The hepatic portal system comprises of a system of veins, including the hepatic portal vein, which directs blood from the gastrointestinal tract to the liver. This includes the lower esophagus to the upper anal canal, as well as the spleen and pancreas. As food is digested, various nutrients are absorbed at the small intestines. These substances, carried by venous blood, will travel to the liver first for processing, before travelling to the heart to be pumped to the pulmonary system for oxygenation. This means that most nutrients (and drugs) will encounter the liver before it is delivered to the rest of the body. This has important implications for drugs, since the liver may metabolize it into an active form, or metabolize it into an inactive form. If the latter is true, oral delivery of the drug will likely be ineffective.
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