The vein that carries blood from the abdominal organs to the liver. The portal vein, which measures about 8 centimeters long, begins at the juncture of the superior mesenteric and splenic veins, passes behind the duodenum (the first part of the small intestine), and then divides into right and left branches that supply blood to the right and left lobes of the liver. The portal vein carries nutrients from the digestive tract, as well as insulin from the pancreas, to the liver.
In people who don’t have diabetes, this physical arrangement allows the liver to play a key role in helping the body to maintain normal blood glucose levels. When blood glucose levels get too high, the extra insulin the pancreas secretes goes by way of the portal vein to the liver, telling it to take up more glucose from the bloodstream and store it, and to produce less glucose. When blood glucose levels fall, the alpha cells of the pancreas make more of a hormone called glucagon, which goes straight to the liver via the portal vein, telling it to make more glucose.
The portal vein also plays an essential role in experimental islet transplantation in people with Type 1 diabetes. Donor islets are infused into the portal vein of transplant recipients. The portal vein carries the islets to the liver’s sinusoids, a network of capillaries, where they take up residence, thrive, and, ideally, secrete insulin just as a healthy pancreas does.