Propulsion and Mixing of Food in the Alimentary Tract PDF

Propulsion and Mixing of Food in the Alimentary Tract Lecture Outline I. Ingestion of Food II. Motor Functions of the Stomach III. Movements of the Small Intestine IV. Movements of the Colon 1 Propulsion and Mixing of Food in the Alimentary Tract Objectives 1.Describe an overview of the assimilation...

Propulsion and Mixing of Food in the Alimentary Tract Lecture Outline I. Ingestion of Food II. Motor Functions of the Stomach III. Movements of the Small Intestine IV. Movements of the Colon 1 Propulsion and Mixing of Food in the Alimentary Tract Objectives 1.Describe an overview of the assimilation of nutrients 2.List the three main motor functions of the GI tract 3.Identify sphincters of the GI system 4.Explain the process of mastication 5.Explain deglutition 6.List features of each portion of the esophagus 7.Explain the anesthesia implications of the LES and barrier pressure 8.Identify pathologies of the esophagus including GERD 9.Identify motor functions of the stomach and regulation of stomach emptying 10.Explain movements of the small intestine including MMCs and hormonal signals 11.Identify principle functions of the ileocecal valve and the colon 12.Explain the process of defecation 2 References Assigned reading from your text: Hall chapter 64 3 I. Ingestion of Food 4 Assimilation of Nutrients in the GI Tract ❑ Or …. The Schedule of events for your next meal Mouth – – Esophagus – Stays in stomach for 1-4 hours while it mixes and grinds food Smaller particles→ pyloric sphincter Exocrine stomach secretions(mucosa) dilute food Gastric acid dissolves food Small intestine – – – – 10 seconds for food to travel through esophagus Stomach – – – – Mastication of food creates bolus that can be swallowed Saliva lubricates and provides enzymes for digestion Entry of food into duodenum coordinates with exocrine secretions of pancreas/liver Pancreas essential for digestive enzymes Pancreas secretes HCO3- to neutralize stomach acid Gallbladder delivers stored bile to the intestine Food moves through small intestine in 7-10 hours – – Food is mixed with digestive enzymes All significant absorption of nutrients occurs here Large intestine transports fluids, electrolytes, and fermented insoluble fiber over 12-24 hours Elimination of fecal waste occurs 1-3 days after ingestion 5 Three Main Functions Of GI Tract: ❑ Three Functions: Churning- enhances digestion and absorption nutrients Propulsion- caudal movement of food/waste Acts as a reservoir- facilitated by sphincters 6 Sphincters ❑7 Sphincters in the GI tract 1.Upper esophageal sphincter 2.Lower esophageal sphincter 3.Pyloric sphincter 4.Sphincter of Oddi 5. Ileocecal sphincter 6.Internal 7.External Anal Sphincters ❑Sphincters: -Specialized circular muscles -Act as one-way valves - Regulate antegrade and retrograde movement Proximal stimuli cause sphincteric relaxation Distal stimuli induce sphincteric contraction -Separate low-pressure organs - Maintain positive resting pressure -Coordination with adjacent organs by intrinsic and neurohumoral stimuli 7 Mastication ❑ Chewing Most muscles of mastication innervated by motor branch of CN V Chewing important for cellulose membranes – Digestive enzymes act on food particle surface only A bolus of food in the mouth causes the jaw to drop→ Stretch reflex and rebound contraction 25 x is commonly sufficient for food bolus to pass through esophagus without pain 8 Deglutition Ganong FIGURE 27–4 Movement of food through the pharynx and upper esophagus during swallowing. A.The tongue pushes the food bolus to the back of the mouth. B.The soft palate elevates to prevent food from entering the nasal passages. C.The epiglottis covers the glottis to prevent food from entering the trachea and the upper esophageal sphincter relaxes. D.Food descends into the esophagus. 9 Three Stages of Deglutition ❑ Voluntary stage- stage 1 Food bolus pushed into oropharynx by elevation of the tongue against the hard palate ❑ Pharyngeal stage- stage 2 (Involuntary) Bolus stimulates sensitive epithelial swallowing receptor areas all around the opening of the pharynx -especially tonsillar pillars- afferently via CNs IX and X to the deglutition center of brainstem and efferently via CNs V, IX, X, and XII to initiate automatic pharyngeal muscle reflex contractions Soft palate pulled up to close posterior nares to prevent food reflux Palatopharyngeal folds impede large particles from passing Larynx pulled upward/anteriorly, vocal cords approximated, epiglottis covers laryngeal inlet 3-4 cm of the esophageal wall of the cricopharyngeus muscle, the UES relaxes Pharyngeal peristalsis forces bolus into the UES Respiration momentarily halted CVA may affect pharyngeal stage ❑ Esophageal stage- stage 3 (Involuntary) When upright, gravity assists swallowing ahead of peristalsis Primary peristalsis caused by stretching of the muscular wall Some cells release VIP in response to stretch to inhibit smooth muscle and relax in front of bolus Secondary peristalsis- Myenteric initiates a secondary peristaltic wave if needed- vagovagal reflex Swallowing possible in any position 10 Structures of the Esophagus ❑ Upper esophageal sphincter (UES) Cricopharyngeus muscle aka pharyngoesophageal sphincter Upper 3-4 cm of esophageal wall Controlled by extrinsic cranial nerves Skeletal muscle UES remains constricted; relaxes during the pharyngeal stage of deglutition ❑ Esophageal body Skeletal and smooth muscle Body remains relaxed Esophagus does not display slow waves Mucosa only secretes mucus ❑ Lower (inferior) esophageal sphincter (LES) aka cardiac sphincter, esophagogastric sphincter/junction Circular smooth muscle 3 cm above juncture with the stomach Controlled by ENS Three parts: – Intrinsic sphincter -esophageal muscle – Extrinsic sphincter- fibers of crural portion of diaphragm – Oblique ”sling” fibers of stomach create a flap-valve LES is tonically constricted with an intraluminal pressure of 30 mmHg – Relaxes on swallowing – Tonic activity prevents reflux of gastric contents into esophagus – Acetylcholine constricts – NO and VIP from interneurons cause it to relax close when gastric pressure high 11 LES and Barrier Pressure ❑ Barrier pressure opposes gastric regurgitation and aspiration of gastric contents The likelihood of gastroesophageal reflux is determined by barrier pressure Barrier pressure (BrP) = LES pressure – Intragastric pressure - Higher barrier pressure lowers the risk of reflux/aspiration - To prevent aspiration, a positive gradient maintains LES tone ❑ Pharmacologic prophylaxis for aspiration aims to: – Increase gastric pH – Lower gastric volume – Reduce the incidence of emesis 12 Influences On Barrier Pressure ❑ Barrier pressure reduced by: – Decreased LES tone – Increased intragastric pressure – Anticholinergics- decrease LES tone – Pregnancy- decreases LES and increases intragastric pressure – Cricoid pressure (Sellick’s maneuver) decreases LES tone- possibly reflex ❑ Barrier pressure increased by: – Metoclopramide increases LES ❑ No effect on barrier pressure: – Succinylcholine – no change -increases LES and IGP 13 Gastroesophageal Reflux Disease (GERD) ❑ Patients with heartburn have a lower barrier pressure – Endotracheal intubation protects the airway from aspiration in anesthetized patients – Hiatal hernia- gastroesophageal junction dysfunction interferes with barrier pressure 14 Pathological Conditions Associated With The Esophagus ❑ Esophageal pathology: Achalasia: failure of smooth muscle motility and failure of the LES to relax – Prevents transmission of food into stomach- (Idiopathic) Esophagitis: inflammation of esophagus caused by infection (bacterial or viral), or acid reflux from stomach Barrett’s esophagitis – GI metaplasia (transformation of epithelium) of lower esophagus r/t chronic GERD→ main risk for adenocarcinoma of lower esophagus Carcinoma of upper esophagus typically r/t smoking/alcohol 15 II. Motor Functions of the Stomach 16 Motor Functions of the Stomach ❑ During fasting, stomach quiescent ❑ With ingestion of food: 1. Vagovagal (receptive) relaxation allows up to 1.5 L 2. Food causes mixing waves initiated by basic electrical rhythm forms semifluid paste- chyme 3. Slowly empties chyme from stomach at a rate suitable for proper digestion – Intense antral peristaltic contractions (systole) create 50-70 cm H2O pressure provide a pumping action- the pyloric pump forces milliliters of chyme into duodenum ❑ Postprandially, hunger contractions (pangs) begin 1224 hours post-prandially; greatest intensity in 3-4 days Pyloric sphincter is tonically contracted, remains open enough for water to empty into duodenum with ease 17 Receptive Relaxation of the Stomach ❑ Receptive relaxation of the stomach Vagally mediated Triggered by movement of pharynx and esophagus Accommodates food with minimal increase in pressure As peristaltic waves approach the stomach, the wave of relaxation is transmitted through the myenteric inhibitory neurons in advance to entire stomach and duodenum to receive food 18 Regulation of Stomach Emptying ❑ Stomach emptying regulated by signals from duodenum and stomach Antrum, pylorus, and upper duodenum function as a unit Signals from the duodenum are potent- control the rate of stomach emptying ❑ Factors promoting emptying: Stretching stomach wall (increased food volume)- elicits myenteric reflexes Accentuates activity of the pyloric pump and inhibits pylorus For liquids- volume is the most important determinant of rate of emptying Large volumes empty faster than small volumes Gastrin causes secretion of highly acidic gastric juice, stimulates motor functions Most importantly- it enhances the activity of the pyloric pump ❑ Factors inhibiting gastric emptying: Inhibitory signals from the duodenum include both: 1. Enterogastric inhibitory nervous reflexes Duodenal distention 2. Hormonal feedback - CCK potent inhibitor Chyme is monitored and can inhibit gastric emptying if it: Is excessively acidic Contains too much unprocessed protein or fat Is hypotonic or hypertonic- sensed by duodenal osmoreceptors Is irritating 19 Gastric emptying and NPO Guidelines Tonicity Food Character Consistency Fast Intermediate Slow Hypotonic Isotonic Hypertonic Sugars/Starch Protein Fats Liquid Semi-solids Solids Compared to Traditional NPO guidelines – After midnight or 6+ hours Clear, carbohydrate containing solution up to 2 hours preoperatively may lower residual gastric volume 20 III. Movements of the Small Intestine 21 Movements of the Small Intestine ❑ Movement Segmentation movements ”chop” the chime Peristalsis (and segmentation) propels chime Migrating motor complexes (MMCs), interrupt at 90 min intervals to move indigestible components from stomach through small intestine Nervous signals- peristalsis greatly increased after meals Hormonal signals- gastrin, CCK, insulin, motilin affect intestinal motility 22 Movements of the Mucosal Muscle ❑ Mucosal muscle Causes short folds to appear in the intestinal mucosa Individual fibers extend into the intestinal villi to “milk” the villi so lymph flows freely from central lacteals into the lymphatic system 23 Ileocecal valve ❑ Ileocecal valve prevents backflow from the colon to the small intestine – Ileocecal valve protrudes into cecum – Forcefully closed when excess pressure builds in cecum – Resists reflux – Normally 1500-2000 ml chyme empty into cecum each day ❑ Feedback control – Degree of contraction of the ileocecal sphincter and intensity peristalsis in terminal ileum controlled by reflexes from cecum 24 IV. Movements of the Colon 25 Principle Functions Of The Colon: ❑ Main functions: 1. Absorption of water and electrolytes from chyme to form solid feces – Progresses from fluid to solid along the colon 2. Storage of fecal matter until expelled ❑ Mixing Movements- Haustrations – Fecal matter in large intestine is slowly dug into and rolled over to expose all fecal material to the mucosal surface of colon ❑ Large circular contractions + teniae coli ❑ Mass Movements – propulsive movements- force feces into the rectum – Stimulated by the gastrocolic and duodenocolic reflexes resulting from distention of stomach and duodenum – – Occur 1-3 x/day (ulcerative colitis all day) Especially for first hour after breakfast 26 ❑ Defecation Defecation Rectum is usually empty of feces Dribble of fecal matter through the anus prevented by tonic constriction of: – An internal anal sphincter Several cms long Circular smooth muscle Feces inhibits via myenteric plexus – An external anal sphincter Striated voluntary muscle Controlled by pudendal nerve Intrinsic myenteric reflex is weak- must be fortified by a parasympathetic defecation reflex People who too often inhibit their natural reflexes are likely to become severely constipated Newborns and people with transected spinal cords lack control of external sphincter 27 1. Mass movements are often stimulated after a meal by distention of the stomach (gastrocolic reflex) and distention of the duodenum (duodenocolic reflex). Mass movements often lead to which of the following? A. B. C. D. E. Bowel movements Gastric movements Haustrations Esophageal contractions Pharyngeal peristalsis 2. Swallowing is a complex process that involves signaling between the pharynx and swallowing center in the brain stem. Which structure is critical for determining whether a bolus of food is small enough to be swallowed? A. B. C. D. E. Epiglottis Larynx Palatopharyngeal folds Soft palate Upper esophageal sphincter 3. Just prior to induction, your patient states they can still feel their dinner from the night before sitting in their stomach. Which sphincter most concerns you re: the risk of aspiration? A. B. C. D. Upper esophageal sphincter Lower esophageal sphincter Pyloric sphincter Sphincter of Oddi 4. Which of the following increases barrier pressure (decreases likelihood of aspiration? A. B. C. D. Administering reglan Administering cricoid pressure during induction Administering succinylcholine Administering glycopyrralate 28