The hepatic portal system is the route blood takes through the liver. Organs of the gut (abdominal cavity) like the stomach, pancreas, and spleen send blood to the liver for detoxification. Other organs empty blood into the inferior vena cava which delivers blood into the right atrium of the heart. When this blood leaves the liver, it re-enters the venous circulation. In addition, glucose in the hepatic portal circulation is removed from the blood and stored as glycogen in the liver. Glycogen acts as a reserve energy source.
This process is extremely important because of the detoxifying action of the hepatic tissue. We unknowingly ingest many toxins that would otherwise do use great harm if not detoxed by the liver.