Superior Mesenteric Vein

The superior mesenteric vein is an important vessel of the digestive system. It carries nutrient-rich and deoxygenated blood from the small intestine, the caecum, and the ascending and transverse colon to the hepatic portal vein. Thus, it is part of the portal system.

The superior mesenteric receives blood from a network of smaller venous branches that drain the intestines. These small branches join to form a long common trunk, which extends to the right of the superior mesenteric artery, with its side branches running side by side with this artery branches. Then,
it runs upwards behind the pancreas and the stomach to meet the splenic vein, with which it forms the hepatic portal vein and the hepatic portal system that runs into the liver. 

The superior mesenteric vein is different from the other veins in the body, because it does not drains straight towards the cava vena but towards the liver. The reason for this is that it conveys blood containing rich nutrients, which are the products of digestion; these nutrients have to be metabolized by the liver first before returning to the heart through the cava vein.

Beginning at the ileocaecal junction, the superior mesenteric vein receives the following secondary tributaries: the jejunal and ileal veins (about 18 in number), the right colic veins, the middle colic vein, and the right gastroepiploic vein. In most individual, the pancreaticoduodenal vein drains directly into the portal vein.

Below, diagram of the superior mesenteric vein and its branches. You can see the portal and splenic vein and the splenic vein. They are not blue as usually are depicted because they do not run into the cava vein but into the portal.