BSC 216 Ch. 25 Notes
BSC 216 Ch. 25 Notes 10617
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This 12 page Class Notes was uploaded by Gretchen Pierce on Sunday April 12, 2015. The Class Notes belongs to 10617 at University of Alabama - Tuscaloosa taught by Jason Pienaar in Spring2015. Since its upload, it has received 88 views.
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Date Created: 04/12/15
Digestive System Chapter 25 4915 544 PM SaHvann Extrinsic salivary glands secrete about 1 to 15 L of saliva per day 0 Cell of Acini filter water and electrolytes from blood and add amylase mucin and lysozyme Salivary Nuclei in the medulla oblongata and pons respond to signals generated by presence of food 0 Tactile pressure and taste receptors 0 Salivary nuclei receive input from higher brain centers as well Odor sight thought of food stimulates salivation 0 Send signals by way of autonomic fibers in the facial and glossopharyngeal nerves to the glands Parasympathetics stimulate the glands to produce an abundance of thin enzymerich saliva Sympathetic stimulation stimulates the glands to produce less and thicker saliva with more mucus a Dry or sticky under stress or dehydration Bolus mass swallowed as a result of saliva binding food particles into a soft slippery easily swallowed mass The Pharynx Pharyngeal constrictors superior middle and inferior circular muscles that force food downward during swallowing 0 When not swallowing the inferior constrictor remains contracted to exclude air from the esophagus o This constriction is considered to be the upper esophagus sphincter although it is not an anatomical feature 0 Disappears at the time of death when the muscles relax so it is a physiological sphincter not an anatomical structure The Esophagus Esophagus a straight muscular tube 2530cm long 0 Begins at level between C6 and cricoid cartilage o Extends from pharynx to cardiac orifice of stomach passing through esophageal hiatus in diaphragm 0 Lower Esophageal Sphincter food passes at this point because of this constriction o Prevents stomach contents from regurgitating into the esophagus o Protects esophageal mucosa from erosive effect of the stomach acid 0 Heartburn burning sensation produced by acid reflux into the esophagus Swallowing Swallowing Track 0 1 Tongue compresses food against palate to form bolus o 2 Bolus passes into pharynx Misdirection of bolus is prevented by tongue blocking oral cavity soft palate blocking nasal cavity and epiglottis blocking larynx o 3 Upper esophageal sphincter constricts and bolus passes downward o 4 Peristalsis drives bolus down esophagus Esophagus constricts above bolus and dilates and shortens below it o 5 Lower esophageal sphincter relaxes to admit bolus to stomach Swallowing deglutition a complex action involving over 22 muscles in the mouth pharynx and esophagus o Swallowing center a pair of nuclei in medulla oblongata that coordinates swallowing Communicates with muscles of pharynx and esophagus by way of trigeminal facial glossopharyngeal and hypoglossal nerve Swallowing occurs in 2 phases 0 Buccal Phase under voluntary control Tongue collects food presses it against the palate forming a bolus and pushes it posteriorly Food accumulates in oropharynx in front of blade of the epiglottis Epiglottis tips posteriorly and food bolus slides around it through the laryngeal opening Bolus enters laryngopharynx and stimulates tactile receptors and activates next phase 0 Pharyngoesophageal Phase involuntary Three actions prevent food and drink from reentering the mouth or entering the nasal cavity or larynx n Root of the tongue blocks oral cavity a Soft palate rises and blocks nasopharynx n Infrahyoid muscles pull larynx up to meet epiglottis while laryngeal folds close airway Food bolus is driven downward by constriction of the upper middle and lower pharyngeal constrictors Bolus enters esophagus stretches it and stimulates peristalsis Peristalsis wave of muscular contraction that pushes bolus ahead of it entirely involuntary reflex When standing or sitting upright the food and liquid drops through the esophagus by gravity faster than peristalsis can keep up with it o Peristalsis ensures you can swallow regardless of body position 0 Liquid reaches the stomach in 12 seconds 0 Food bolus in 48 seconds 0 When it reaches lower end of the esophagus the lower esophageal sphincter relaxes lets food pass into stomach The Stomach Stomach a muscular sac in upper left abdominal cavity immediately inferior to diaphragm o Primarily functions as a food storage organ Internal volume of about 50mL when empty 10 to 15L when extremely full and extend nearly as far as the pelvis Mechanically breaks up food particles liquefies the food and begins chemical digestion to protein and fat 0 Chyme soupy or pasty mixture of semi digested food in the stomach 0 Most digestion occurs after the chyme passes on to the small intestine Microscopic Anatomy 0 Gastric Pits depressions in gastric mucosa o Lined with simple columnar epithelium o 23 tubular glands open into the bottom of each gastric pit Cardiac glands in cardiac region Pyloric glands in pyloric regions Gastric glands in the rest of the stomach Mucous cells secrete mucus o Predominate in cardiac and pyloric glands o In gastric glands called mucous neck cells since they are concentrated at the neck of the gland Regenerative stem cells found in the base of the pit and in the neck of the gland 0 Divide rapidly and produce a continual supply of new cells to replace cells that die 0 Parietal cells found mostly in the upper half of the gland o Secrete hydrochloric acid HCI intrinsic factor and a hunger hormone called ghrelin Chief Cells most numerous o Secrete gastric lipase and pepsinogen o Dominate lower half of gastric glands 0 Absent in pyloric and cardiac glands Enteroendocrine cells concentrated in lower end of gland o Secrete hormones and paracrine messengers that regulate digestion Gastric Secretions Gastric Juice 23L per day produced by gastric glands o Mainly a mixture of water hydrochloric acid and pepsin 5 main secretions by gastric glands 1 Hydrochloric acid HCI 2 Pepsin Gastric Lipase Intrinsic Factor Chemical Messengers WP OOOOO Hydrochloric Acid 1 O Gastric juice has a high concentration of hydrochloric acid pH as low as 08 Parietal cells produce HCI and contain carbonic anhydrase CAH C02 H20 H2CO3 HCO3 H H is pumped into gastric gland lumen by H KATPase pump Antiporter uses ATP to pump H out and K in HCO3 exchanged for CI chloride shift from blood plasma Cl chloride ion pumped into the lumen of gastric gland to join H forming HCI Elevated HCO3 bicarbonate ion in blood causes alkaline tide increasing blood pH Functions of HCI 1 Activates pepsin and Linguinal lipase 2 Breaks up CT and plant cell walls a helps liquefy food to form chyme 3 Converts ingested ferric ions Fe3 to ferrous ions Fe2 n Fe2 absorbed and used for hemoglobin synthesis 4 Contributes to nonspecific disease resistance by destroying most ingested pathogens 0 Pepsin 2 o Zymogens digestive enzymes secreted as inactive proteins 0 O Converted to active enzymes by removing some of their amino acids Pepsinogen zymogen secreted by the chief cells Hydrochloric acid removed some of its amino acids and forms pepsin active form that digests proteins Autocatalytic effect as some pepsin is formed it converts more pepsinogen into more pepsin Pepsin digests dietary proteins into shorter peptide chains Protein digestion is completed in the small intestine Acidic environment needed to change pepsinogen to pepsin Parietal cell Removed Dlelary peptide proteins gt HCI A J a j 4 3 35 3 L we J a Chief cell Pepelnogen a 39 Imogen 4 a 3 0 Partially digesled protein 39Ccrgrpiglil C Tlm Mr nuHll 0019 th In Putnamquot mun21 1 39Mprmlmtum izv xlaney Gastric gland Gastric Lipase 3 o Gastric lipase produced by chief cells 0 Gastric lipase and Linguinal lipase play a minor role in digesting dietary fats Digests 1015 of dietary fats in the stomach Rest digested in the small intestine Intrinsic Factor 4 o Intrinsic Factor a glycoprotein secreted by parietal cells 0 Essential to absorption of vitamin B12 by the small intestine Binds vitamin 812 and then intestinal cells absorb this complex by receptormediated endocytosis 0 Vitamin 812 is needed to synthesize hemoglobin Prevents pernicious anemia o Secretion of intrinsic factor is the only indispensable function of the stomach Digestion can continue if stomach is removed gastrectomy but 812 supplements will be needed 0 Chemical Messengers 5 o Gastric and pyloric glands have various kinds of enteroendocrine G cells that produce as many as 20 chemical messengers Most are hormones that enter blood and stimulate distant cells Others are paracrine secretions that stimulate neighboring cells Several are peptides produced in both the digestive tract and the CNS gutbrain peptides n Secretin cholecystokinin are the ones we will focus on Gastric Motility Swallowing center of medulla oblongata signals stomach to relax 0 Basically getting ready to accept food 0 Food stretches the stomach activating a receptiverelaxation response 0 Resists stretching briefly but relaxes to hold more food 0 Soon stomach shows a rhythm of peristaltic contractions controlled by pacemaker cells in longitudinal layer of muscularis externa o Gentle ripple of contraction every 20 seconds churns and mixes food with gastric juice 0 Becomes stronger contraction at pyloric region 0 After 30 min or so these contractions become quite strong They churn the food mix it with gastric juice and promote its physical breakup and chemical digestion Digestion and Absorption in the Stomach Digestion salivary and gastric enzymes partially digest protein and lesser amounts of starch and fat in the stomach Stomach not really involved with absorption of nutrients 0 Most digestion and nearly all absorption occur after the chyme has passed into the small intestine Stomach does not absorb any significant amount of nutrients o Aspirin 0 Some lipidsoluble dugs Alcohol is absorbed mainly by small intestine o Intoxicating effects depend partly on how rapidly the stomach is emptied Protection of the Stomach 0 Why does the stomach NOT digest itself 0 Living stomach is protected in 3 ways from the acidic enzymatic environment it creates 1 Mucous Coat thick highly alkaline mucus resists action of acid and enzymes 2 Tight Junctions between epithelial cells prevent gastric juice from seeping between them and digesting the CT of the lamina propria and beyond 3 Epithelial Cell Replacement Stomach epithelial cells live only 36 days a Sloughed off into the chyme and digested with the food a Replaced rapidly by cell division in the gastric pits o Breakdown of these protective measures can result in inflammation and peptic ulcer Peptic Ulcer Gastritis inflammation of the stomach can lead to a peptic ulcer as pepsin and hydrochloric acid erode the stomach wall 0 Most ulcers are caused by acidresistant bacteria Helicobacter pylori that can be treated with antibiotics and PeptoBismol Regulation of Gastric Function Nervous and endocrine systems collaborate o Increases gastric secretions and motility when food is eaten o Suppresses them when the stomach empties Gastric activity is divided into 3 phases 0 Cephalic Phase stomach being controlled by brain 0 Gastric Phase stomach controlling itself o Intestinal Phase stomach being controlled by small intestine Phases overlap and can occur simultaneously 0 Cephalic Phase 1 o Stomach responds to sight smell taste or thought of food 0 Sensory and mental inputs converge on the hypothalamus Relays signals to medulla oblongata o Vagus nerve fibers from medulla oblongata stimulate the enteric nervous system ENS of stomach In turn stimulates gastric secretion 0 Gastric Phase 2 0 Period in which swallowed food and semi digested protein activate gastric actively 23 gastric secretion occurs in this phase 0 Ingested food stimulates gastric activity in 2 ways Stretching the stomach I Activates short reflex mediated through myenteric nerve plexus n Activates long reflex mediated through vagus nerves and brain stem By increasing the pH of its contents 0 Gastric Secretion is stimulated by 3 chemicals 1 Acetylcholine Ach secreted by parasympathetic nerve fibers of both reflexes 2 Histamine a paracrine secretion from enteroendocrine cells in the gastric glands 3 Gastrin a hormone produced by the enteroendocrine G cells in pyloric glands Intestinal Phase 3 0 Stage in which the duodenum responds to arriving chyme and moderates gastric activity through hormones and nervous re exes o Duodenum initially enhances gastric secretion but soon inhibits it Enhancing mechanisms Stretching of the duodenum accentuates vagovagal reflex that stimulates the stomach Peptides and amino acids in chyme stimulate G cells of the duodenum to secrete more gastrin which further stimulates the stomach Inhibitory mechanisms Enterogastric reflex duodenum sends inhibitory signals to the stomach by way of the ENS and signals to the medulla oblongata triggered by acid and semi digested fats in the duodenum n Inhibits vagal nuclei reducing vagal stimulation of the stomach I Stimulate sympathetic neurons sends inhibitory signals to the stomach Chyme also stimulates duodenal enteroendocrine G cells to release secretin and cholecystokinin a They stimulate the pancreas and gallbladder important in digestion in the small intestine I Also suppress gastric secretion 4915 544 PM 4915 544 PM