Portal circulation is the flow of blood from one organ to another, without going through the heart. The term is most often used to refer to how blood moves through the network of veins in the gut and digestive organs, such as the spleen and pancreas, and is carried to the liver. This particular system is known as the hepatic portal system, although it's also sometimes — and somewhat inaccurately — called the portal venous system.
Blood within the hepatic portal system contains all the nutrients absorbed by the digestive tract, which can then be processed by the liver. Useful substances may be adapted for use or stored, while harmful substances are removed and may be converted into less toxic forms. Sometimes, obstructions occur in the portal circulation and pressure builds up, leading to a condition known as portal hypertension.
Networks of tiny blood vessels, known as capillaries, form the beginning of the portal circulation. These capillaries drain blood from the digestive system, all the way from the lowest part of the esophagus to the last section of gut which leads to the anus. Similar networks of capillaries drain the pancreas, spleen and gallbladder.
After leaving the capillaries, blood in the portal circulation empties into larger veins. The splenic vein drains the spleen, and the superior and inferior mesenteric veins carry blood from the large and small intestine. Veins known as the right and left gastric veins take blood from the stomach and esophagus, while the gallbladder is drained by the cystic veins. All of these veins empty into a larger blood vessel known as the portal vein, which splits into left and right branches, both of which enter the liver.
Inside the liver, the portal circulation drains into spaces known as hepatic sinusoids, where glucose and other sugars are processed. Any excess amino acids, the building blocks of proteins, are removed and used to make a substance called urea. This is later processed by the kidneys and eliminated from the body in urine. In the sinusoids, immune system cells are present which remove any harmful microbes from the blood.
After passing through the portal circulation and the liver, blood drains into either the left or right hepatic vein. The hepatic veins leave the liver, emptying into a vein called the inferior vena cava, which is part of the systemic blood circulation that carries blood back to the heart. From the inferior vena cava, blood is delivered to the right upper chamber of the heart, or right atrium, before being ejected from the right lower chamber, or ventricle, into the pulmonary circulation of the lungs, where oxygen is added.