The Liver: Anatomical First Stop for Orally administered Drugs: The first pass effect:

Oral drugs -> absorbed by small intestines -> Portal venous system -> Delivers absorbed molecules -> to the liver -> transforms most of the chemicals by a series of liver enzymes -> breaks the drugs into metabolites -> some will cause effect and some will be deactivated and readily excreted from the body -> as a result, large percentage of the oral dose is destroyed and never reaches the tissues. -> the portion of the drug that gets through is delivered to the circulatory system for transport throughout the body.