RT Book, Section A1 Barrett, Kim E. A1 Barman, Susan M. A1 Boitano, Scott A1 Brooks, Heddwen L. SR Print(0) ID 1115831033 T1 Gastrointestinal Physiology: Introduction T2 Ganong’s Review of Medical Physiology, 25e YR 2018 FD 2018 PB McGraw-Hill Education PP New York, NY SN 9780071825108 LK accessmedicine.mhmedical.com/content.aspx?aid=1115831033 RD 2024/03/28 AB For unicellular organisms that exist in a sea of nutrients, it is possible to satisfy nutritional requirements simply with the activity of membrane transport proteins that permit the uptake of specific molecules into the cytosol. However, for multicellular organisms, including humans, the challenges of delivering nutrients to appropriate sites in the body are significantly greater, particularly if the organisms are terrestrial. Further, most of the food we eat is in the form of macromolecules, and even when these are digested to their component monomers, most of the end products are water-soluble and do not readily cross cell membranes (a notable exception are the constituents of dietary lipids). Thus, the gastrointestinal system has evolved to permit nutrient acquisition and assimilation into the body, while prohibiting the uptake of undesirable substances (toxins and microbial products, as well as microbes themselves). The latter situation is complicated by the fact that the intestine maintains a lifelong relationship with a rich microbial ecosystem residing in its lumen, a relationship that is largely mutually beneficial if the microbes are excluded from the systemic compartment.