Portal vein

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.

Terminology

A portal venous system is one in which veins connect two capillary beds; or in other words drain one organ/organ system and pass into another organ/organ system rather than being directly returned to the heart. The hepatic portal vein is the only portal system many are aware of. However, there is also a hypophyseal portal system which passes blood from the hypothalamus to the anterior pituitary⁸.

Gross anatomy

The portal vein usually measures approximately 8 cm in length in adults with a maximum diameter of 13 mm. 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 confluence). 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 posterior margin of the free edge of the lesser omentum with the other structures of the portal triad (common hepatic duct and common hepatic artery) anteriorly. Anterior to the inferior vena cava, the portal vein forms the anterior border of the epiploic foramen (of Winslow). Along its length, it receives various tributaries including the inferior mesenteric, gastric, and cystic veins.

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 transverseand umbilical portions, as delineated by the ligamentum venosum, and is mostly extrahepatic in 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.

Relations

• anterior - the neck and body of the pancreas, D1 segment of duodenum
  • anterolateral - common bile duct
  • anteromedial - common hepatic artery
• posterior - inferior vena cava

Variant anatomy

The overall incidence of portal vein variation is reported to be ~25% (range 20-30%), which should be recognised prior to procedures such as liver transplantation, complex hepatectomy and portal vein embolisation ^2-4: 

• 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 ⁷
  • frequently associated with other abnormalities and situs inversus 

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

Development

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 ⁶.

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

Related pathology

• Abernethy malformation
• cavernous transformation of the portal vein
• portal hypertension
• portal vein thrombosis
• portal venous gas