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Chapter 17 Digestive System 1 Major Functions • Ingestion • Taking food into body • Propulsion • Passage of food along the gastrointestinal tract • Digestion • Mechanical & Chemical 2 Major Functions • Absorption • Nutrients of food from GI tract diffuses into cardiovascular & lymphatic systems for distribution to cells • Defecation • Elimination of indigestible substances from GI tract 3 Basic Organization • Gastrointestinal Tract (GI tract) – Alimentary canal – Continuous tube running through the ventral cavity from the mouth to the anus – 9 meters – Mouth, pharynx, esophagus, stomach, small intestine, large intestine 4 Basic Organization • Accessory Structures – Teeth, tongue, salivary glands, liver, gall bladder, pancreas (all lie outside GI tract except teeth and tongue) – Produce or store secretions that aid in the chemical breakdown of food – Secretions reach the GI tract through ducts 5 Major Organs 6 Alimentary Canal 7 Alimentary Canal Wall • Mucosa (3 tissues) – inner lining of the GI tract, mucous membrane – Epithelium • stratified in mouth and esophagus and simple throughout the rest of the GI tract 8 Alimentary Canal Wall • Mucosa (3 tissues) – Lamina propria • Loose connective tissue connecting epithelial layer to muscularis mucosa • provides space for blood and lymphatic vessels – Muscularis mucosa • Smooth muscle that makes folds in the intestine to increase the surface area 9 Alimentary Canal Wall • Submucosa – Dense connective tissue that binds the mucosa to the muscularis – Highly vascular – Contains part of the autonomic nerve supply to the muscularis mucosa 10 Alimentary Canal Wall • Serosa – Outermost layer of the GI tract – Peritoneum • Parietal peritoneum – Lines the wall of the abdominal cavity • Visceral peritoneum – Lines some organs • Peritoneal cavity – Open space between the two peritoneums 11 Alimentary Canal Wall • One extension of the parietal peritoneum is called the mesentery – Tip of the mesentery is attached to the posterior abdominal wall – Binds the small intestine to the wall – Supply the small intestine with blood and nerves 12 Alimentary Canal Wall 13 Movements of the Tube • mixing movements • peristalsis 14 Mouth • Oral cavity/buccal cavity • Formed by the cheeks, hard palate, soft palate and tongue • Ingestion • Mechanical digestion • Prepares food for chemical digestion 15 Mouth • Uvula – Tissue that protrudes past the end of the soft palate – Covers opening to nasopharynx during swallowing 16 Mouth • Mechanical Digestion – Mastication • Tongue moves food, teeth grind food, and food mixes with saliva • Food reduced to a soft flexible mass called a bolus 17 Tongue 18 Tongue • Skeletal muscle covered by mucous membranes – Medial septum • Divides tongue into two equal halves • Tongue attached inferiorly to the hyoid bone – Lingual frenulum • Attaches tongue to the floor of the mouth • Limits the posterior movement of the tongue – Superior surface covered by papillae--some of which are “taste buds” 19 Palate • roof of oral cavity 20 Primary Teeth •Start appearing about 6 months • 8 incisors • 4 cuspids • 8 molars 21 Secondary Teeth 22 Secondary Teeth • • • • • • Start appearing about 6 years 32 total Jaw continues to grow until maturity Age 6- 1st set of molars Age 12- 2nd set of molars Age 18- 3rd set of molars – “wisdom teeth” 23 Section of a Tooth 24 Salivary Glands 25 Secretions of Salivary Glands • Parotid glands • Clear • Water, serous fluid • Rich in amylase •Enzyme that starts breakdown of starch • Submandibular glands • Primarily serous fluid • Some mucus • Sublingual glands • Primarily mucus • Most viscous 26 Pharynx 27 Swallowing Mechanism •Deglutition •Soft palate and uvula raise • •Hyoid bone and larynx elevate •Epiglottis closes off top of trachea •Longitudinal muscles of pharynx contract •Inferior constrictor muscles relax and esophagus opens •Peristaltic waves push food through pharynx 28 Swallowing Mechanism • Voluntary Stage – Bolus forced to the back of the mouth cavity and into the oropharynx by the tongue pressing up and back against the palates • Pharyngeal Stage – Bolus stimulates nerves in the oropharynx – Impulses cause soft palate and uvula to move up and seal off the nasopharynx • Esophageal Stage – Bolus pushed through the esophagus by involuntary muscular movements called peristalsis 29 Swallowing Mechanism – Peristalsis • Function of the muscular that is controlled by the medulla • Circular fibers contract forcing food down • Longitudinal fibers below bolus contract and shorten the portion of the esophagus ahead of bolus, increasing the opening above • Solid food through esophagus about 4 to 8 seconds • Liquids about 1 second 30 Swallowing Mechanism 31 Esophagus 32 Esophagus • Muscular collapsible tube that lies posterior to the trachea • 22-26 cm long • Passes trough the mediastinum just anterior to the vertebral column • Pierces the diaphragm through the opening called the esophageal hiatus 33 Esophagus 34 Stomach 35 Stomach • J-shaped • Inferior to the diaphragm • Sphincter between esophagus and stomach is the cardiac sphincter 36 Stomach • Composed of 4 parts – Cardia • Closest to the esophagus – Fundus • Rounded portion above the cardia – Body • Central portion – Pylorus • Narrow, inferior region prior to the small intestine 37 Stomach • Stomach can only absorb some water, electrolytes, certain drugs (aspirin) and alcohol 38 Lining of Stomach 39 Lining of Stomach • Made of the same four layers as the rest of the GI tract (these layers are modified) • Rugae – large folds on an empty stomach 40 Lining of Stomach • Chief cells – secrete pepsin, which is a protein splitting enzyme • Parietal cells – secrete HCl • converts pepsinogen to pepsin – intrinsic factor • helps absorption of vitamin B12 • Mucous cells 41 Lining of Stomach • Enteroendocrine cells – Produce gastrin – Hormone that stimulates HCl & pepsin production – Closes cardiac sphincter and relaxes pyloric sphincter – Gastric juice 42 Phases of Gastric Secretion • Cephalic phase • Triggered by smell, taste, sight, or thought of food • Parasympathetic impulses trigger gastric juice secretion • Gastric phase • Triggered by presence of food in stomach and distention of stomach • Partially digested proteins and caffeine stimulate secretion of gastrin • Gastrin increases production of gastric juice, increases motility of GI tract, relaxes pyloric sphincter 43 Phases of Gastric Secretion • Intestinal phase Triggered by movement of food into small intestine 2-6 hours after ingestion • Intestinal cells release intestinal gastrin • Inhibits secretion of gastric juice • Secretin • Stimulates pancreatic secretions • • Cholecystokinin • Induces the contraction of the gallbladder, which releases which releases bile • Gastric inhibiting peptide (GIP) • Stimulates the release of insulin, which maintains proper blood sugar level 44 Regulation of Gastric Secretions 45 Mixing and Emptying Actions 46 Enterogastric Reflex Regulates the rate at which chyme leaves the stomach 47 Pancreas 48 Pancreas • Accessory structure of the GI tract • Oblong gland 12.5 cm long and 2.5 cm thick • Lies posterior to the greater curvature of the stomach • Connected by 2 ducts to the upper part of the small intestine 49 Pancreas • Pancreatic duct– Joins with the common bile duct from the gallbladder to form the hepatopancreatic ampulla, which empties into the small intestine about 10 cm below the pylorus • 2.5 cm above the hepatopancreatic ampulla is the other duct 50 Pancreas • Histology – Groups of glandular epithelial cells – About 1% of these cells are called the Islets of Langerhan • Endocrine part of the pancreas that secrete hormones (insulin) – Other 99% are called acini – Exocrine glands that secrete digestive enzymes called Pancreatic juice 51 Pancreatic Juice • Pancreatic amylase • splits glycogen into disaccharides • Pancreatic lipase • breaks down triglycerides • Trypsin, Chymotrypsin, and Carboxypeptidase • digest proteins • Nucleases • Digest nucleic acids • Bicarbonate ions • Make pancreatic juice alkaline 52 Regulation of Pancreatic Secretions • Acidic chyme stimulates release of secretin • Stimulates release of pancreatic juice 53 Liver 54 Liver • Average size is about 1.4 kg • Located under diaphragm in the right hypochondrium and part of the epigastrium of the abdomen • 2 principal lobes – Left and right 55 Hepatic Lobule 56 Hepatic Lobule • Functional unit of the liver • Each lobule surrounds a central vein • Kupffer’s cells – Line spaces between cells – Destroy worn out RBC and WBC, bacteria & toxic substances – Filters blood • all things absorbed from the GI tract go to the liver first 57 The Paths of Blood and Bile in Hepatic Lobule 58 Liver Functions • Carbohydrate metabolism • Maintains normal blood glucose levels • Produces glycogen from glucose • Breaks down glycogen into glucose • Converts noncarbohydrates to glucose • Fat metabolism • Synthesizes lipoproteins, phospholipids, cholesterol • Digest cholesterol • Stores fat • Converts carbohydrates and proteins into fats 59 Liver Functions • Protein metabolism • Loss of this function results in death with in a few days • Removal of nitrates • Conversion of NH3 into urea • Synthesis of plasma proteins • Synthesis of fibrinogen (blood coagulant) • Synthesis of anticoagulants • Conversion of one amino acid to another 60 Liver Functions • Removal of drugs and hormones • Detoxify penicillin, ampicillin • Excrete or alter into bile, estrogen, or aldosterene • Excretion of bile • Synthesis of bile salts 61 Liver Functions • Storage • glycogen, vitamins A, B12, D, E, K, iron, copper • Phagocytosis • Kupffers cells • Activation of vitamin D 62 Bile • Yellow, brown, olive green liquid with a pH of 7.6-8.6 • Mostly water • Bile salts • Emulsification of fats • Absorption of fatty acids, cholesterol, and fat-soluble vitamins • Bile pigments • Cholesterol • Electrolytes 63 Gallbladder 64 Gallbladder • • • • Pear shaped sac 7-10 cm long Accessory structure Located in the fossa of the visceral surface of the liver 65 Gallbladder • Stores and concentrates bile until small intestine needs it • Empty small intestine = closed hepatopancreatic sphincter • Bile flows back to gallbladder • Chyme in small intestine opens the sphincter 66 Regulation of Bile Release • Fatty chyme entering duodenum stimulates gallbladder to release bile • Hormonal stimulation causes smooth muscle contraction & squeezes contents into the cystic duct & common bile duct 67 Small Intestine • 2.5 cm in diameter • 6.35 m long • Absorption and most digestion occur here 68 Three Parts of Small Intestine 69 Three Parts of Small Intestine • Duodenum (means 12) • 1st 12 fingers width from pyloric sphincter (about 25cm) • Jejunum (means empty) • Ileum • Longest part at 3.6m long • Joins the large intestine at the ileocecal sphincter 70 Small Intestine Histology • Same 4 layers as the rest of the GI tract – Mucosa and submucosa modified for digestion & absorption • Intestinal glands – secrete juices • Duodenal glands – secrete alkaline mucous 71 Small Intestine Histology • Large surface area = better absorption • 4 things help the small intestine absorb better • Length • Microvilli • Villi – Projections about 1mm high – 10 to 40 every square mm – Gives lining of the small intestine a velvety appearance – Arteriole, venule, capillary network, and lymphatic vessels 72 Small Intestine Histology 73 Small Intestine Histology 74 Small Intestine Histology • Circular folds (plicae circulares) • Permanent ridges about 10 mm high • Start near the proximal portion of duodenum and stop at the midpoint of the ileum • Causes chyme to spiral, which enhances absorption by increasing the contact with villi 75 Small Intestine Histology 76 Physiology of Digestion • Mechanical • Segmentation • Major movement of small intestine • Localized contraction to mix the chyme • DOES NOT move chyme downward • Peristalsis • • • • Moves chyme down Weak compared to in the esophagus Chyme moves at 1 cm per minute Food stays in the small intestine about 3 to 5 hours • Over-distended wall triggers peristaltic rush resulting in diarrhea 77 Physiology of Digestion • Chemical • Pancreatic juice, bile and intestinal juice complete the breakdown of carbohydrates, proteins and lipids 78 Secretions of Small Intestine • Peptidase • Breaks down peptides into amino acids • Sucrase, maltase, lactase • Break down disaccharides into monosaccharides • Lipase • Breaks down fats into fatty acids and glycerol • Enterokinase • Converts trypsinogen to trypsin 79 Secretions of Small Intestine • Somatostatin • Hormone that inhibits acid secretion by stomach • Cholecystokinin • Hormone that inhibits gastric glands, stimulates pancreas to release enzymes in pancreatic juice, stimulates gallbladder to release bile • Secretin • Stimulates pancreas to release bicarbonate ions in pancreatic juice 80 Physiology of Absorption • Food pieces need to be small enough to pass through epithelial tissue in villi • Depends on diffusion, facilated diffusion, osmosis, and active transport • Absorbed into blood • Carbohydrates • Monosacharides • Proteins • amino acids • Fats, lipids • fatty acids, glycerol, and glycerides 81 Physiology of Absorption • Micelles – Container for fatty acids and glycerides – When they come in contact with epithelial cells they allow contents to diffuse out of the “bubble” – The micelle stays in the small intestine • Lipids hitch rides with protein transporters to form lipoproteins 82 Physiology of Absorption • HDL’s (high density lipoproteins) – Remove cholesterol from arteriole walls to take to the liver – Good • LDL’s (low density lipoproteins) – Transport cholesterol to tissues for hormone and membrane production – Cause cholesterol to build up in your arteries – Bad 83 Physiology of Absorption • Water – Absorbed by osmosis – Average of 9 liters of water enter the small intestine a day – All but 1 liter is absorbed 84 Mesentery • Suspends portions of the small intestine from the posterior abdominal wall 85 Large Intestine 86 Large Intestine • 1.5 Meters in length and 6.5 cm in diameter • Cecum – Portion below the ileocecal sphincter – The appendix is attached to the pouch of the mecum • Colon (4 parts) – – – – Ascending- up Transverse- across Descending- down Sigmoid- angles toward the midline of the body 87 Large Intestine • Rectum • last 20 cm of the gastrointestinal tract • Anal Canal • last 2 to 4 cm of the rectum • Anus • Opening of the canal • Internal anal sphincter- smooth “involuntary muscle” • External anal sphincter- skeletal muscle 88 Large Intestine • Histology – – – – Same four layers as the rest of the GI tract No villi No circular folds Simple columnar epithelial cells absorb water – Goblet cells secrete mucous 89 Large Intestinal Wall 90 Functions of Large Intestine • Little or no digestive function • Absorbs water and electrolytes •Less than 10% of total amount • Secretes mucus • Houses intestinal flora 91 Functions of Large Intestine • Forms feces • Chyme stays in the large intestine for 3 to 10 hours • Becomes a semi-solid due to the absorption of water • All but about 100 mL of water is absorbed • Water, inorganic salts, sloughed off epithelial cells, products of bacterial decomposition and undigested parts of food • Bile pigments altered by bacteria provide color • Smell produced by bacterial compounds 92 Functions of Large Intestine • Carries out defecation • Emptying of rectum • Triggered by distention of the rectum 93 Movements of Large Intestine • Slower and less frequent than those of small intestine • Mixing movements • Peristalsis • Mass movements usually follow meals 94 Life-Span Changes • Teeth become sensitive • Gums recede • Teeth may loosen or fall out • Heartburn more frequent • Constipation more frequent • Nutrient absorption decreases • Accessory organs age but the effects are less noticeable 95 Clinical Application Hepatitis • inflammation of the liver • most commonly caused by viral infection • can be caused by reactions to drug, alcoholism or autoimmunity Signs and Symptoms • headache • low fever • fatigue • vomiting • rash • foamy urine • pale feces • jaundice • pain Hepatitis A – not washing hands or eating raw shellfish Hepatitis B – chronic; serum Hepatitis C – serum Hepatitis D – very severe; only produces symptoms if infected with B; serum Hepatitis E, F, G – more rare 96