The portal vein is formed by the union of the superior mesenteric vein and the splenic vein.
Immediately before reaching the liver, the portal vein divides into right and left branches. It ramifies further, forming smaller venous branches and ultimately portal venules. Each portal venule courses alongside a hepatic arteriole and the two vessels form the vascular components of the portal triad. These vessels ultimately empty into the hepatic sinusoids to supply blood to the liver.