Veins generally arise from capillary beds at the tail ends of arteries.
The portal vein receives primary inputs from the splenic vein, the superior mesenteric vein, the inferior mesenteric vein, and the gatric veins. Blood in the portal vein is deoxygenated and contains toxins. The portal vein flows into the liver where another capillary bed exists. Here the blood is detoxified. This blood is both deoxygenated and detoxified.