1 Falciform ligament 2 Inferior vena cava 3 Hepatic veins 4 Portal vein 5 Pancreas 6 Superior mesenteric vein 7 Inferior mesenteric vein 8 Splenic vein 9 Spleen 10 Left lobe of liver

PORTAL CIRCULATION

This is the largest closed circulation in the human body. It is performed via the portal vein and its tributaries. All the veins of the alimentary tract beginning from the abdominal esophagus to the anal valves in the anal canal are drained into the portal vein. The portal vein carries blood from the alimentary canal to the liver in order to detoxify, through metabolism, foreign bodies, which may be toxic to the body. Paradoxically, such metabolic activity may sometimes lead to generation of poisons, which may kill the body. When the liver fails, poisonous products then accumulate in the body and embarrass the brain, causing hepatic encephalopathy. When blood is shunted from the portal circulation in large amount to the systemic circulation, then we also expect some amount of encephalopathy.

The main tributaries of the portal vein are as follows

	Superior mesenteric vein
	Splenic vein.

These two meet behind the neck of the pancreas to form the portal vein, which the ascends in the free edge of the lesser omentum to reach the porta hepatis in order to enter the liver a two branches (left and right).

The superior mesenteric vein receives the inferior mesenteric vein behind the body of the pancreas. The veins, which accompany the branches of the celiac trunk drain into the superior mesenteric vein, e.g. left gastric vein.

But there is a communication between the portal circulation and the rest of the systemic circulation in the so-called portocaval anastomoses, because the portal vein joins the inferior vena cava in this circulation. Ordinarily these anastomotic channels are silent, i.e. to say they are not prominent. But if there is a portal obstruction, increase venous pressure occurs in the portal circulation called portal hypertension. The effect of this would be the opening up of these collateral anastomoses, leading to abnormal varices. The anastomoses be grouped into six types.

1.Esopago-esophageal anastomoses. Esophageal tributaries of the left gastric vein, which drain into the superior mesenteric vein and from their to the portal vein, interconnect (anastomose) with esophageal tributaries of the azygos and the accessory hemiazygos veins, which drain into the superior vena cava (systemic) circulation).

2.Rectal anastomoses: This is between the inferior rectal vein, which is a tributary of the internal pudendal vein that drains into the internal iliac vein. The internal iliac vein finds its way into the common iliac vein and the into the inferior vena cava (systemic circulation). The anastomoses connect with the superior rectal vein, which is a tributary of the inferior mesenteric vein that drains into the superior mesenteric vein and then to the portal vein. The middle rectal vein is a tributary of the internal iliac vein and it also joins the anastomoses in a minimal way. It drains the muscles of the rectum and anus, while the inferior rectal vein drains only the part of the anal canal below the anal valves. The superior rectal vein drains the rectum and the cloacal part of anal canal up to the anal valves.

3.Anterior abdominal wall veins, which accompany the ligamentum teres drains in the portal circulation. Also veins of the upper part of the anterior abdominal wall drain into the lateral thoracic veins, from where they drain into the axillary vein, to the subclavian and finally to the superior vena cava. Also the superior and inferior epigastric veins drain into the internal thoracic vein and to external iliac veins respectively. These find their way into systemic circulation, superior and inferior vena cavae. Anastomoses between these veins in the anterior abdominal wall can open up in portal hypertension, causing caput medusae formation, i.e. veins radiating away from the umbilicus like the spoke  of bicycle wheel. Also these may lead to enlargement of an otherwise simple thoraco-epigastric vein (inguino-axillary  vein).

4. Extraperitoneal veins at bare area of liver where veins anastomose across the diaphragm. These are the phrenic veins, which drain into the inferior vena cava, while the hepatic veins drain into the portal circulation.

5.Retroperiotenal veins, which are renal, lumbar and phrenic veins, which drain into the systemic circulation, communicate with mesenteric veins that drain into the portal circulation

6.Patent ductus arteriosus may interconnect the left branch of portal vein with the inferior vena cava.

Surgically the portal circulation can be decompressed by a shunt operation. These manouvre shuts the blood from entering the portal circulation and passes it to the systemic circulation instead, thereby easing the increased tension. Any of the following procedures can decompress a congest a portal circulation

	Splenorenal shunt
	Portocaval shunt- i.e. anastomosing portal vein directly to the inferior vena cava. The disadvantage of this procedure is the fact that it can no longer be able to take blood to the liver for detoxification. Hence the individual accumulates toxins, which may prove fatal.

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Exposure of peritoneal cavity

Esophago-esophageal anastomosis (11)

1 inferior mesenteric vein 2 superior mesenteric vein3 right gastric vein 4 splenic vein 5 left gastric vein 6 portal vein 7 inferior vena cava 8 stomach 9 esophageal tributary of left gastric vein 10 esophageal tributary of azygos vein, which drains into systemic circulation

Rectal anastomosis (9)

1 inferior mesenteric vein 2 superior mesenteric vein 3 portal vein 4 splenic vein 5 superior rectal vein 6 inferior rectal vein 7 inferior vena cava 8 common iliac vein

Umbilical anastomoses (10)

1 inferior mesenteric vein 2 superior mesenteric vein 3 portal vein 4 splenic vein 5 lower part of inferior mesenteric vein 6 superficial veins of anterior abdominal wall (systemic circulation) 7 inferior vena cava 8 common iliac vein 9 paraumbilical veins (portal circulation) 

Retroperitoneal anastomoses (11)

1 inferior mesenteric vein 2 superior mesenteric vein 3 portal vein 5 intestine (colon) 6 spleen 7 inferior vena cava 8 colic (intestinal) veins (portal circulation) 9 retroperitoneal veins (systemic circulation) 10 common iliac vein

Extraperitoneal anastomoses (12)

1 inferior mesenteric vein 2 superior mesenteric vein 3 portal vein 4 splenic vein 5 hepatic veins (portal circulation) 6 spleen 7 inferior vena cava 8 phrenic veins (systemic circulation) 9 left lobe of liver 10 bare area of liver 11 diaphragm