Portal vein

Last revised by Henry Knipe on 19 Oct 2023

The portal vein (PV) (sometimes referred to as the main or hepatic portal vein) is the main vessel in the portal venous system and drains blood from the gastrointestinal tract and spleen to the liver.

A portal venous system connects two capillary beds, meaning one organ / organ system will drain blood into another organ / organ system, before returning to the heart. In addition to the hepatic portal venous system, there is also a hypophyseal portal system that passes blood from the hypothalamus to the anterior pituitary 1.

The portal vein usually measures approximately 8 cm in length in adults with a normal diameter of 7-13 mm 2,11 but may vary up to 15 mm 11. It originates posterior to the neck of the pancreas where it is classically formed by the union of the superior mesenteric and splenic veins (the portovenous/portomesenteric confluence) 3. The origin of the vein defines the location of the pancreatic neck.

As it courses to the right towards the liver it inclines superiorly running in the free edge of the lesser omentum 4 (hepatoduodenal ligament) with the other structures of the portal triad (common hepatic duct and common hepatic artery) anterior to it 3. The portal vein is separated from the inferior vena cava by the epiploic foramen (of Winslow) 4.

Immediately before reaching the liver, the portal vein divides in the porta hepatis into left and right portal veins. The right portal vein divides into anterior (supplying segments 5 and 8) and posterior (supplying segments 6 and 7) branches. The left portal vein may be divided into transverse and umbilical portions, as delineated by the ligamentum venosum, and is mostly extrahepatic in its course. The main branches of the left portal vein originate from the umbilical portion, and supply liver segments 2, 3 and 4 5.

The portal vein ramifies further, forming smaller venous branches and ultimately portal venules. Each portal venule courses alongside a hepatic arteriole and the two vessels form the vascular components of the portal triad. These vessels ultimately empty into the hepatic sinusoids to supply blood to the liver.

75% of the blood supplied to the liver comes from the portal vein, but it only supplies 50% of the oxygen supply to the liver.

Along its length, the portal vein receives various tributaries including:

The inferior mesenteric vein joins the splenic vein 40% of the time, the superior mesenteric vein 40% of the time, and portomesenteric confluence at 20% of the time 3. When there is an obstruction in the portal vein or hepatic veinsportosystemic collateral pathways open to allow the drainage of excessive portal system blood into the systemic venous system 4.

The overall incidence of portal vein variation is reported to be ~25% (range 20-30%), which should be recognized prior to procedures such as liver transplantation, complex hepatectomy and portal vein embolization 3,6-8

  • portal vein trifurcation (most common)

    • portal vein divides into three branches: left portal vein, right anterior portal vein, and right posterior portal vein

  • absent right portal vein (rare) 5

    • right sectional portal veins originate independently from the common portal vein

    • if the right anterior section portal vein branches higher from the common portal vein vs posterior sectional portal vein, the surgeon may mistake the posterior sectional portal vein for the right portal vein

  • portal vein duplication (rare)

  • absent left extrahepatic portal vein (rare) 5

    • a single right portal vein originates from the porta hepatis, supplying the right hemiliver, then following an intrahepatic course with distalmost branches supplying the left liver

  • preduodenal portal vein 9

  • hepatic artery passing posterior to the portal vein (10%) rather than anteriorly (90%) 4

There is an increased risk of bile duct hilar anatomical variation in the presence of portal vein variants.

The normal velocity of portal venous flow is 20cm/s to 40cm/s on spectral Doppler ultrasound. The waveform shows gentle undulations with hepatopetal flow direction 12.

The embryonic vitelline veins drain from the yolk sac to the sinus venosus. The two vitelline veins develop anastomosing cross-communications around the duodenum, in the developing liver and the septum transversum. Selective involution of these veins leads to the formation of the portal vein and rotation of the foregut contributes to the formation of its extra-hepatic course 10.

Abnormal involution can lead to congenital abnormalities, including a congenital portosystemic shunt.

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Cases and figures

  • Figure 1: portal vein
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  • Figure 2: portal vein cross-section image
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  • Figure 3: pancreas and duodenum in relation to PV
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  • Figure 4: CTA portal vein anatomy
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