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For unicellular organisms, nutritional requirements can be satisfied by membrane transport proteins that permit the uptake of specific molecules. However, for multicellular organisms, the challenges of delivering nutrients to appropriate sites in the body are significantly greater. Further, most of the food we eat is in the form of macromolecules, and even when these are digested, most of the end products are water-soluble and do not readily cross cell membranes. Thus, the gastrointestinal system has evolved to permit nutrient acquisition and assimilation into the body, while prohibiting the uptake of undesirable substances.

The intestine is a continuous tube that extends from mouth to anus and is formally contiguous with the external environment. A single layer of epithelial cells comprises the semipermeable barrier across which controlled uptake of nutrients takes place. Various glandular structures empty into the intestinal lumen along its length, providing for digestion of food components, signaling to distal segments, and regulation of the intestinal microbiota. Motility functions move the intestinal contents and resulting waste products along the length of the gut, and a rich innervation regulates motility, secretion, and nutrient uptake, in many cases independent of the central nervous system. There are also many endocrine cells that release hormones that work together with neurotransmitters to coordinate overall regulation of the gastrointestinal system. In general, there is considerable redundancy of control systems as well as excess capacity for nutrient digestion and uptake. This served humans well in ancient times but may now contribute to the modern epidemic of obesity.

The liver, while playing important roles in whole body metabolism, is usually considered a part of the gastrointestinal system for two main reasons. First, it provides for excretion from the body of lipid-soluble waste products that cannot enter the urine. Second, the blood flow draining the intestine is arranged such that substances that are absorbed pass first through the liver, allowing for the removal and metabolism of any toxins that have been taken up, as well as clearance of particulates, such as small numbers of enteric bacteria

In this section, the function of the gastrointestinal system and liver will be considered, and the ways in which the various segments communicate to provide an integrated response to a meal. The relevance of gastrointestinal physiology for the development of digestive diseases will also be considered. Digestive diseases represent a substantial burden in terms of morbidity and lost productivity. A 2009 report of the US National Institutes of Diabetes, Digestive and Kidney Diseases found that on an annual basis, for every 100 US residents, there were 35 ambulatory care visits and nearly five overnight hospital stays that involved a gastrointestinal diagnosis. Digestive diseases also appear to be increasing in this population (although mortality, principally from cancers, is thankfully in decline). On the other hand, digestive diseases, and in particular infectious diarrhea, remain important causes of mortality in developing countries where clean sources ...

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