Portal vein anatomy: the hepatic portal system, porta hepatis and portosystemic anastomoses

What it is

The large vein carrying blood from the digestive organs to the liver.

Location

It forms behind the neck of the pancreas by the union of the superior mesenteric and splenic veins and ascends in the hepatoduodenal ligament to the porta hepatis, where it divides into right and left branches.

Function

It carries nutrient-rich blood from the gastrointestinal tract, spleen and pancreas to the liver.

Body system

Cardiovascular · paired (left and right)

FMA ID

FMA15415 (Foundational Model of Anatomy)

Key facts

• It carries about 75 percent of the liver's blood supply.
• It has no valves.
• Its main tributaries are the splenic, superior mesenteric, left and right gastric veins, and it usually receives the inferior mesenteric vein indirectly through the splenic vein.

Clinical relevance

Portal hypertension in cirrhosis causes varices and ascites.

The hepatic portal system is the hepatic portal vein and all the veins that drain into it. A portal system is one that begins and ends in capillaries: here, the capillaries of the digestive organs at one end and the liver at the other. Instead of entering the general circulation directly, absorbed nutrients and some wastes go first to the liver, which processes them. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

Kenhub describes the same idea in terms of vessels: the system connects the capillaries of the intestines and other digestive organs to the modified capillaries of the liver, the hepatic sinusoids. From the sinusoids blood passes to the central veins, then the hepatic veins, and finally the inferior vena cava. (Kenhub: Hepatic portal vein.)

The portal system collects the venous drainage of the spleen, pancreas, gallbladder and the abdominal part of the gastrointestinal tract, so the venous blood of these organs reaches the heart only after passing through the liver. (TeachMeAnatomy: Venous Drainage of the Abdomen.)

The portal vein is classically formed behind the neck of the pancreas by the union of the superior mesenteric vein and the splenic vein (the splenic-mesenteric confluence). OpenStax and TeachMeAnatomy place this at the level of L2. In adults it is about 8 cm long, with a normal diameter of about 7 to 13 mm. (Radiopaedia: Portal vein.)

It runs upwards and to the right towards the liver in the free edge of the lesser omentum (the hepatoduodenal ligament). Inside the ligament the common bile duct lies in front and to the right of the portal vein, and the common hepatic artery in front and to the left. The first part of the duodenum is among its anterior relations. (Radiopaedia: Portal vein.)

Behind it lies the inferior vena cava, and the two great veins are separated only by the epiploic foramen (foramen of Winslow), the opening into the lesser sac. (Radiopaedia: Portal vein.)

The splenic vein is formed from branches from the spleen, pancreas and parts of the stomach. The superior mesenteric vein drains the small intestine and about two-thirds of the large intestine. The inferior mesenteric vein drains the distal third of the large intestine. The portal vein also receives the gastric veins of the stomach and the cystic veins of the gallbladder. (OpenStax Anatomy and Physiology 2e, 20.5: Circulatory Pathways.)

Along its own length the portal vein receives the posterior superior pancreaticoduodenal vein, the right and left gastric veins, the cystic vein and the paraumbilical veins (veins of Sappey). (Radiopaedia: Portal vein.)

The porta hepatis (hepatis porta, transverse fissure of the liver) is a deep fissure on the inferior (visceral) surface of the liver, between the caudate lobe behind and the quadrate lobe in front. Every vessel, nerve and duct of the liver passes through it except the hepatic veins, and the lesser omentum attaches to its edges. (Radiopaedia: Porta hepatis.)

Its contents are arranged duct, artery, vein from front to back: the common hepatic duct in front on the right, the hepatic artery proper in front on the left, and the portal vein behind both. Nerves of the hepatic plexus and lymphatics pass through as well. (Radiopaedia: Porta hepatis.)

On the underside of the liver, the porta hepatis forms the crossbar of an 'H'. The fissures for the ligamentum venosum and ligamentum teres form the left limb, and the grooves for the inferior vena cava and the gallbladder form the right limb. (Radiopaedia: Porta hepatis.)

At the porta hepatis the portal vein divides into the right and left portal veins. The right portal vein divides again into an anterior branch, supplying liver segments 5 and 8, and a posterior branch. (Radiopaedia: Portal vein.)

The branches end as small portal venules. Each portal venule runs with a branch of the hepatic artery (a hepatic arteriole) and a bile duct, and the three together form the portal triad. (Kenhub: Hepatic portal vein.)

The portal vein supplies about 75% of the blood that reaches the liver, but only about 50% of its oxygen; the hepatic artery provides the rest of the oxygen. (Radiopaedia: Portal vein.)

In the 3D model the portal system is shown with the BodyParts3D segmental branches of the right and left portal veins, the splenic, superior mesenteric, inferior mesenteric and gastric veins, with the digestive organs faint around them.

Portal veins and hepatic veins are often confused. The portal vein brings blood into the liver through the porta hepatis. The three hepatic veins (right, middle and left) take blood out of the liver into the inferior vena cava. (Radiopaedia: Hepatic veins.)

The hepatic veins run between the liver segments and are used as landmarks to define them: the right hepatic vein drains segments 5 to 8, the middle hepatic vein segments 4a, 4b, 5 and 8, and the left hepatic vein segments 2, 3, 4a and 4b. The left hepatic vein always lies in front of the left portal vein. (Radiopaedia: Hepatic veins.)

There is no 'portal artery'. The artery that travels with the portal vein in the lesser omentum is the hepatic artery proper. In about 10% of people the hepatic artery runs behind the portal vein rather than in front of it. (Radiopaedia: Portal vein.)

A portosystemic anastomosis is a place where veins of the portal system connect with veins of the systemic circulation. TeachMeAnatomy lists four main sites: the lower oesophagus, the rectum, the retroperitoneum and the paraumbilical region of the anterior abdominal wall. (TeachMeAnatomy: Venous Drainage of the Abdomen.)

Normally little blood crosses these connections. When portal flow is obstructed, blood is diverted through them, and the veins around the anastomosis enlarge into varices. Rupture of oesophageal varices can cause fatal bleeding. (TeachMeAnatomy: Venous Drainage of the Abdomen.)

On imaging, Radiopaedia also describes splenorenal collaterals between the splenic vein and the left renal vein, which can become a spontaneous splenorenal shunt, and the paraumbilical vein running from the left portal vein along the falciform ligament. Dilated veins of the anterior abdominal wall around the umbilicus form the caput medusae, and Radiopaedia links the connection in the anal canal to haemorrhoids. (Radiopaedia: Portosystemic collateral pathways.)

Portal hypertension is raised pressure in the portal venous system. It is defined as a hepatic venous pressure gradient (HVPG) above 5 mmHg, and is called clinically significant at 10 mmHg or more. (Radiopaedia: Portal hypertension.)

Causes are grouped by where the resistance is. Prehepatic causes include portal vein thrombosis. Intrahepatic causes include cirrhosis, the most common cause overall, and schistosomiasis. Posthepatic causes include Budd-Chiari syndrome, congestive heart failure and constrictive pericarditis. (Radiopaedia: Portal hypertension.)

Features include splenomegaly, ascites and portosystemic collaterals such as oesophageal (esophageal) varices. On ultrasound a portal vein wider than about 12 to 13 mm suggests portal hypertension, although this sign alone is insensitive. (Radiopaedia: Portal hypertension.)

In people with liver disease, Kenhub lists ascites, oesophageal varices, spider naevi, caput medusae and palmar erythema as signs associated with portal hypertension. (Kenhub: Hepatic portal vein.)

In the embryo the vitelline veins drain the yolk sac to the sinus venosus of the heart. Parts of them later involute, and what remains forms the portal vein. (Radiopaedia: Portal vein.)

Variant branching is seen in about 25% of people (range 20 to 30%). The most common is portal vein trifurcation, where the right anterior, right posterior and left portal veins arise together. Rare variants include an absent right portal vein, a duplicated portal vein and a preduodenal portal vein. (Radiopaedia: Portal vein.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Hepatovenous segment IV · digestive
• Gastric vein · cardiovascular
• Hepatovenous segment III · digestive
• Gastric artery · cardiovascular
• Middle hepatic vein · cardiovascular
• Hepatic artery proper · cardiovascular
• Hepatic vein · cardiovascular
• Stomach · digestive
• Caudate lobe of liver · digestive
• Hepatic portal vein · cardiovascular