Vena Porta: What is it?
The portal vein is a large venous trunk that collects blood from the spleen and the subdiaphragmatic portion of the digestive tract to carry it to the liver.
The portal vein originates from the confluence of two main vessels: the superior mesenteric vein and the splenic vein.The latter drains the blood from the spleen and, to a lesser extent, that from the stomach, duodenum and pancreas. The vessels of the small intestine, the right half of the large intestine and the head of the pancreas and stomach.
A third portal vein root, the inferior mesenteric vein, collects blood from the left colon and rectum. This vein generally flows into the terminal tract of the splenic vein; at other times it joins directly to the portal vein in the tract where the other two main branches converge or in the terminal portion of the superior mesenteric vein.
Other minor vessels from the stomach and gallbladder also converge in the portal vein.
Taken together, these vessels participate in the hepatic portal system (a portal system is, by definition, a venous device that collects the waste blood from one or more organs and carries it to another organ from which it is made to flow into the general circulation).
The portal vein lumen has an internal diameter of about one centimeter (8-12 mm) and is traversed by one liter of blood per minute, a quantity that corresponds to approximately 70-80% of the blood supply to the liver. 20-30% is covered by the hepatic artery, which carries oxygenated blood taken directly from the aorta; that coming from the digestive organs, while still containing a fair percentage of oxygen, is instead particularly rich in nutrients and other substances absorbed in the intestine .
Once the hepatic hilum is reached (fissure - point of insertion - from which the blood vessels and the nerves to the organ enter and exit), the portal vein divides almost in a T shape into the two main intrahepatic branches, right to left. These branches in turn subdivide repeatedly until they embrace each single hepatic lobule. The hepatic artery also shares with the portal vein the entry point and the dense branching inside the organ; thus two clearly distinct vascular districts are formed, one on the right and one on the left. The blood coming from the superior mesenteric vein, rich in digestive products, it mainly runs along the right branch, while that coming from the splenic vein goes mainly to the left. In fact, in the short path of the portal vein, the blood flows coming from the two main efferent branches (mesenteric and splenic) manage to mix only partially.
The blood that irrigates the hepatocytes is then collected from the terminal hepatic venules of the hepatic vein and is conveyed into the inferior vena cava and from there to the heart.
Portal hypertension
For further information: Portal Hypertension - Causes and Symptoms
Portal hypertension often results from liver cirrhosis and alcoholic hepatitis; in this case the structural alterations of the liver obstruct the blood flow inside it, increasing the blood pressure in the portal vein. Even the presence of an "obstruction" inside it (thrombosis of the portal vein) determines the same result, exactly as when the obstacle is downstream and prevents a normal outflow of blood from the liver to the general circulation (for example due to thrombosis hepatic veins or congestive heart failure).
In the presence of portal hypertension, the organism tries to compensate for the blockage of circulation by developing or strengthening collateral circulation; if the obstruction is inherent in the liver, consequently, part of the toxic substances usually inactivated by the organ (both endogenous and exogenous, as drugs taken orally) "skip" the hepatic passage and find themselves unchanged in the circulation. In addition, there are phenomena of suffering of the vessels in which a greater influx of blood is conveyed due to the obstruction, with the appearance of lesions to the varices esophageal and hemorrhoids, pathological aspect of the umbilical veins (caput medusae) and enlargement of the spleen. In the presence of portal hypertension, ascites (accumulation of fluid in the peritoneum) is also common; there may also be an abnormal enlargement of the spleen (splenomegaly) and suffering phenomena in the brain (hepatic encephalopathy) and kidney (hepatorenal syndrome) .
