In the discussion of extravascular administration, it mentioned that it is possible that not all of the administered dose of a drug is absorbed. The percentage of the dose that is absorbed and reaches the circulation is called the bioavailability (F). A drug's bioavailability is typically less than 100. This can be due to poor absorption, or more commonly, metabolic degradation prior to the reaching the systemic circulation. Drugs given enterally pass through the portal vein and the liver prior to reaching the systemic circulation.
(F) = AUC oral / AUC intravenous
See how bioavailability effects the AUC by clicking here and selecting the AUC (module 3).