Exsc223- Chapter 23 - Lecture notes ch 23 PDF

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CHAPTER 23: DIGESTIVE SYSTEM Function of digestive system: 1. Take in food 2. Break it down into nutrient molecules 3. Absorb molecules into the bloodstream 4. Rid body of any indigestible remains Types of digestive systems 1. Alimentary canal (gastrointestinal or GI tract or gut) a. Continuous muscular tube that runs from the mouth to anus b. Digest food: breaks down into smaller fragments c. Absorbs fragments thru lining into blood d. Organs: mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus 2. Accessory digestive organs a. Teeth, tongue, gallbladder b. Digestive glands- produce secretions Digestive processes 1. Ingestion: eating 2. Propulsion: movement of food thru the alimentary canal a. Swallowing b. Peristalsis: major means of propulsion of food that involves alternating waves of contraction and relaxation 3. Mechanical breakdown: chewing, mixing food w/ saliva, churning food in stomach, and segmentation 4. Digestion: series of catabolic steps w/ enzymes that break down complex food molecules into chemical building blocks 5. Absorption: passage of digested fragments from lumen of GI traction into blood or lymph 6. Defecation: elimination of indigestible substances via anus in form of fences

Segmentation- mix food with enzymes

Digestive Organs and Peritoneum - Peritoneum: serous membranes of abdominal cavity o Visceral: membrane on external surface of most digestive organs o Parietal: membrane that lines the body wall (outside) - Peritoneal cavity o Fluid-filled space between two peritonea’s o Fluid lubricates mobile organs - Mesentery: double-layer of peritoneum; layers are fused back to back o Extends from body wall to digestive organs o Provides route for blood vessels, lymphatic, and nerves o Holds organs in place and stores fat - Intraperitoneal organs: organs that are located within the peritoneum - Retroperitoneal organs: located outside, or posterior to the peritoneum o Includes most pancreas, duodenum, and parts of large intestine Peritonitis - Inflammation of peritoneum - Can be caused by piercing abdominal wound, perforating ulcer, or ruptured appendix - Peritoneal covering stick together, which helps localize infections - Dangerous and lethal if it becomes widespread - Treatment: debris removal and mega doses of antibiotics Four Basic layers in Digestive Organs 1. Mucosa a. Innermost layer that lines lumen b. Functions: different layers perform one or all three i. Secretes mucus, digestive enzymes, and hormones ii. Absorbs end products of digestion iii. Protects against infectious disease 2. Submucosa a. External to mucosa, areolar CT b. Contains blood and lymphatic vessels, lymphoid follicles, and submucosal nerve plexus that supply surrounding GI tract tissues c. Elastic tissues that help organ to regain shape after storing large meal 3. Muscular externa a. Muscle layer responsible for segmentation and peristalsis b. Contains inner circular muscle layer and outer longitudinal layers i. Circular layer thickens in some areas to form sphincters, acting as a valve 4. Serosa a. Outermost layer, visceral peritoneum i. Formed from areolar CT in most organs ii. Replaced by fibrous adventitia (dense CT) in esophagus iii. Retroperitoneal Splanchnic Circulation

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Arteries that branch off aorta to serve digestive organs Hepatic portal circulation o Drains nutrient-rich blood from digestive organs o Delivers blood to live for processing

Control of Digestive System - Enteric nervous system (gut brain) o GI tract’s own nervous system, more neurons than spinal cord - Made up of enteric neurons o Submucosal nerve plexus  Regulates glands & smooth muscle in mucosa o Myenteric nerve plexus  Controls GI tract motility - Participates in both short and long reflex arcs o Short reflexes: mediated by enteric nerve system (inside) o Long reflexes: respond to inside or autonomic nervous system (outside)  Parasympathetic system: stimulation  Sympathetic system: inhibition Functional Anatomy of Digestive System 1. Mouth and associated organs 2. Pharynx and esophagus 3. Stomach 4. Liver, gallbladder, pancreas 5. Small intestine 6. Large intestine 1. Mouth & Associated organs - Mouth is where food is chewed and mixed w/ enzyme-containing saliva that begins process of digestion, and swallowing process is initialed - Associated organs include mouth, tong, teeth Mouth - Lips & cheeks: keeps food between teeth - Palate: roof of mouth o Forms rigid surface against which the tongue forces food during chewing - Tong: floor of mouth o Gripping, repositioning, mixing food - Salivary glands: produce and secrete saliva o Clean mouth, dissolve food, moisten food o begins breakdown of starch via amylase - teeth: tear and grind food, physically breaking down into smaller fragments

Digestive Processes in Mouth

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Ingest Begins mechanical breakdown by chewing Initiated propulsion by swallowing Starts the digestion of polysaccharides

2. Pharynx and Esophagus - Pharynx: food passed from mouth into oropharynx then into laryngopharynx o Allows passage of food, fluids, and air - Esophagus: flat muscular tube that runs from laryngopharynx to stomach o Is collapsed when not involved in food propulsion Digestive Processes: Swallowing - Conduits to pass food from mouth to stomach - Food propulsion that starts w/ deglutition (swallowing) o Buccal phase: voluntary contraction of tongue o Pharyngeal-esophageal phase: involuntary phase controlled by swallowing center 3. Stomach - Temporary storage tank that starts chemical breakdown of protein digestion o Converts bolus to food to paste-like chyme - Types of gland cells o Glands produce most gastric juice o Glands include secretory cells  Mucous neck cells  Parietal cells: secrete hydrochloric acid and intrinsic factor  Chief cells: pepsinogen and lipases  Enteroendocrine cells Mucosal Barrier - Harsh digestive conditions require stomach to be protected - Protects stomach by three factors: 1. Thick layer of bicarbonate-rich mucus 2. Tight junctions between epithelial cells a. Prevent juice seeping underneath tissue 3. Damaged epithelial cells are quickly replaced by division of stem cells a. Surface cells replaced every 3-6 days Gastritis and Gastric Ulcers - Gastritis: inflammation caused by anything that breaches stomach’s mucosal barrier - Peptic or gastric ulcers o Can cause erosions in stomach wall o Most ulcers caused by bacterium Helicobacter pylori o Can also be caused by non-steroid anti-inflammatory drugs (NSAIDs), such as aspirin Digestive Processes in Stomach

1. 2. 3. 4. 5. 6.

Breakdown of food Holding area for food Delivers chyme to small intestine Denatures proteins by HCL Pepsin carries out enzymatic digestion of proteins Secretion of intrinsic factor for vitamin B12 absorption

Regulation of Gastric Secretion 1. Cephalic (reflex) phase- conditioned to reflex triggered by aroma, taste, sight, thought 2. Gastric phase- local neural and hormonal regulation upon food in stomach 3. Intestinal phase- begins with a brief stimulatory component followed by inhibition Mechanism of HCL Formation - Parietal cells pump H+ into stomach lumen via proton pumps (H+/K+ ATPase) o Cl- is pumped out of lumen to join with H+, forming HCL

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If too much HCL in your stomach, how would you inhibit HCL production? H+ K+ ATPase pump is blocked inhibiting H+ to enter the stomach lumen Why do we need HCL in our stomach? Liver, Gallbladder, and Pancreas Accessory organs associated with small intestine Liver: produce bile (fat emulsifier, breaks down one big lipid into multiple little lipids) Gallbladder: storage of bile produced by liver (so they should be located close together) Pancreas: supplies most enzymes needed to digest chyme, as well as bicarbonate to neutralize stomach acid

Homeostatic Imbalance of Liver - Hepatitis (inflammation of liver)- caused by viral infection, drug toxicity, wild mushroom poisoning - Cirrhosis (fibrosis of liver)- progressive, chronic inflammation from chronic hepatitis or alcoholism

o Liverfatty, fibrous liver portal hypertension Gallbladder - Thin-walled muscular sac on ventral surface of liver - Functions to store and concentrate bile by absorbing water and ions - Contains many honeycomb folds that allow it to expand as it fills with bile - Muscular contractions release bile via cystic duct, which flows into bile duct Pancreas - Location: retroperitoneal area, deep to greater curvature of stomach - Exocrine function: produce pancreatic juice o Acini: produce zymogen granules containing proenzymes (not active yet) o Ducts: produce water and bicarbonate - Endocrine function: secretion of insulin and glucagon by pancreatic islet cells Pancreatic Juice - Watery, alkaline solution (pH 8) to neutralize acidic chyme coming from stomach - Electrolytes, primarily HCO3 – (bicarbonate ion) - Digestive enzymes: o Proteases (for proteins): secreted in inactive form to prevent self-digestion  How to make active? When they meet HCL o Amylase (for carbs) o Lipases (for lipids) o Nucleases (for nucleic acids) All three organs (liver, gallbladder, and pancreas) are connected by cystic duct Sphincter can regulate how much bile the gallbladder or liver can release 5. -

Small intestine Major organ of digestion and absorption 2-4 m long (7-13 ft), diameter of 2.5-4 cm (1.0-1.6 in) subdivisions: duodenum, jejunum, Ileum length and other structural modifications (circular folds, villi, and microvilli) provide huge surface area for nutrient absorption)

Small Intestine Wall - Intestinal crypts: tubular glands scattered between villi - 5 main types of cells found in villi and crypts 1. enterocytes: absorptive cell 2. goblet cells: mucus- secreting cells 3. enterendocrine cells: source of enterogastrones 4. Paneth cells: secrete antimicrobial agents 5. Stem cells: divide to produce other cell types

Chemotherapy and GI Tract Epithelium - Chemotherapy targets rapid dividing cells - Negative side effects are that it also targets rapidly dividing GI Tract Epithelium - Reason why many patients undergoing chemotherapy have symptoms of nausea, vomiting, and diarrhea Digestive Processes in SI - Chyme from stomach contains partially digested carbs and proteins and undigested fats - Absorb all nutrients and most water - Sources of enzymes for digestion o Substances such as bile, bicarbonate, digestive enzymes are important from liver and pancreas o Brush border enzymes bound to plasma membrane perform final digestion of chyme

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6. Large intestine Subdivisions 1. Cecum: 1st part of large intestine 2. Appendix: masses of lymphoid tissue a. Twisted shape makes it susceptible to blockages 3. Colon: a. Ascending: travels up right side of abdominal cavity b. Transverse: travels across abdominal cavity c. Descending: travels down left side of abdominal cavity d. Sigmoid: S-shaped portion that travels thru pelvis 4. Rectum: three rectal valves stops fences from being passed with gas (flatus) 5. Anal canal: last segment of LI, has two sphincters a. Internal anal sphincter: smooth muscle b. External anal sphincter: skeletal muscle

Clinical- Homeostatic Imbalance - Appendicitis: acute inflammation of appendix; usually results from a blockage by fences that traps infectious bacteria - Ruptured appendix can cause peritonitis - Symptoms: pain in umbilical region, moving lower right abdominal quadrant o Loss of appetite, nausea, and vomiting are also seen - Treatment: surgical removal (appendectomy) Bacterial Flora - Consists of 10000+ different types of bacteria o Outnumber our own cells 10 to 1 - Enter from SI or anus to colonize colon - Metabolic functions o Fermentation  Ferment indigestible carbs and mucin  Release irritating acids and gasses

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o Vitamin synthesis: B complex and some K Keeping pathogenic bacteria in check o Beneficial bacteria outnumber and suppress pathogenic bacteria o Immune system destroys any bacteria that try to breach mucosal barrier

Digestive Process in LI - No food breakdown occurs except what enteric bacteria digest - Vitamins (made by bacterial flora), water, and electrolytes (especially Na+ and Cl-) are reclaimed - Major functions of LI are propulsion of fences to anus and defecation - Low-fiber diet can narrow colon and causes strong contractions that increase pressure on walls o Can result in diverticula, hern Clinical- homeostatic imbalance - Irritable bowel syndrome o Functional GI disorder, affects LI o Causes cramping, abdominal pain, bloating, gas, diarrhea, and constipation o Stress is common precipitating factor  Stress management is important in treatment - Diarrhea, watery stools, no sufficient time to absorb remaining water in LI o Prolonged diarrhea may result in dehydration and electrolyte imbalance - Constipation, extended periods of time and too much water is absorbed o Stool becomes hard and difficult to pass o Due to insufficient fiber or fluid in diet, improper bowel habits, lack of exercise, or laxative abuse Mechanism of Digestion and Absorption - Digestion: catabolic process that breaks down macromolecules down into monomers small enough for absorption o Intrinsic and accessory gland enzymes o Enzymes carry out hydrolysis - Absorption: moving substances from lumen of gut into body o Tight junctions ensure molecules to pass through epithelial cell o Materials enter cell through apical membrane (lumen side) and exit through basolateral membrane (blood side) - Lipid molecules can be absorbed passively through membrane, but other polar molecules are absorbed by active transport - Most nutrients are absorbed before chyme reaches ileum Digestion of Carbohydrates - Only monosaccharides can be absorbed - Breakdown begins in mouth with salivary amylase - Final breakdown into monosaccharides (glucose, fructose, galactose)

Clinical- Homeostatic Imbalance - People with lactose intolerance cannot consume lactose - Any lactose eaten remains undigested and creates osmotic gradient in intestine that prevents water from being absorbed, resulting in diarrhea Digestion of Proteins - Proteins are broken into: o Large polypeptides o Small polypeptides and small peptides o Finally, into amino acid monomers, with some dipeptides and tripeptides Digestion of Lipids - Steps in lipid digestion in intestine 1. Emulsification: break large fats into smaller ones 2. Digestion: breakdown fat into monoglyceride and two free fatty acids via lipase 3. Micelle formation: coated with bile salts and lecithin 4. Diffusion: cross epithelial membrane via diffusion 5. Chylomicron formation: lipid products are converted back into triglycerides and packaged with lecithin and lipoproteins, forming chylomicron 6. Chylomicron transport: chylomicrons are exocytosed from basolateral side and enter lymphatic lacteal Digestion of Nucleic Acids - Nuclei of ingested cells in food contain DNA and RNA - Pancreatic nucleases hydrolyze nucleic acid to nucleotide monomers - Brush border enzymes, nucleosidases, and phosphatases break nucleotides down into free nitrogenous bases, pentose sugars, and phosphate ions - Breakdown products are actively transported by special carriers in epithelium of villi Absorption of Vitamins - In small intestine o Fat soluble vitamins (A, D, E, K) are carried by micelles; diffuse into absorptive cells o Water soluble vitamins (C and B) are absorbed by diffusion or by passive or active transporters o Vitamin B12 (large, charged molecule) binds with intrinsic factor and is absorbed by endocytosis - In large intestine: vitamin K and B vitamins from bacterial metabolism are absorbed Absorption of Electrolytes - Most ions are transported actively along length of small intestine - Iron and calcium are absorbed in duodenum - Na+ absorption is coupled with active absorption of glucose and amino acids - Cl- is transported actively - K+ diffuses in response to osmotic gradients; lost if water absorption is poor

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Usually amount in intestine is amount absorbed Iron and calcium absorption is related to need o Ionic iron is stored in mucosa cells with ferritin o When needed transported in blood by transferrin o Ca2+ absorption is regulated by vitamin D and parathyroid hormone

Absorption of Water - 95% is absorbed in the small intestine by osmosis o most of rest is absorbed in large intestine - Net osmosis occurs if concertation gradient is established by active transport of solutes - Water uptake is coupled by solute uptake...