a storage compartment (such as bone or adipose tissue). Lastly, the xenobiotic or its metabolites may pass through into target cells causing toxic effects. Distribution varies with the route of absorption. Substances absorbed from the GI tract enter the portal circulation and go directly to the liver. Once in the liver they are subject to immediate biotransformation. This is known as the first pass effect. In contrast, substances that are absorbed by the lung or skin go into the circulatory system and distribute to all organs at the same time.