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Unit 6 - Digestive System - Full Textbook Notes
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Department
Biology
Course
BIOL 373
Professor
Heidi Engelhardt
Semester
Summer

Description
Unit 6 Digestive System Digestive Function and ProcessesThe gastrointestinal tract GI tract is a long tube passing through the body o The lumen and its contents are actually part of the external environmentThe primary function of the GI tract is to move nutrients water and electrolytes from the external environment into the bodys internal environmentPeptic ulcers are raw patches on the walls of the GI tract due to the failure of protective mechanisms against autodigestionThe body naturally excretes more fluid than it intakes therefore fluid must be reabsorbed or the body would rapidly dehydrateThe digestive tract must also repel foreign invaders defense mechanisms include o mucus digestive enzymes acid and the largest collection of lymphoid tissue in the body the gutassociated lymphoid tissue GALT4 basic processes of the digestive system 1 Digestion the chemical and mechanical breakdown of foods into smaller units that can be taken across the intestinal epithelium into the body 2 Absorption the active or passive transfer of substances from the lumen of the GI tract to the ECFEvidence shows that changing longterm environmental conditions effects nutrient absorption 3 Motility movement of material in the GI tract as a result of muscle contractionRegulated to maximize the availability of absorbable material if food moves through the system too rapidly there is not enough time for everything in the lumen to be digested and absorbed 4 Secretion the transfer of water and ions from the ECF to the digestive tract lumen Regulated to maximize the availability of absorbable material if digestive enzymes are not secreted in adequate amounts food in the GI tract cannot be broken down into an absorbable form Anatomy of the Digestive SystemOral Cavity mouth and pharynx o The first stages of digestion begins with chewing and the secretion of saliva by 3 pairs of Salivary glands sublingual glands under the tongue submandibular glands under the mandible jawbone and parotid glands near the hinge of the jawFood moves from the oral cavity to the GI tract o moves by waves of muscle contraction o Chyme a soupy mixture of foodsecretions from secretory epithelium the liver and the pancreasDigestion takes place primarily in the lumen of the tubeEsophagus a narrow tube that travels through the thorax to the abdomen o the walls start out as skeletal muscle and move into smooth muscle about 23 of the way down o it ends at the stomach just below the diaphragmStomach a baglike organ that can hold as much as 2 liters of food and fluid when fully expanded o Divided into 3 sections 1 The upper fundus 2 The central body 3 The lower atrium o Pylorus the opening between the stomach and the small intestine which is guarded by the pyloric valve o Pyloric valve a thickened band of smooth muscle which relaxes to allow only small amounts of chyme into the small intestine at any one timeIntegrated signals and feedback loops between the intestine and stomach regulate the rate at which chyme enters the duodenum this ensures that the intestine is not overwhelmed with more than it can digest and absorbSmall Intestine the segment of the gastrointestinal tract where most absorption and digestion take place o Divided into 3 sections 1 The duodenumsecretions from the pancreasliver enter the duodenum through ductsA contracted sphincter the sphincter of Oddi keeps pancreatic fluid and bile from entering the small intestine except during a meal 2 The Jejunum 3 The Ileum o Digestion is essentially completed in the small intestine leaving about 15 liters of chyme per day to pass into the large intestineLarge intestine the terminal portion of the intestine o colon proximal sectionwatery chyme is converted into semisolid feces as water and electrolytes are absorbed out of the chyme and into the ECF o rectum terminal section feces are propelled and distension of the rectal wall triggers a defecation reflexFeces leave the GI tract through the anus with its external anal sphincter of skeletal muscle which is under voluntary controlGut the portion of the GI tract running from the stomach to the anusThe gastrointestinal wall consists of four layers 1 An inner mucosa facing the lumenThe entire wall is crumpled into folds called rugae in the stomach and plicae in the small intestineThe intestinal mucosa also projects into the lumen in small fingerlike extensions known as villiInvaginations in the stomach are called gastric glands and in the intestine they are cryptsThe deepest invaginations form secretory submucosal glands that open into the lumen through ductsMade up ofA Epithelial cells are the most variable feature of the GI tracttransporting epithelial cells enterocytes in the small intestine secrete ions and water into the lumen and absorb ions water and nutrients into the ECF o at the mucosal apical surface cells release enzymes mucus and paracrine molecules into the lumen o at the serosal basolateral cells secrete hormones into the blood or paracrine messengers into the interstitial fluid where they act on neighboring cellsendocrine and exocrine secretory cells o leaky intestinal epithelium in the small intestine due to loose junctions water and solutes can be absorbed between the cells instead of through themCells have plasticity their tightness and selectivity can be regulatedstem cells rapidly dividing undifferentiated cells that continuously produce new epithelium in the crypts and gastric glands o as division occurs the new cells are pushed toward the luminal surface o GI epithelial cells live for only a few daysthe rapid turnover and cell division rate make it susceptible to developing cancersB Lamina Propria
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