The portal circulation (fig 2.15)
There is one portion of the systemic circulation which differs in an important respect from the general circulation pattern, namely bloodflow in the veins from the stomach and intestines.
The blood to the stomach and intestines is supplied by arteries derived from the aorta. The arteries break up in the ordinary way into capillaries in the walls of these organs and the capillaries reunite again into veins. Ultimately all the veins coming from the stomach and intestines unite to form a single large vein called the portal vein (fig 2.15).
So far this is in accordance with the general circulation pattern. But the portal vein is peculiar in that instead of joining the inferior vena cava as might be expected, it enters the liver, where it branches off just as an artery does, into smaller branches and these again branch off into a second set of capillaries in which the blood circulates through the liver. Only after the blood has passed through the liver these capillaries reunite into several large veins (hepatic veins, fig 2.15(6)) which enter the inferior vena cava. This special arrangement is named the portal circulation. Its object is to enable nutrients (excluding fats) absorbed from the stomach and intestines to pass through the liver and be processed there, before they enter the general circulation.