The hepatic portal system refers to the area of the gastrointestinal system that is serviced by veins which take substances to the liver to be processed before sending them to the heart. This system of veins starts with the lower part of the esophagus and extends down to the top part of the anal canal. The significance of this is if you have a medicine or substance that is supposed to go directly to all the cells of the body, you better get it past the liver first. For example, nitrocglycerin can not be swallowed, because it would be captured by the hepatic portal system and sent to the liver, where it would be inactivated. Nitroglycerin must be placed under the tongue, where it would be absorbed directly into the blood stream, or administered transdermally with a patch on the skin. Another way of bypassing the hepatic portal system is with the use of suppositories, which would get the medicine directly into the blood stream.