| Back | The digestive system isdesigned to process food toselectively take up nutrientsand prevent the entry ofharmful substancesThe digestive system performs 5 basic digestive processes•Motility: contractions ofskeletal muscle chop andgrind food and then smoothmuscle in the wall of the tubemixes and propels the luminalcontents.•Secretion: excretion ofenzymes, mucus, and ionsinto the lumen, and secretionof hormones into the blood.•Digestion: food is brokendown into smaller moleculessuitable for absorption (alsodetoxification).•Absorption: transport ofwater, ions and nutrientsfrom the lumen, across theepithelium and into the blood.•Excretion: elimination ofundigested food and somemetabolic waste products.
 The brain-gut axis is a bidirectional system that controls GIfunction via the Autonomic/Enteric Nervous System, GIhormones, and the immune system
 The motility and secretory activity in the GI tract.of the various parts of the GI tractare controlled and coordinated byneural, endocrine, and paracrinefactors• Neural: regulation by the brain,extrinsic innervation by theautonomic nervous system, and theactivity of an enteric nervoussystem.• Endocrine: regulation via releaseof hormones by triggers associatedwith the meal. The hormones travelthrough the bloodstream to changethe activity of a distant segment ofthe GI tract, and/or organ draininginto it (e.g. the pancreas).• Paracrine: regulation by chemicalmediators that alter the function ofcells in the local area where theyare released (e.g. histamine releasefrom the cells in the stomach tocontrol acid secretion).
 The digestive tract wall has four layers andits own nervous systemEnteric Nervous System• The myenteric plexus is mainlyinvolved with control of gut motility.• The submucosal plexus coordinatesintestinal absorption and secretion.Mucosa: highly folded mucousmembrane with epithelial cellsspecialized for absorption,exocrine, and endocrinefunctions.Submucosa: connective tissuecontaining nerves and vessels.Muscularis externa: innercircular and outer longitudinalsmooth muscle layers.Serosa: outer connective tissuewith a mesothelium thatsecretes serous fluid tolubricate and prevent friction.
 The enteric nervoussystem containsneural pathways thatwork with theautonomic nervoussystem, sensoryreceptors, hormones,and paracrine factorsto coordinate themotility and secretoryactivity of the GI tractin response tochanges in theluminal contents
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 The nervous system,hormones, and paracrinefactors regulate theproduction of secretionsby the salivary glands,esophageal glands,gastric glands, GImucosa, hepatobiliarysystem, and pancreasApproximately 6 - 7 litersper day of digestive fluidsare secreted into the GIlumen
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