The Digestive System PDF

The Digestive System General Anatomy and Digestive Processes Mouth Through Esophagus Stomach Liver, Gallbladder and Pancreas Small Intestine Large Intestine 25-1  The Digestive System most nutrients we eat cannot be used in their existing form...

The Digestive System General Anatomy and Digestive Processes Mouth Through Esophagus Stomach Liver, Gallbladder and Pancreas Small Intestine Large Intestine 25-1  The Digestive System most nutrients we eat cannot be used in their existing form – must be broken down into smaller parts the digestive system is essentially a “disassembly line” – to break down nutrients and absorb them gastroenterology – the study of the digestive tract and the diagnosis and treatment of its disorders 25-2  Digestive Function digestive system – the organ system that processes food, extracts nutrients from it, and eliminates the residue 25-3  Facets of Digestion mechanical digestion – physical breakdown of food – cutting and grinding action of the teeth – churning action of stomach and small intestines chemical digestion – a series of hydrolysis reactions that breaks dietary macromolecules into their monomers – carried out by digestive enzymes polysaccharides into monosaccharides proteins into amino acids fats into monoglycerides and fatty acids nucleic acids into nucleotides some nutrients in food are in a usable form: – vitamins, free amino acids, minerals, cholesterol 25-4  General Anatomy digestive system has two Oral cavity Tongue Parotid anatomical subdivisions Teeth gland Sublingual gland Pharynx Submandibular digestive tract (alimentary gland Esophagus canal) – 30-foot long muscular tube Diaphragm from mouth to anus – mouth, pharynx, esophagus, Liver Stomach Pancreas stomach, small intestine, Gallbladder Transverse Bile duct and large intestine Ascending colon colon Descending Small intestine colon accessory organs Cecum Appendix – teeth, tongue, salivary Sigmoid colon Rectum glands, liver, gallbladder, Anal canal Anus and pancreas 25-5 Relationship to Peritoneum mesenteries – connective tissue sheets that loosely suspend the stomach and intestines from the abdominal wall – allows stomach and intestines to undergo strenuous contractions – hold abdominal viscera in proper relationship to each other – prevents the intestines from becoming twisted and tangled – passage for blood vessels and nerves – contain many lymph nodes and lymphatic vessels 25-6 Relationship to Peritoneum parietal peritoneum – serous membrane that lines the wall of abdominal cavity – turns inward along posterior midline – forms dorsal mesentery – a translucent two- layered membrane extending to the digestive tract – the two layers of the mesentery separate and pass around opposite sides of the organs forming the serosa – come together on the far side of the organs and continue as another sheet of tissue – the ventral mesentery 25-7 Mesentery and Mesocolon Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Greater omentum (retracted) Transverse colon Mesocolon Descending colon Jejunum Mesentery Sigmoid colon (b) mesentery holds many blood vessels mesocolon anchors colon to posterior body wall Lesser & Greater Omentum Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Liver Stomach Gallbladder Lesser omentum Greater Ascending colon omentum Small intestine (a) lesser omentum- attaches stomach to liver greater omentum- covers intestines like an apron  The Tongue – manipulates food between teeth while it avoids being bitten – extracts food particles from the teeth after a meal – very sensitive – nonkeratinized stratified squamous epithelium covers its surface – lingual papillae – bumps and projections on the tongue that are the sites of the taste buds; gustation – the act of tasting 25-10  Teeth (know 4 types) Humans grow two sets of teeth: primary (baby) teeth (20) secondary (permanent) teeth (32) 25-11  Tooth and Gum Disease the human mouth is home to more than 700 species of microorganisms, especially bacteria plaque – sticky residue made of bacteria and sugars – calculus – calcified plaque – Bacteria eat sugars and release acids that dissolve minerals of tooth (dental caries or cavities) – root canal therapy is necessary if cavity reaches pulp calculus in the gingival sulcus: – allows bacterial invasion of the sulcus – gingivitis – inflammation of the gums – periodontal disease – destruction of the supporting bone around the teeth (may result in tooth loss) 25-12  Mastication mastication (chewing) - breaks food into smaller pieces to be swallowed and exposes more surface to the action of digestive enzymes – first step in mechanical digestion – food stimulates oral receptors that trigger an involuntary chewing reflex 25-13  Saliva – moistens mouth – begins starch and fat digestion – cleanses teeth and inhibits bacterial growth – dissolves molecules so they can stimulate the taste buds – moistens food and binds it together to be swallowed 25-14  Saliva hypotonic solution of water and the following solutes: – salivary amylase – enzyme that begins starch digestion – lingual lipase – enzyme that is activated by stomach acid and digests fat after the food is swallowed – mucus – binds and lubricates the mass of food – lysozyme – enzyme that kills bacteria – immunoglobulin A (IgA) – an antibody – electrolytes - Na+, K+, Cl-, phosphate and bicarbonate 25-15  Salivary Glands Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Parotid gland Parotid duct Tongue Sublingual ducts Masseter muscle Submandibular duct Submandibular gland Sublingual Opening of gland submandibular 25-16 duct  Pharynx pharynx – a muscular funnel that connects oral cavity to esophagus and allows air flow from nasal cavity to larynx pharyngeal constrictors (superior, middle, and inferior) - circular muscles that force food downward during swallowing – when not swallowing, the inferior constrictor remains contracted to exclude air from the esophagus 25-17 Pharynx 25-18  Esophagus a straight muscular tube 25-30 cm long – extends from pharynx to cardiac orifice of stomach, passing through esophageal hiatus in diaphragm – lower esophageal sphincter – food pauses here prevents stomach contents from regurgitating protects esophageal mucosa from harsh stomach acid heartburn – acid reflux into the esophagus – esophageal glands in submucosa secrete mucus – deeply folded into longitudinal ridges when empty – skeletal muscle in upper third, mixture in middle third, and only smooth muscle in the bottom third 25-19  Swallowing (Deglutition) swallowing (deglutition) – a complex action involving over 22 muscles in the mouth, pharynx, and esophagus swallowing occurs in two phases: – buccal phase – under voluntary control – pharyngoesophageal phase – is involuntary peristalsis – wave of muscular contraction that pushes the bolus ahead of it 25-20  Soft palate Uvula Epiglottis Bolus of food Relaxation Pharynx Esophagus Tongue2 Bolus passes into pharynx. Misdirection of bolus is Epiglottis prevented by tongue blocking oral cavity , soft Glottis palate blocking nasal cavity, and epiglottis blocking larynx. 1 Tongue compresses Trachea food against palate to form a bolus. 3 Upper esophageal sphincter constricts and bolus passes Constriction downward. Peristaltic 4Peristalsis drives bolus down wave esophagus. Esophagus Bolus constricts above bolus and dilates and shortens below it. Relaxation Shortening Swallowing Lower esophageal 5 sphincter relaxes to admit bolus to stomach. Constriction Stomach Lower esophageal Cardiac sphincter orifice 25-21 (a) Relaxation X-ray: Swallowing in Esophagus Upper esophagus Peristaltic contraction Bolus of ingested matter passing down esophagus (b) 25-22 © The McGraw-Hill Companies, Inc.,/Jim Shaffer, photographer  Upper GI Disorders GERD: gastroesophageal reflux disease: frequent reflux of stomach contents into esophagus causes heartburn, esophageal erosion treatment: lifestyle changes, many prescription drugs Hiatal hernia: part of stomach protrudes superior to the diaphragm may cause heartburn, in some cases may “strangle” part of the stomach tissue surgery needed if symptoms are severe 25-23  Stomach a muscular sac in upper left abdominal cavity just inferior to the diaphragm – primarily a food storage organ 1.0 – 1.5 L after a typical meal up to 4 L when extremely full mechanically breaks up food, liquifies it, & begins chemical digestion of protein/fat – chyme – soupy or pasty mixture of semi-digested food in the stomach most digestion occurs after the chyme passes on to the small intestine 25-24 Gross Anatomy of Stomach J-shaped muscular organ with lesser and greater curvatures – divided into four regions cardiac region (cardia) –area near cardiac orifice fundic region (fundus) – dome-shaped portion superior to esophageal attachment body (corpus) – greatest part of the stomach pyloric region – narrower pouch at inferior end – pyloric (gastroduodenal) sphincter – regulates the passage of chyme into the duodenum 25-25  Gross Anatomy of Stomach Diaphragm Lesser omentum Fundic region Cardiac region Lesser curvature Body Pyloric region: Longitudinal Antrum muscle 3 Pyloric canal Circular muscle muscle Pylorus Pyloric layers sphincter Oblique muscle Gastric rugae Greater curvature Greater omentum Duodenum (a) 25-26 ? 25-27  Microscopic Anatomy simple columnar epithelium covers mucosa – apical regions of its surface cells are filled with mucin – swells with water and becomes mucus after it is secreted mucosa and submucosa flat when stomach is full, but form longitudinal wrinkles called gastric rugae when empty 25-28  Microscopic Anatomy gastric pits – depressions in gastric mucosa – lined with simple columnar epithelium – two or three tubular glands open into the bottom of each pit – cardiac glands in cardiac region – pyloric glands in pyloric regions – gastric glands in the rest of the stomach 25-29  Microscopic Anatomy Lumen of stomach Epithelium Gastric pit Mucosa Gastric gland Lamina propria Submucosa Lymphatic nodule Muscularis Muscularis mucosae externa Serosa Artery Vein Oblique layer of muscle Circular layer of muscle Longitudinal layer of muscle (a) Stomach wall 25-30 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Opening of Gastric Pit Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Visuals Unlimited 25-31 Pyloric and Gastric Glands Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Mucous neck cell Parietal cell Mucous cell Chief cell G cell (b) Pyloric gland (c) Gastric gland 25-32  Cells of Gastric Glands mucous cells – secrete mucus Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. G (endocrine) cells Mucous neck cell – secrete hormone gastrin Parietal cell parietal cells – secrete HCl, intrinsic factor, and a hunger hormone called ghrelin Chief cell chief cells – most numerous – secrete gastric lipase and pepsinogen G cell (c) Gastric gland 25-33  Gastric Secretions gastric juice – 2 – 3 liters per day produced by the gastric glands mainly a mixture of water, hydrochloric acid, and pepsin 25-34  Hydrochloric Acid gastric juice has a high conc. of hydrochloric acid parietal cells produce HCl & contain the enzyme carbonic anhydrase (CAH) CAH – CO + H O H CO HCO - + H+ 2 2 2 3 3 – H+ is pumped into gastric gland lumen by H+- K+ ATPase pump (proton pump) – HCO3- exchanged for Cl- (chloride shift) from blood plasma Cl- joins H+ forming HCl alkaline tide from elevated HCO3- (bicarbonate ion) in blood leaving stomach 25-35  Hydrochloric Acid Blood Parietal cell Lumen of gastric gland Drugs for Alkaline tide chronic heartburn Cl– Cl– called proton-pump Stomach inhibitors acid K+ slow this step HCO–3 – HCO3 H+ H+–K+ ATPase CO2 CO2+H2O H2CO3 25-36  Functions of HCl activates pepsin and lingual lipase breaks up connective tissues and plant cell walls – helps liquefy food to form chyme converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+) – Fe2+ used for hemoglobin synthesis contributes to disease resistance by destroying many pathogens 25-37  Pepsin zymogens – digestive enzymes secreted as inactive proteins – converted to active enzymes by removing some of their amino acids – pepsinogen – zymogen secreted by chief cells HCl removes some of its amino acids and forms pepsin that digests proteins autocatalytic effect – as some pepsin is formed, it converts more pepsinogen into more pepsin 25-38  Production and Action of Pepsin Parietal cell Removed Dietary peptide proteins HCl Pepsin (active enzyme) Chief cell Pepsinogen (zymogen) Partially digested protein Gastric gland Pepsin cuts dietary proteins into shorter peptide chains - protein digestion is completed in the small intestine  Gastric Lipase gastric lipase – produced by chief cells gastric and lingual lipase play a minor role in digesting dietary fats – digests 10% - 15% of dietary fats in the stomach – rest digested in the small intestine 25-40  Intrinsic Factor a glycoprotein secreted by parietal cells needed for absorption of vitamin B12 by the small intestine – binds vitamin B12 and then intestinal cells absorb this complex by receptor-mediated endocytosis vitamin B12 needed to make hemoglobin – prevents pernicious anemia secretion of intrinsic factor is the only indispensable function of the stomach 25-41  Chemical Messengers gastric and pyloric glands have various kinds of enteroendocrine cells that produce 20+ chemical messengers – some are hormones - stimulate distant cells – others are paracrine secretions that stimulate neighboring cells – several are peptides produced in both the digestive tract and the central nervous system – gut-brain peptides 25-42  Gastric Motility When we eat, swallowing center of medulla oblongata signals stomach to relax food stretches stomach, activating a receptive- relaxation response soon stomach shows a rhythm of peristaltic contractions controlled by pacemaker cells – after 30 minutes or so these contractions become quite strong they churn the food, mix it with gastric juice, and promote its physical breakup and chemical digestion 25-43  Gastric Motility – antrum of pyloric region holds ~ 30 mL of chyme – as a parastaltic wave passes down the antrum, it squirts ~3 mL of chyme into the duodenum at a time – allowing only a small amount enables duodenum to: neutralize the stomach acid digest nutrients little by little – if duodenum is overfilled it inhibits gastric motility – typical meal emptied from stomach in 4 hours less time if the meal is more liquid as long as 6 hours for a high-fat meal 25-44  Vomiting vomiting (emesis) – the forceful ejection of stomach and intestinal contents (chyme) from the mouth induced by: – overstretching of the stomach or duodenum – chemical irritants such as alcohol and bacterial toxins – visceral trauma – intense pain or psychological and sensory stimuli – controlled by center in medulla oblongata 25-45  Vomiting abdominal contractions and rising thoracic pressure force the upper esophageal sphincter to open – chyme is driven upward by abdominal contractions combined with reverse peristalsis chronic vomiting causes: – dangerous fluid, electrolyte, and acid-base imbalances – bulimia – eating disorder - tooth enamel becomes eroded by the hydrochloric acid in the chyme – aspiration (inhalation) of acid is very destructive to the respiratory tract 25-46  Protection of the Stomach stomach is protected in three ways from its harsh acidic and enzymatic environment – mucous coat – thick, highly alkaline mucus resists action of acid and enzymes – tight junctions - between epithelial cells prevent gastric juice from seeping between them and digesting the connective tissue – epithelial cell replacement – stomach epithelial cells live only 3 to 6 days breakdown of these protective measures can result in inflammation and peptic ulcer 25-47  Healthy Mucosa and Peptic Ulcer (a) Normal (b) Peptic ulcer gastritis, inflammation of the stomach, can lead to a peptic ulcer as pepsin and HCl erode the stomach wall most ulcers are caused by acid-resistant bacteria, Helicobacter pylori , that can be treated with antibiotics and Pepto-Bismol. 25-48 ? 25-49 Digestion and Absorption most digestion and nearly all absorption occur after the chyme has passed into small intestine stomach does not absorb any significant amount of nutrients alcohol is absorbed mainly by small intestine – intoxicating effects depend partly on how rapidly the stomach is emptied 25-50  Liver, Gallbladder, and Pancreas small intestine receives chyme from stomach also secretions from liver and pancreas – enter digestive tract near the junction of stomach and small intestine 25-51  The Liver liver – reddish brown gland located immediately inferior to diaphragm the body’s largest gland – weighs about 1.4 kg (3 pounds) variety of functions – secretes bile which contributes to digestion 25-52  Gross Anatomy of Liver four lobes - right, left, quadrate, and caudate gall bladder – adheres to a depression on the inferior surface of the liver, between right and quadrate lobes 25-53  Gross Anatomy of Liver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Inferior vena cava Bare Right lobe Caudate lobe area Left lobe Falciform ligament Quadrate lobe Gallbladder Right lobe (b) Anterior view (c) Posterior view 25-54  Microscopic Anatomy of Liver hepatic lobules – tiny cylinders that fill interior of the liver – about 2 mm long and 1 mm in diameter – consist of: central vein – passing down the core hepatocytes – cuboidal cells surrounding central vein in radiating sheets or plates hepatic sinusoids – blood-filled channels that fill spaces between the plates – lined by a fenestrated endothelium (good for adding liver proteins to blood) – blood flowing through the sinusoids comes directly from intestines 25-55  Microscopic Anatomy of Liver Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Hepatocytes Central vein Bile canalicul Hepatic triad: Branch of Hepatic hepatic sinusoid portal Branch vein of hepatic artery Bile ductule (a) 25-56 Functions of Hepatocytes after a meal, hepatocytes absorb from the blood: – glucose, amino acids, iron, vitamins, & other nutrients for use or storage remove and degrade: – hormones, toxins, bile pigments, and drugs secrete into the blood: – albumin, lipoproteins, clotting factors, angiotensinogen, and more between meals, hepatocytes break down stored glycogen and release glucose into the blood 25-57  Bile Ducts bile canaliculi – narrow channels into which the liver secretes bile – merge into common hepatic duct – cystic duct from gall bladder joins c. hepatic duct – bile duct - union of cystic & common hepatic ducts – near duodenum, bile duct joins pancreatic duct – forms expanded chamber – hepatopancreatic ampulla ends in major duodenal papilla on duodenal wall – major duodenal papilla contains muscular hepatopancreatic sphincter regulates passage of bile and pancreatic juice into duodenum 25-58  Gross Anatomy of Gallbladder, Pancreas, & Bile Passages Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Gallbladder: Neck Hepatic ducts Body Common hepatic duct Head Cystic duct Bile duct Accessory Pancreatic duct pancreatic duct Duodenum Pancreas: Minor duodenal Tail papilla Body Head Hepatopancreatic sphincter Major duodenal papilla Jejunum Hepatopancreatic ampulla 25-59  Gallbladder pear-shaped sac on underside of liver – serves to store and concentrate bile – about 10 cm long – head (fundus) usually projects slightly beyond inferior margin of liver – neck (cervix) leads into cystic duct 25-60  Bile yellow-green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments, & bile acids – bile acids (bile salts) – steroids (from cholesterol) bile acids and lecithin, a phospholipid, aid in fat digestion by emulsifying fats – gallstones may form if bile becomes too concentrated – 80% of bile acids are reabsorbed in ileum & returned to liver – 20% of the bile acids are excreted in the feces the body’s only way of eliminating excess cholesterol 25-61  Gallstones gallstones (biliary calculi) - hard masses in either the gallbladder or bile ducts – made of cholesterol, Ca carbonate, & bilirubin cholelithiasis - formation of gallstones – most common in obese women over 40 due to excess cholesterol obstruct ducts and cause pain – cause jaundice - yellowing of skin due to bile pigment accumulation; also poor fat digestion, and impaired absorption of fat-soluble vitamins lithotripsy - use of ultrasonic vibration to pulverize stones without surgery 25-62  Other Liver Disorders Cholecystitis: inflammation of the gall bladder, often due to blockage may cause rupture, often requires surgery Hepatitis: inflammation of the liver due to viral infection Type A : usually food-borne, self limiting Type B and Type C: more serious, contracted through body fluid contact some people recover from Type B, but Type C is usually incurable and can lead to cirrhosis and liver cancer Cirrhosis: inflammation and scarring of the liver  The Pancreas spongy gland embedded in mesentary adjacent to duodenum – 12 to 15 cm long, and 2.5 cm thick – head encircled by duodenum, body, and a tail on the left – both an endocrine and exocrine gland endocrine portion – pancreatic islets that secrete insulin and glucagon exocrine portion – 99% of pancreas - secretes pancreatic juice – small ducts converge on the main pancreatic duct 25-64  The Pancreas – pancreatic duct runs lengthwise through the middle – joins the bile duct at hepatopancreatic ampulla – hepatopancreatic sphincter controls release of both bile & pancreatic juice into the duodenum – accessory pancreatic duct – smaller duct that branches from the main pancreatic duct opens independently into the duodenum bypasses the sphincter and allows pancreatic juice to be released without bile – pancreatic juice – alkaline mixture of water, enzymes, zymogens, sodium bicarbonate, and other electrolytes 25-65  Pancreatic Zymogens: – Trypsinogen secreted into intestinal lumen converted to trypsin by enterokinase, an enzyme secreted by mucosa of small intestine trypsin is autocatalytic – makes more trypsin – Chymotrypsinogen: converted to chymotrypsin – Procarboxypeptidase: converted to carboxypeptidase – Proelastase: converted to elastase Trypsin catalyzes the conversion of all of the above 25-66  Pancreatic Enzymes Other pancreatic enzymes that don’t digest protein are also released These are not zymogens – don’t have an inactive form – pancreatic amylase – digests starch – pancreatic lipase – digests fat – ribonuclease and deoxyribonuclease – digest RNA and DNA 25-67 Activation of Pancreatic Enzymes in the Sm. Intestine Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chymotrypsinogen Chymotrypsin Trypsinogen Procarboxypeptidase Carboxypeptidase Trypsin Enterokinase 25-68  Pancreatitis Inflammation of the pancreas often from alcohol abuse, or untreated gallstones sometimes cause is unknown common in dogs after a large, high fat meal pancreas releases trypsin and starts digesting itself – can be life-threatening diagnosis can be made by detecting high levels of amylase and lipase in the blood 25-69  Small Intestine – coiled mass filling most of the abdominal cavity inferior to the stomach & liver – nearly all chemical digestion & nutrient absorption occurs in sm. intestine – the longest part of the digestive tract 2.7 to 4.5 m long in a living person – “small” intestine refers to the diameter, not length 25-70  Small Intestine Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Stomach Duodenum Jejunum Ascending colon Ileocecal junction Cecum Appendix Ileum 25-71  Gross Anatomy Small intestine divided into 3 regions: – duodenum – the first 25 cm begins at the pyloric valve ends at a sharp bend called the duodenojejunal flexure receives stomach contents, pancreatic juice, and bile stomach acid is neutralized here fats are physically broken up (emulsified) by the bile acids pepsin is inactivated by increased pH pancreatic enzymes take over the job of chemical digestion 25-72  Gross Anatomy – jejunum – first 40% of small intestine beyond duodenum has large, tall, closely spaced circular folds its wall is relatively thick and muscular especially rich blood supply - red color most digestion & nutrient absorption occurs here – ileum – last 60% of the postduodenal small intestine thinner, less muscular, less vascular, and paler pink color Peyer patches – prominent lymphatic nodules in clusters on the side opposite the mesenteric attachment 25-73  Gross Anatomy ileocecal junction – the end of the small intestine – where the ileum joins the cecum of the large intestine ileocecal valve – a sphincter formed by the thickened muscularis of the ileum – regulates passage of food residue into the large intestine 25-74  Microscopic Anatomy: Sm. Int. tissue layers have modifications for nutrient digestion and absorption – lumen lined with simple columnar epithelium – large internal surface area for effective digestion and absorption great length and three types of internal folds or projections – circular folds (plicae circulares) – villi – increase surface area by a factor of 10 – microvilli – increase the surface area by a factor of 20 25-75  Microscopic Anatomy circular folds – from the duodenum to the middle of the ileum – cause chyme flow in spiral path – promote more thorough mixing & nutrient absorp. villi - fingerlike projections 0.5 to 1 mm tall – make mucosa look fuzzy – covered with two types of epithelial cells: absorptive cells (enterocytes) goblet cells – secrete mucus – core of villus filled with areolar tissue embedded in this tissue are an arteriole, a capillary network, a venule, and a lymphatic capillary called a lacteal 25-76  Villi Villi Absorptive cell Brush border of microvilli Capillary network Goblet cell Lacteal Intestinal crypts Venule Arteriole Lymphatic vessel 25-77 (c)  Microscopic Anatomy microvilli – fuzzy border of microvilli on apical surface of each absorptive cell – “the brush border” - increases absorptive surface area – brush border enzymes – in the plasma membrane of microvilli carry out some of the final stages of enzymatic digestion not released into the lumen contact digestion – the chyme must contact the brush border for digestion to occur intestinal churning of chyme ensures contact with the mucosa 25-78  Intestinal Motility contractions of small intestine serve 3 functions: – to mix chyme with intestinal juice, bile, & pancreatic juice – to churn chyme and bring it in contact w/ mucosa – to move residue toward large intestine segmentation – movement in which stationary ringlike constrictions appear in places along the intestine – they relax and new constrictions form elsewhere – most common kind of intestinal contraction – when most nutrients have been absorbed and little remains but undigested residue, segmentation declines and peristalsis begins 25-79  Segmentation in Small Intestine Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) Segmentation purpose of segmentation is to mix and churn, not to move material along as in peristalsis 25-80  Peristalsis gradual movement of contents towards colon peristaltic wave begins in the duodenum, travels 10-70 cm and dies out followed by another wave starting further down the tract ileocecal valve usually closed – food in stomach triggers gastroileal reflex that enhances segmentation in the ileum and relaxes the valve (b) Peristalsis 25-81 ? 25-82  Lactose Intolerance lactose passes undigested into large intestine – increases osmolarity of intestinal contents – causes water retention in the colon and diarrhea – gas production by bacterial fermentation of the lactose occurs in many parts of the pop. – 15% American whites, 90% of American blacks, 70% of Mediterraneans; and nearly all of Asian descent 25-83  Gross Anatomy of Large Intestine large intestine receives ~ 500 mL indigestible residue/day – reduces it to about 150 mL of feces by absorbing water and salts – eliminates feces by defecation 25-84  Gross Anatomy of Large Intestine – begins as cecum inferior to ileocecal valve – appendix attached to the lower end of the cecum densely populated with lymphocytes; it’s a source of immune cells and “good” bacteria – ascending colon, right colic (hepatic) flexure, transverse colon, left colic (splenic) flexure, and descending colon frame the small intestine – sigmoid colon: S-shaped portion leading into pelvis 25-85 Gross Anatomy of L. Intestine – rectum - portion ending at anal canal has small curves and infoldings – anal canal – final 3 cm of the large intestine terminates at the anus anal columns and sinuses – exude mucus into anal canal during defecation hemorrhoids – permanently distended veins that protrude into the anal canal or form bulges external to anus 25-86 Gross Anatomy of L. Intestine – muscularis externa of colon haustra – pouches in the colon caused by muscle tone taenia coli – longitudinal bands of muscle that maintain the haustra internal anal sphincter - smooth muscle external anal sphincter - skeletal muscle 25-87 Anatomy of Large Intestine Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Right colic Greater omentum flexure (retracted) Left colic Transverse flexure colon Taenia coli Mesocolon Haustrum Ascending Descending colon colon Ileocecal valve Omental appendages Ileum Cecum Appendix Sigmoid Rectum colon Anal canal External anal sphincter 25-88 (a) Gross anatomy Anatomy of Anal Canal Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Rectum Anal canal Hemorrhoidal veins Internal anal sphincter External anal sphincter Anus Anal columns Anal sinuses 25-89 (b) Anal canal  Microscopic Anatomy mucosa - simple columnar epithelium through entire large intestine – anal canal has nonkeratinized stratified squamous epithelium in its lower half no circular folds or villi lamina propria and submucosal layers have large amount of lymphatic tissue – provide protection from the bacteria that densely populate the LI 25-90  Intestinal Flora & Gas bacterial flora populate large intestine – about 800+ species of bacteria – ferment cellulose & other undigested carbs we absorb resulting sugars – help in synthesis of vitamins B and K flatus - intestinal gas – average person produces 500 mL per day from 7 to 10 L of gas present but reabsorbed – most is swallowed air, but hydrogen sulfide, indole and skatole produce odor hydrogen gas may explode during electrical cauterization used in surgery 25-91  Absorption and Motility LI takes ~ 12-24 hours to reduce residue to feces – reabsorbs water & electrolytes, but doesn’t digest feces consist of 75% water and 25% solids - bacteria, fiber, fat, mucus and sloughed epithelial cells haustral contractions occur every 30 minutes – a form of segmentation – distension of a haustrum stimulates it to contract mass movements occur 1 to 3 times a day – triggered by gastrocolic and duodenocolic reflexes filling of the stomach and duodenum stimulates  Defecation stretching of the rectum stimulates defecation reflex – parasympathetic defecation reflex involves spinal cord stretching of rectum sends sensory signals to spinal cord pelvic nerves return signals, intensifying peristalsis & relaxing internal anal sphincter defecation occurs only if external anal sphincter is voluntarily relaxed 25-93  Disorders of L. Intestine Crohn’s Disease: inflammation of portions of the small or large intestine caused by inappropriate immune response; runs in families one of a group of chronic conditions called IBD’s (inflammatory bowel disorders) that cause digestive upset, weight loss, fever Diverticulitis: small pouches form in the lining of the intestine; food may get trapped in these pouches, causing inflammation and pain correlated with a low-fiber diet 25-94  Disorders of L. Intestine Colorectal cancer: common and deadly form of cancer starts as precancerous polyps runs in families correlated with a high-animal-fat diet curable if caught early using digital rectal exams or colonoscopy 25-95 Endocrine and ANS Control of Digestion Three overlapping phases of digestion: 1. Cephalic phase 2. Gastric phase 3. Intestinal phase Cephalic phase: stimuli from food (smell, sight, taste) reaches cerebral cortex, hypothalamus, and brain stem - Brain stem signals cranial nerves VII and IX to signal salivary glands, and cranial nerve X stimulates gastric glands 25-96 Endocrine and ANS Control of Digestion Gastric phase: stretch receptors and chemoreceptors respond to food in stomach, and signal parasympathetic and enteric neurons to trigger peristalsis at the same time, gastrin is released by G cells in the stomach (because of presence of food, acid, and acetylcholine from ANS) gastrin enters blood, comes back to stomach, and signals release of gastric juice, plus increases gastric motility 25-97  Endocrine and ANS Control of Digestion Intestinal phase: slows chyme entering sm. intestine stretch receptors cause enterogastric reflex – send impulses to medulla to inhibit parasympathetic stimulation, so gastric motility is reduced cholecystokinin (CCK) and secretin secreted by intestinal glands (each enhances the other) CCK causes secretion of enzyme-rich pancreatic juice, release of bile, and causes feeling of satiety secretin causes flow of bicarbonate-rich pancreatic juice, inhibits gastric juice 25-98