PORTAL VEIN: NORMAL PHYSIOLOGY AND ANATOMY
The portal vein carries approximately 1.5 L/min of blood from the splanchnic circulation to the liver. Unlike most veins in the body, the portal vein lacks valves. It constitutes around 80% of blood supply to the liver, with portal venous flow estimated to be around 2 cm/sec. The normal pressure of the portal vein is around 5 mm Hg, and pressure greater than 10 mm Hg results in the development of portosystemic shunts such as gastroesophageal varices.
The portal vein is approximately 8 cm in length, and the normal diameter is around 1 cm. It is formed by the confluence of the superior mesenteric vein (SMV) and the splenic vein. It runs behind the neck of the pancreas and the first part of the duodenum and enters the hepaticoduodenal ligament to the right of common hepatic artery and behind the common bile duct. The retropancreatic portion of the portal vein/SMV lacks venous tributaries and allows dissection of the pancreas off the venous structure during pancreaticoduodenectomy. Several important venous branches, such as anterior and posterior superior and inferior pancreaticoduodenal veins, drain directly into the portal vein/SMV. On the other hand, the left gastric vein and the inferior mesenteric vein usually join the portal vein via the splenic vein. Left gastroepiploic vein and middle colic vein can either drain directly to the SMV or sometimes form the so-called gastrocolic trunk of Henle before joining the SMV.
In the porta hepatis, the portal vein bifurcates into the right and left portal veins. The right portal vein usually receives the cystic vein before dividing into right anterior and right posterior branches. The right anterior branch divides further to supply portal venous blood to liver segments V and VIII, and the right posterior branch bifurcates into segmental branches to supply liver segments VI and VII. The left portal vein takes more horizontal course under the hilar plate, and it runs below the quadrate lobe before it enters the umbilical fissure. It usually supplies the caudate lobe (segment I) before dividing into segmental branches of the lateral segments (II and III) and medial segment (IV) of the left hepatic lobe.
The left portal vein is connected by a fibrous remnant of the obliterated umbilical vein called the ligamentum teres to the systemic periumbilical veins. This connection can sometimes recanalize in cases of portal hypertension. The left portal vein is also connected to the inferior vena cava (IVC) by the obliterated ductus venosus called the ligamentum venosum.
NORMAL VARIATIONS AND CONGENITAL ANOMALIES OF THE PORTAL VEIN
The most common variation of the portal vein is trifurcation of the main portal vein.1 It usually takes the form of the right anterior and posterior portal veins arising directly from the main portal ...