The hepatic portal system is the network of veins that drains oxygen-poor and nutrients-rich blood into the liver. This system of venous blood vessels is made up of three main veins: 1) the splenic; 2) the superior mesenteric; and 3) the portal vein, with the first two ones joining one another to give rise to the third one. However, there are important venous tributaries which drain into each one of these three big veins, contributing to the portal system network of special veins.
The splenic vein also receives blood from the pancreatic, the left gastroepiploic, short gastric, and the inferior mesenteric vein. Let us remember that the splenic drains the spleen, and the others the stomach and large intestine respectively. Meanwhile, secondary tributaries carry nutrient-rich blood from the small intestine into the superior mesenteric vein, which joins the splenic.
As mentioned above, the portal vein arises from the union of the superior mesenteric and splenic vein. The portal, in turn, also receives blood directly from the left gastric and right gastric vein just before it enters the liver, where it divides into two main branches that keep giving off smaller branches until they form a network of venules and capillaries.
All these venous blood vessels that constitute the hepatic portal system are special veins. They are special for two reasons; firstly, they carry nutrients and hormones-rich blood. Secondly, they do not drain towards and into the cava vein, as it is the case with all the other ordinary veins. They drain into the liver, where all the nutrients (proteins, fatty acids, glycerol, glucose, minerals) are metabolized by the liver into the vital substances our body cells need. Only when they have been metabolically processed, these essential contents are carried in the venous blood by the hepatic veins into the inferior cava.
Below, diagram drawing of the veins that compose the hepatic portal system.
