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DIGESTIVE SYSTEM • • • • • • • • • • • • • Digestive System muscular tube digestive tract GI tract gastrointestinal tract alimentary canal runs from oral cavity pharynx, esophagus, stomach, small & large intestines, rectum to anus includes accessory organs teeth tongue salivary glands liver gall bladder pancreas Functions • provides fuel to keep cells running • provides building blocks for growth & repair • removes residue • protective function for bacteria – largest immune organ • • • • • Functions four integrated steps Ingestion – intake of food Digestion – mechanical • physical manipulations • teeth tear & crush • stomach mixes & churns – chemical breakdown • physically manipulated materials broken into smaller fragments by enzymes & acids Absorption – uptake of nutrients – movement of organic substances, ions, vitamins & water into blood Defecation – excretion of undigested residue – removal of waste products Histological Organization • located in peritoneal cavity • lined by serous membrane • visceral peritoneum covers organs • parietal peritoneum lines inner body wall surface • 4 layers • mucosa • submucosa • muscularis externa • serosa • • • • • Layers of Wall Mucosa – innermost epithelial lining – consists of inner epithelium & thin layer of smooth muscle-muscularis mucosae Epithelium-simple columnar in most of tract – from oral cavity through esophagus & lower anal canal-stratified Submucosa – loose connective tissue containing blood & lymph vessels with nerve plexus Muscularis externa – 2 layers of smooth muscle arranged in inner, circular layer & outer, longitudinal layer – important in mechanical processing & movement of materials along tract. – lining of muscularis externa is thrown into foldsincreases surface area Serosa – inner layer of loose connective tissue – adipose tissue & outer epithelial layer Nervous System Control of Digestive Functions • sympathetic stimulation inhibits gastrointestinal secretion, motor activity & contraction of gastrointestinal sphincters & blood vessels • parasympathetic stimuli stimulate these Nervous Control of Digestive Functions • digestive system has its own, local nervous system • enteric nervous system • can function independently of CNS • regulates motility, secretion & blood flow in tract • has more neurons than spinal cord • comprised of two nerve networks • submucosal plexus in submucosa • myenteric plexus between two layers of muscularis externa • Motility in Digestive Tract • Peristalsis – progressive contraction of circular & longitudinal muscles – propels bolus along tract – circular muscles contract behind bolus while circular muscle ahead relaxes Motility in Digestive Tract • Segmentation – contraction & relaxation of nonadjacent segments of tract – moves contents forwards & backwards – mixes & churns bolus – breaks into fragments – mixes with intestinal secretions Ingestion • eating • begins food processing • begins chemical & mechanical digestion • oral or buccal cavity • responsible for analysis of material prior to swallowing • mechanical processing via tongue & teeth • lubrication-mixing ingested material with saliva & mucus • limited digestion of carbohydrates & lipids • lined with stratified squamous epithelium • roof-hard & soft palate • floor-tongue Tongue • mechanical processing • manipulation to assist chewing • sensory analysis – touch, taste, temperature • secretion of lingual lipase – begins lipid breakdown Salivary Glands • make saliva • controlled by ANSparasympathetic nervessalivary reflex • Lubricating • Moistening • Parotid – serous, watery secretion containing salivary amylase (starchmaltose) • Submandibular – secretion contains mucus & amylase • Sublinguals – mucus secretion – buffer & lubricant Teeth • aid in mechanical digestion by mastication or chewing • breaks down connective tissues in plant fibers & meat • helps saturate materials with salivary secretions & enzymes • permits easier deglutition • during mastication, salivary glands secrete salivasoften food into a bolus (semi-solid lump) Swallowing-Deglutition • involves over 22 muscles in mouth, pharynx & esophagus • controlled by swallowing center in medulla & pons • occurs in three phases • Buccal • Pharyngeal • Esophageal Buccal Phase • voluntary • tongue pushes just formed bolus toward oropharynx • bolus stimulates tactile receptors • activates next phase Pharyngeal Phase • tactile receptors send impulses to deglutition center in medulla • impulses returning from center cause soft palate & uvula to more upward closing off nasopharynxprevents food from entering nasal cavity • epiglottis covers glottisopening to larynx • bolus driven downward by constriction of upperthen middle then lower pharyngeal constrictors • as bolus slides into esophagus Esophageal Phase • esophagus stretchestriggers peristalsis pushes bolus ahead of it • peristalsis carries bolus from upper esophageal sphincter through esophagus to lower esophageal sphincterstomach Stomach • bolus passes through lower esophageal sphincter into stomach • expanded section of digestive tube between esophagus & small intestine Stomach Functions • bulk storage – stores ingested food in upper part • digestion – mechanical breakdown of ingested food-lower parts – disrupts chemical bonds by acids & enzymes • produces intrinsic factor – for vitamin B12 absorption • • • • • Stomach Parts Cardia – smallest region – contains mucous glands • protects from stomach acids & enzymes Fundus – makes contact with diaphragm Body – largest region – mixing tank – contains gastric glandsacids & enzymes Pyloris – leads into duodenum of small intestine inside empty stomach mucosa & submucosa are thrown into folds called rugae – distended when food is in stomach – allow stomach to expand Stomach Wall • covered with simple columnar glandular epithelium • gastric mucosa is covered with numerous small holes – openings of gastric pits • two or three tubular glands open into bottom of each gastric pit • gastric glands – secrete mucus – secrete acid & enzymes Gastric Gland Cells • Mucous Neck Cells • Parietal • Chief • Enteroendocrine Parietal Cells • secrete hydrochloric acid • assists in break down of food • not made in cytoplasm – too strong – would dissolve secretory vessel & destroy cell • H+ & Cl- are made & then secreted out of cell & assembled – maintains pH between 0.8 & 2.0 – kills microorganisms – breaks down plant cell walls & connective tissue in meats – essential for pepsin • converts pepsinogen to pepsin • make intrinsic factor – needed for absorption of vitamin B12 Cells of Gastric Pit • Chief Cells • secrete pepsinogen – inactive precursor of pepsin – digests proteins • Enteroendocrine Cells-G cells – make gastrin Absorption in Stomach • little absorption – aspirin & ethanol • absorption does not occur because • cells are covered by mucus blanket & therefore never contact chyme directly • cells do not have transport mechanisms needed to absorb materials • gastric lining is impermeable to water • digestion has not been completed – digested food pieces too big Digestion in Stomach • mechanical digestion-churns bolus & mixes it with digestive juices • chemical digestionbreaks bonds • food digests in stomach for several hours • preliminary digestion of proteins by pepsin • not completed – limited time substances are in stomach – pepsin attacks only specific types of peptide bonds • digestion of carbohydrates & lipids by salivary amylase & lingual lipase • enzymes continue to digest until pH falls below 4.5 Chyme Formation • bolus + secretions soupy mixture-chyme • each peristaltic wave delivers a bit of chyme to small intestine through pyloric sphincter-gastric emptying • chemical digestion in small intestine depends on activity of pancreas, liver & gall bladderaccessory digestive organs • • • • • • Pancreas lies posterior to greater curvature of stomach exocrine acini cells secrete 2001500 ml of juice/day secreted into small ducts-unite to form larger ducts- pancreatic & accessory pancreatic duct joins common bile duct from liver & gall bladder enters duodenum as hepatopancreatic ampulla passage of pancreatic juice & bile through this into small intestine is controlled by sphincter of the hepatopancreatic ampulla or Sphincter of Oddi. Pancreatic Juice • mixture of water, salt, enzymes, zymogens & sodium bicarbonate • secretions controlled by hormones of duodenum • chymeduodenum secretin pancreas watery buffer pH 7.58.8 raises pH of chyme • chymeduodenumC CK pancreas pancreatic enzymes Pancreatic Secretions • pancratic amylase – starch breakdown • ribonuclease & deoxyribonuclease – nucleic acid breakdown • pancreatic lipase – lipid breakdown • zymogens: trypsinogen & carboxypeptidase • a brush border enzymeenterokinase cleaves trypsinogentrypsin • trypsin then works on other inactive precursorsactive ones Liver & Gall Baldder • accessory digestive organs • liver-inferior to diaphragm • gall bladder-in a depression on posterior surface of liver • Liver-two principle lobes-a larger right & a smaller left lobe • connected by a mesentery fold-falciform ligament • right lobe includes an inferior quadrate & a posterior caudate lobe Liver & Gall Bladder • hepatocytes-major functioning cells of liver make 800 -1000 mls of bile each day • bile leaves liver via right & left hepatic ducts which unite as common hepatic duct • these join with the cystic duct from the gall bladder to form the common bile duct • bile is stored & modified in the gall bladder • enters small intestine via cystic duct • does not enter small intestine until gallbladder contracts • principal stimulus for releasecholecystokinin CCK Emulsification • bile contains water, bile salts, bile pigments, cholesterol, lecithin & several ions • bile salts are important in digestion of lipids • lipids are not water soluble • mechanical processing results in large drops • bile salts breaks down large lipid globules into a suspension of smaller lipid globules • process called emulsification • increases surface area available for enzymatic attack • digested lipids are absorbed in lacteals of the small intestine • • • • • • • • • Liver Functions carbohydrate metabolism – stabilizes blood glucose by glycogenolysis & gluconeogenesis lipid metabolism – removes lipids for storage or breaks down lipids when needed amino acid metabolism – removes excess amino acids removes waste products – amino acidsammonia – neutralizes ammonia by converting it to urea – important in drug inactivation vitamin storage fat soluble vitamins-A, D, E & K & B12 mineral storage – stores iron bound to ferritin phagocytosis & antigen presentation – Kupffer cells engulf old RBCs, debris, etc, & stimulates immune system synthesis of plasma proteins Small Intestine • about 20 feet long – 90% of nutrient absorption • Duodenum – next to stomach – mixing bowl • Jejunum – bulk of chemical digestion & absorption • Ileum – longest part – ends at ileocecal valve • sphincter controlling release of substances into large intestine Small Intestine Lining • folded into transverse foldsplicae • permanent • increase surface area for absorption • covered by simple columnar epithelium • microvilli project from cells of epithelium forming brush border • increase surface area more • allow chyme to contact more of small intestine wall • increased contact means more efficient food absorption Brush Border Small Intestine Epithelium • absorptive cells • digest & absorb nutrients in chyme • goblet cells • make mucus Intestinal Villi • mucosa is thrown into folds forming intestinal villi • Core contains lymph vessel- lacteal – absorbs products of fat digestion • at base-entrance to intestinal glandscrypts of Lieberkuhn – secrete 1-2 liters of intestinal juice each day – produce brush border enzymes • Paneth cells – secrete lysozyme • Enteroendocrine cells – S cells • make secretin – CCK cells • make cholecystokine – K cells • make GIP-glucose-dependentinsulinotropic peptide Duodenum • contains duodenal or Brunners glandsmucous • primary function of duodenum • receive chyme & neutralize acids Ileum • contains Peyer’s patches • aggregates of lymphoid nodules • protection from bacteria Movement in Small Intestine • as chyme enters duodenumperistaltic contractions (migrating motility complexes) move it toward jejunum • segmentation-contraction & relaxation of nonadjacent segments of the tract mixes & churns the material breaking it into fragments & mixing it with intestinal secretions Chemical Digestion & Absorption • begins in mouth • salivary glandssalivary amylase • polysaccharides broken into di- & tri- saccharides • salivary amylase & pancreatic amylase continue to break down in stomach • a brush border enzyme-α dextrinase clips off one glucose at a time making di & tri saccharides • monosaccharides are made by brush border enzymes specific for specific disacchardies • maltase splits maltose • sucrase splits sucrose • absorbed by facilitated transport Chemical Digestion & Absorption • fat digestion begins in mouthlingual lipase • continues in stomach • fat enters small intestine as coarse emulsion created by lipase digestion & mechanical mixing • lipid droplets too big to be absorbed or further broken down • bile salts (phospholipids) coat emulsion stabilizing fat droplets • pancreatic lipase digests triglycerides in emulsion • triglyceridesmonoglycerides & free fatty acids • absorbed by simple diffusion into lacteals Chemical Digestion & Absorption • protein digestion begins in mouth – mechanical processing • chemical processing – takes place in stomach with HCl • pepsin continues digestion in stomach breaking peptide bonds • other proteases & peptidases found on brush border of intestinal villa cells-trypsin, elastin, chymotrypsin continue to breakdown protein bonds • carboxypeptidases break off individual amino acids from ends of peptides • absorbed via facilitated diffusion Large Intestine • begins at end of ileum • ends at anus • attached to posterior abdominal wall by mesocolon • ileocecal sphincter regulates passage of chyme from the small to the large intestine • relaxes when food leaves stomachgastroileal reflex • cecum-first part • hangs inferior to ileocecal valve • attached-appendix • functions: reabsorbs water • compacts intestinal contents into feces • absorbs vitamins • stores fecal materials Large Intestine-Colon • • • • • • colon-largest part ascending colon transverse colon descending colon sigmoid colon muscularis layer has inner circular layer & longitudinal muscle layer concentrated into 3 bands called teniae coli • contraction of these pull wall into bulging pockets or haustra • permits expansion & elongation of colon Large Intestine-Rectum & Anus • Rectum – short, terminal expandable part of large intestine • Anus – separates large intestine from outside • closed by 2 sphincters • internal smooth muscle sphincter – not under voluntary control • external skeletal muscle sphincter – under voluntary control Digestion & Absorption in Large Intestine • takes 12 -24 hours to reduce residue of meal into feces • no significant digestion of organic molecules-less than 10% • reabsorbs electrolytes • reabsorbs water • digestive system is one way body maintains water balance • receives about 9 liters of water each day • 7.5 L reabsorbed by small intestine • 1500 ml of material enters colon each day & only 200 ml are excreted with feces Motility in Large Intestine • movement begins when • . substances pass ileocecal sphincter • hastral churning – haustra remain relaxed & begin to fill upwhen distension reaches certain pointwalls contract & squeeze contents of one haustra into another • mass peristalsis • occur 3-4X/day • called the gastrocoli reflexes • • • • • • • • Defecation final digestive process stage processed feces (undigested waste products) carried from sigmoid colon to rectum distends rectal wallstimulates stretch receptorssends nerve impulses to sacral spinal cord impulses return via parasympathetic fibers to colon, rectum & anus causes contraction of longitudinal muscles causes rectum to shorten increases pressure in it & opens internal anal sphincter external anal sphincter must relax to complete the process feces are expelled Phases of Gastric Activity • gastric activity occurs in three overlapping stages • Cephalic • Gastric • Intestinal Cephalic Phase • directed by CNS • see, smell or anticipate foodcerebral cortex, hypothalamus and brain stemfacial, glossopharyngeal & vagus nerves • facial, glossopharyngeal nervessalivary glandssaliva • vagus nervegastric glandsgastric juice production • lasts only a few minutes Gastric Phase • begins with arrival of food in stomach • stomach distendsstretch receptors • increased pH & undigested materials in stomachchemoreceptors • Stretch & chemoreceptorssubmucos al plexus parasympathetic & enteric neureonsincrease peristaltic waves & stimulates secretion of gastric juice • phase Intestinal Phase • begins when chyme enters duodenum • phase controls rate of gastric emptying • important for small intestine to slow down gastric emptying to give time to neutralize acid & to efficiently absorb incoming nutrients – duodenum, jejunum & ileum are able to process only small amounts of material at any one time • stretching duodenum activates the enterogastric reflex • stretch receptorsmedullainhibit parasympathetic & stimulate sympathetic fibersinhibits gastric motility Intestinal Phase • controlled by : secretin & cholecystokinin • chyme containing amino acids & fatty acidsIntestinal glands (CCK cells) CCKstsimulates secretion of pancreatic juice, causes gall bladder to contract, relaxes Sphincter of Oddi-slows gastric emptying • immediate resultsno more chyme released from stomach • lowered pH (from chyme) Intestinal glands (S cells)secretin stimulates flow of pancreatic juicesbicarbonate buffers the acidic chyme • secretininhibits secretion of gastric juice and enhances effects of CCK Other Gut Hormones • tract makes at least 10 other gut hormones • motilin, substance P, bombesinaffect motility of intestine • VIP (vasoactive intestinal peptide)stimulates secretion of ions & water;inhibits acid production by stomach • gastrin releasing peptide stimulates gastrin release • somatostatininhibits gastrin release