The hepatic portal circulation is important in that it captures substances from the digestive system and sends them to the liver to be metabolized. Depending on the substance, such as medication, different methods may be used to introduce them into the body. Nitroglycerin, for example, may not be take orally because it would be absorbed by the hepatic portal vein and taken to the liver, which would inactivate it. Nitroglycerin, therefore is placed under the tongue and allowed to dissolve directly into the bloodstream. Either that, or it may be administered transdermally, by a patch applied to the skin. Suppositories are another way to bypass the hepatic portal system, because the lower two-thirds of the digestive system drains into another vein, which feeds directly to the inferior vena cava.