, venous blood is mixed with arterial (oxygenated) blood from the hepatic artery (arteria hepatica propria) and passed through the liver for processing of nutrients. After toxins are broken down, the blood returns to the greater circulation. Thus, the portal circulation is a bypass circulation that is a branch from the great circulation. Some substances pass through the portal circulation several times via the enterohepatic circulation.
Anatomy and structure
The portal vein is located horizontally behind the pancreas and runs to the upper right in the hepatoduodenal ligament (connecting the liver to the duodenum). From there it enters the hepatic orifice. It represents a junction of several veins from the gastrointestinal tract. It collects blood from the gastric veins, the pyloric vein, the gallbladder vein, the venae paraumbilicales, the superior mesenteric vein, the inferior mesenteric vein and the splenic vein. Thereby, the confluence (confluens) from the splenic vein (Vena splenica) and the superior intestinal vein (Vena mesenterica superior) is the actual beginning of the hepatic portal vein. After the portal vein passes through the hepatic orifice, it divides into one branch to each of the right and left lobes of the liver. It then divides further 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 ventricle of the heart. In this process, the latter is no longer part of the portal vein system.
Function and Tasks
The portal vein circulation is not directly involved in the major blood circulation, but represents a secondary branch. However, this subsidiary circulation provides for the hepatic passage of blood. As part of this system, nutrients absorbed in the intestine enter the liver. At the same time, the liver also breaks down toxins that have entered the body. Only after detoxification and processing of the nutrients is the blood returned to the large bloodstream. From there, the processed nutrients are absorbed into the appropriate target organs. The portal circulation also includes the enterohepatic circulation. Some substances are not completely broken down in the liver and can re-enter the intestine via the bile. From there, these substances are again transported via the portal vein to the liver and further metabolized. In the process, 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 efficacy through degradation in the liver. For other active ingredients, efficacy is reduced because they only partially enter the large bloodstream. This so-called first-pass effect (initial liver passage of the active ingredient) has an impact on the dosage or even the form of their administration. The first-pass effect depends 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 include portal hypertension and portal vein thrombosis. Portal hypertension is an increase in blood pressure in the portal vein. This results in outflow obstruction and the blood backs up. Bypassing the portal circulation, the blood now drains through the veins of the esophagus or stomach.This results in gastritis (stasis gastritis), indigestion, enlargement of the spleen, esophageal varices (varicose veins of the esophagus) or caput medusae (abdominal varicose veins). In addition, ascites (accumulation of fluid in the abdominal cavity) may occur. The causes of this condition are varied. Among others, it can be caused by occlusions in the portal vein (portal vein thrombosis) in addition to liver diseases, such as cirrhosis. Portal vein thrombosis represents an occlusion of the portal vein by a blood clot. Depending on the extent of the thrombosis, the disease may go completely unnoticed or show the typical symptoms of portal hypertension. Esophageal varices, gastric inflammation, splenomegaly or ascites may occur. However, due to the special nature of the portal circulation, there is no risk of pulmonary embolism. Both portal hypertension and portal vein thrombosis can be independent but also sequelae of other diseases. For example, pancreatic diseases, liver diseases or blood coagulation disorders should be considered as causes. Since the portal veins can also transport parasites, parasitic diseases caused by the fox tapeworm (alveolar echinococcosis) or liver flukes are possible. Some pathogens can even migrate to other body organs via the gallbladder or blood. A birth-related disease is the so-called persistent ductus venosus. Here, the connection between the portal vein and the inferior vena cava remains, so that toxins absorbed in the intestine can enter the large bloodstream directly. Limited life expectancy necessitates surgical intervention in this case.
