The portal vein is the large vein that collects blood from the abdominal part of the gastrointestinal tract from the lower third of esophagus to halfway down the anal canal including spleen, pancreas and gall bladder.
It is also called hepatic portal vein.
It is called portal vein because it begins in one set of capillaries [in the gut] and ends in another set of capillaries [in the liver].
Formation of Portal Vein
Portal vein is about 8 cm or less in length. It is formed by the union of superior mesenteric vein and splenic vein behind the neck of the pancreas at the level of vertebra L2.
It ascends to the right behind the first part of the duodenum and enters the free margin of the lesser omentum. These parts are known as infraduodenal, retroduodenal and supraduodenal parts.
The vein ends at the right end of porta hepatis by dividing into right and left terminal branches which further break up into sinusoids. The blood from the sinusoids is collected by hepatic veins that join the inferior vena cava.
The right branchis shorter and wider. It receives cystic vein before entering porta hepatis.
Left branch traverses ports hepatis from its right end to left and provides branches to caudate and quadrate lobes. Before entering the left lobe, it receives paraumbilical veins, ligamentum teres and ligamentum venosum
Tributaries of the portal vein
- Splenic and superior mesenteric veins – the formative tributaries
- Left gastric vein
- Right gastric vein
- Cystic veins
- Paraumbilical veins
- Superior pancreatoduodenal vein
Relations of Portal Vein
Neck of pancreas
Inferior vena cava
First part of duodenum
Inferior vena cava
Bile duct within margin of lesser omentum
Inferior vena cava [separated by foramen epiploicum]
Portocaval or Portosystemic anastomosis
Under normal conditions, the portal venous blood traverses the liver and drains into the inferior vena cava of the systemic venous circulation through the hepatic veins. This is the direct route. However, other, smaller communications exist between the portal and systemic systems,.
These small communications form important routes of collateral circulation in event of portal obstruction.
These communications are as follows
The paraumbilical veins connect the left branch of the portal vein with the superficial veins of the anterior abdominal wall (systemic). In portal obstruction, the veins around the umbilicus enlarge to for caput medusae
Lower third of the esophagus
Esophageal tributaries of the left gastric vein (portal tributary) anastomose with the esophageal veins draining the middle third of the esophagus into the azygos veins (systemic tributary).
Superior rectal veins [portal] draining the upper half of the anal canal anastomose with the middle and inferior rectal veins [systemic]
Bare Area of Liver
Hepatic venules [portal] anastomose with phrenic and intercostals veins [systemic]
Posterior Abdominal Wall
The veins of retropelvic organs like the duodenum, ascending colon, descending colon, pancreas, and liver (portal tributary) anastomose with the renal, lumbar, and phrenic veins (systemic tributaries).
The portal vein is not a true vein, because it conducts blood to capillary beds in the liver and not directly to the heart. It is a major component of the hepatic portal system, one of only two portal venous systems in the body – with the hypophyseal portal system being the other.
Function of Portal Vein
The portal vein and hepatic arteries form the liver’s dual blood supply. Approximately 75% of hepatic blood flow is derived from the portal vein. The remaining remainder is from the hepatic arteries.
In carrying venous blood from the gastrointestinal tract to the liver, the portal supplies the liver with metabolic substrates and it ensures that substances are first processed by the liver before reaching the systemic circulation.
This enables detoxification of toxins too.
Clinical significance of Portal Vein
Increased blood pressure in the portal vein, called portal hypertension, is a major complication of liver disease, most commonly cirrhosis. A dilated portal vein is a sign of portal hypertension. Portal hypertension may result in clinically observable signs like ascites, esophageal varices, spider nevi, caput medusae, and palmar erythema.
Pylephlebitis is infection of the portal vein.