Gastrointestinal anatomy, physiology and permeation pathways

The oral route is by far the most common means for ingesting drugs into the body. It is also the favored route due to the low cost of drug treatment management and patient compliance resulting from the convenience of oral drug administration. Because the alimentary canal is the functional organ, constructed naturally to absorb nutrition of diverse chemical complexity, oral administration of xenobiotics appears to be simple. However, as pharmacokinetics (PK) developed since 1953 when Gold and coworkers measured digoxin bioavailability after oral administration [1,2] and as the disciplines of drug discovery and drug delivery expanded enormously, it became evident that a profound understanding of the biology and physiology of the gastrointestinal (GI) tract is crucial for optimizing the bioavailability of orally administered drugs.