Anatomical and physiological parameters affecting gastrointestinal absorption in humans and rats

Anatomical and physiological parameters of the gastrointestinal (GI) tract dramatically affect the rate and extent of absorption of ingested compounds . These parameters must be considered bq nutritionists, pharmacologists and toxicologists when describing or modeling absorption. Likewise, interspeci es extrapolation (e,g. from rat to human) requires species-to-species compa rison of these parameters, The present paper(l) describes the alimentary ca nal and the barrier to absorption; (2) relates the major sites of absorptio n: (3) compares the dimensions and surface areas of human and rat intestina l tracts; (3) discusses motility of the gut and transit times through regio ns of the alimentary canal: (5) explains how luminal contents are altered b y physical, chemical and metabolic processes: and (6) describes the flow of blood and lymph from the GT tract to the systemic circulation, including t he enterohepatic circulation. Despite strong morphological similarities bet ween humans and rats at the microscopic level, gross anatomical differences in the relative absorptive surface areas provide a basis for concluding th at the human CI tract is capable of absorbing materials faster and to a gre ater extent than that of the rat. Differences in the environment of the GI lumen of the two species make it possible to infer which substances are mor e likely to be present in a dissolved;non-ionized state for each species, T aken together, these differences may be of sufficient magnitude to alter th e assessment of risks,benefits for a given compound when those risks/benefi ts are based on interspecies extrapolations. (C) 2001 Elsevier Science Ltd. All rights reserved.