Corpus: Portal venous system

The portal vein circulation is a venous blood circulation that supplies blood from the entire gastrointestinal tract and the spleen via the portal vein (vena portae) first to the liver and from there to the inferior vena cava. It belongs to the so-called portal vascular system.

The portal circulation is a "branch" of the large systemic circulation. It ensures that nutrient-rich blood from the superior mesenteric vein and the inferior mesenteric vein, as well as blood from the lienal vein, first passes through the liver before it reaches the inferior vena cava and thus the large systemic circulation. This means that the blood passes through a capillary bed twice in the portal circulation: first the capillary bed of the intestine, then the capillary bed of the liver.

The purpose of the portal circulation is to supply the liver directly with the nutrients absorbed in the intestine for metabolisation. The same applies to any toxins: they are broken down in the liver before they enter the bloodstream.

In the case of medicines, the passage through the liver can result in them being partially or completely metabolised before they reach their target organs. This is known as the "first-pass effect".

Pathological changes in the liver parenchyma, such as liver cirrhosis, lead to portal hypertension. As a result, the blood flows via bypass circuits, so-called portocaval anastomoses, towards the heart. Examples are the dilated periumbilical veins (caput medusae), oesophageal varices and the dilated rectal veins.

In addition to the classic portal vein system of the liver, there are 2 other such systems in humans:

• Pituitary portal system: conducts the releasing hormones and release-inhibiting hormones of the hypothalamus to the endocrine cells of the anterior pituitary.
• Pancreatic portal system: The venous blood of the islets of Langerhans reaches the capillary system of the exocrine pancreas via vasa efferentia. The acini of the exocrine pancreas are thus supplied directly with active substances from the endocrine pancreas.

In birds, reptiles and amphibians, there is also another portal vein circulation in the kidney. It does not occur in humans.