Digestion notes


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Themes from other units with overlapping functions: Transepithelial transport Types of epithelial cells Kidney function Key points from this chapter: ¾ The four digestive processes are: digestion, absorption, motility, and secretion. ¾ The three phases of digestion are: cephalic phase, gastric phase, intestinal phase. ¾ Digestion of carbohydrates, proteins, and fats takes place in multiple regions of the GI tract. Digestion of each type of biomolecule requires specific enzymes and is sensitive to pH. ¾ Absorption processes differ for each biomolecule, but absorption of most nutrients takes place in the small intestine. ¾ Motility involves peristaltic and circular (circumferential, segmental) contractions of the GI tract smooth muscle. Motility is under nervous, hormonal, and local chemical control. ¾ The GI tract secretes 7L of fluid a day. These secretions include: fluids rich in ions, enzymes and mucus. Ion secretion and absorption in the digestive system are similar to ion transport in the kidneys. ¾ There are two reflex patterns that are involved in the regulation of GI function: short reflexes and long reflexes. ¾ Numerous digestive hormones also control GI function. FUNCTION AND PROCESSES OF THE DIGESTIVE SYSTEM 1) digestion 2) absorption 3) motility 4) secretion ANATOMY OF THE DIGESTIVE SYSTEM Tissue types: Epithelial cells, smooth muscle cells, mucosa, submucosa, submuscosal plexus, myenteric plexus The Digestive System Consists of the GI Tract and Accessory Glandular Organs GI tract: esophagus, stomach, small intestine, large intestine, colon, feces, rectum, anus Accessory organs: pancreas, liver, gall bladder The GI Tract Wall Has Four Layers 1) Mucosa (mucosa, lamina propria, muscularis mucosa, villi, microvilli, gastric glands, crypts, submucosal glands) 2) Submucosa (submucosal plexus- enteric nervous system) 3) Muscularis Externa (myenteric plexus-enteric nervous system, peritoneal membrane, mesentery) 4) Serosa MOTILITY Gastrointestinal Smooth Muscle Contracts Spontaneously Key words: tonic contractions, phasic contractions, slow-wave potentials, interstitial cells of Cajal GI Smooth Muscle Exhibits Different Patterns of Contraction Key words: migrating motor complex, peristaltic contractions, segmental contractions GASTROINTESTINAL PHYSIOLOGY / DIGESTION
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SECRETION Digestive Enzymes Are Secreted in the Mouth, Stomach, and Intestine Specialized Cells Secrete Mucus Mucins, like enzymes, are made on the rough endoplasmic reticulum. They are glycosylated in the lumen of the endoplasmic reticulum and Golgi apparatus, packaged into secretory vesicles, and shuttled to the apical region of the cell by microtubules. Once at the apical surface, some vesicles release their contents immediately while others store their contents, waiting for a release signal.
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