This
note describes the formation, tributaries and major site for the portocaval
anastomosis (also called portosystemic anastomosis or portal-systemic anastomosis).
Portal Vein
The
portal vein is the vein of the gut that drains 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.
Formation
It
is about 2 inches long and formed by the union of superior mesenteric vein and
splenic vein behind the neck of the pancreas. It ascends to the right behind
the first part of the duodenum and enters the free margin of the lesser
omentum. Entering through the porta hepatis, it divides 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 tributaries of the
portal vein are:
Splenic
and superior mesenteric veins - the formative tributaries
Left
gastric,
Rright
gastric and
Cystic
veins
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, and they become important when the direct
route becomes blocked.
These
communications are as follows:
1.
At the lower third of the esophagus, the esophageal branches 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).
2.
Halfway down the anal canal , the superior rectal veins (portal tributary) draining the
upper half of the anal canal anastomose with the middle and inferior rectal
veins (systemic tributaries), which are tributaries of the internal iliac and
internal pudendal veins, respectively.
3.
The
paraumbilical veins connect the left branch of the portal vein with the
superficial veins of the anterior abdominal wall (systemic tributaries).
4.
The
veins of the ascending colon, descending colon, duodenum, pancreas, and liver
(portal tributary) anastomose with the renal, lumbar, and phrenic veins
(systemic tributaries).
REFERENCES:
Following resources are used while preparing this post (readers are strongly recommended to go through them for more details):
R. Snell's Clinical Anatomy
K. L. Moore's Clinically Oriented Anatomy