Liver Portal System: Definition, Function, Related Veins and Clinical Relevance

Human anatomy is the system of veins that comprises the hepatic portal vein and its tributaries.

It is also known as the portal venous system. However, it is not the only example of a portal venous system and splanchnic veins, which is not synonymous with the hepatic portal system and is imprecise.

Since it means visceral veins and not necessarily the veins of the abdominal viscera.

Essentially, it drains structures ultimately supplied by celiac (except for the gonads), anterior mesenteric, gastrosplenic, and posterior mesenteric arteries.

Therefore, the branches of the hepatic portal system closely follow many of the components of these arteries, and it is convenient to study them after they have been identified on the streets.

The liver portal system is injected with yellow latex in some samples, which greatly facilitates its study. You can follow its branches in a non-injected selection and briefly examine a shark with the system injected.

The main vessel of the hepatic portal system is the hepatic portal vein, a large vein found in the gastrohepatoduodenal ligament next to the hepatic artery and the anterior part of the bile duct.

 

The hepatic portal vein is formed by the confluence of three main vessels, the gastric, pancreaticomesenteric, and lyenomesenteric veins. They join to form the hepatic portal vein near the anterior tip of the dorsal lobe of the pancreas.

Remember that the celiac artery also divides into its branches very close. Occasionally, the gastric and lyenomesenteric veins join to form a very short vessel that enters with the pancreaticomesenteric to form the hepatic portal vein.

The gastric vein accompanies the gastric artery on the dorsal and ventral surfaces of the stomach. The lyenomesenteric vein extends along the dorsal lobe of the pancreas.

To the left of the spleen, the vein is formed by the confluence, near the posterior end of the dorsal lobe, of the relatively short posterior lienogastric vein and the posterior intestinal vein.

The first comes from the spleen and the back of the stomach (that is, in parallel with the line gastric artery), the latter from the back of the intestine (remember that this region is supplied by the anterior mesenteric artery, which becomes the posterior intestinal artery).

The pancreaticomesenteric vein accompanies the pancreaticomesenteric artery to the beginning of the intestine. Here is formed by several tributaries. Among these are the anterior intestinal veins and the anterior liganogastric veins.

As expected, the anterior vein extends parallel to the anterior intestinal artery. It is larger than the anterior lienogastric vein, which comes from the spleen and adjacent regions of the pyloric part of the stomach.

Function

The portal venous system is responsible for directing blood from the parts of the gastrointestinal tract to the liver. The substances absorbed in the small intestine travel first to the liver for processing before continuing to the heart.

Not all the gastrointestinal tract is part of this system. The system extends from approximately the lower portion of the esophagus to the upper part of the anal canal. It also includes the venous drainage of the spleen and pancreas.

Many drugs absorbed through the gastrointestinal tract are metabolized substantially in the liver before reaching the general circulation.

This is known as the first-pass effect. As a result, certain medications can only be taken through specific routes.

For example, nitroglycerin can not be swallowed because the liver will deactivate the medication. Still, it can be taken under the tongue or transdermally (through the skin) and, therefore, absorbed in a way that avoids the portal venous system.

Conversely, dextromethorphan, a cough suppressant, is best taken orally because the liver must metabolize it into dextrorphan to be effective. This last principle is that of most prodrugs.

Suppositories are a way to bypass the portal vein partially: the upper 1/3 of the rectum drains into the portal vein, while the lower 2/3 drains into the internal iliac vein that goes directly into the inferior vena cava (thus avoiding the liver).

The blood flow to the liver is unique since it receives oxygenated and (partially) deoxygenated blood. As a result, the partial oxygen gas pressure (pO2) and the perfusion pressure of the portal blood are lower than in other organs of the body.

The blood passes from the portal vein branches through the cavities between “plates” of hepatocytes called sinusoids. The blood also flows from the components of the hepatic artery and mixes in the sinusoids to supply oxygen to the hepatocytes.

This mixture is filtered through the sinusoids and accumulates in a central vein that drains into the hepatic vein. The hepatic vein drains posteriorly into the inferior vena cava.

The hepatic artery provides 30 to 40% of the oxygen to the liver, while it only accounts for 25% of the total blood flow of the liver. The rest comes from the partially deoxygenated blood of the portal vein.

The liver consumes approximately 20% of total body oxygen when at rest. That is why the total blood flow of the liver is relatively high, about 1 liter per minute and up to two liters per minute.

Related veins

That’s, on average, a quarter of the average resting heart rate. The large veins that are considered part of the portal venous system are:

  • Hepatic portal vein.
  • Splenic vein
  • Vena mesentérica superior.
  • Vena mesentérica inferior.

The superior mesenteric vein and the splenic vein join to form the actual hepatic portal vein. The inferior mesenteric vein connects most people in the splenic vein. Still, it is known to communicate in the portal vein or the superior mesenteric vein in some people.

The portal venous system corresponds to areas supplied by the celiac trunk, the superior mesenteric artery, and the inferior mesenteric artery.

Clinical relevance

Portal hypertension is a condition in which blood pressure portal venous system is too high. It is often the result of cirrhosis of the liver.