Digestive System
Chapter 23
Overview
Digestive Processes
Functional Concepts
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Mechanical and Chemical Stimuli
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Intrinsic and extrinsic Controls
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Two types of reflexes
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Short =enteric (local)
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Long =CNS and autonomic nerves
Digestive Anatomy Overview
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Hepatic portal system
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4 layers
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Mucosa -mucous membrane
Submucosa -areolar tissue with
blood and lymphatic vessels
Muscularis Externa -circular and
longitudinal muscle layers
Serosa -areolar and simple
squamous
Enteric Nervous System
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Submucosal plexus
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Myenteric plexus
Functional Anatomy
Functional Anatomy
Saliva
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Greater than 97% water
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PH 6.75-7.0
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Electrolytes
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Enzymes: Salivary amylase and lingual lipase
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Mucin -glycoprotein
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Immune proteins: Lysozyme, definsin, IgA
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Urea and Uric acid
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Naturally occurring bacteria
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http://www.textbookofbacteriology.net/normalflora.ht
Salivary Control
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Intrinsic salivary glands keep mouth moist
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Extrinsic glands moisten food
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Food entering mouth activates
Parasympathetic innervation triggers enzyme rich
saliva
Sympathetic triggers thick saliva (mucin rich)
Chemical and mechanical stimulate nuclei in
pons and medulla
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Sight and smell of food also activate
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Lower GI irritation, spicy food, acidic food
Teeth and Gums
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Mechanical breakdown of food
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Oral disease may cause:
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Atherosclerotic plaques
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Clotting within coronary and cerebral arteries
Pharynx and Esophagus
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Pharynx contains friction free stratified
squamous epithelia
Esophagus is a muscular tube
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GERD
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Hiatal hernia
Digestive Process:
Mouth to Esophagus
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Mechanical breakdown
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Deglutition (swallowing) uses 22 muscle groups
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Buccal phase -voluntary movement within mouth
Pharyngeal-esophageal phase -involuntary
movement controlled by pons and medulla
The Stomach
Stomach
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Simple columnar epithelium -secretes thick
alkaline mucous
Gastric pits and glands
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Secretions depend upon location
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Mucous neck cells -thin mucous
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Parietal cells -secretes H+, Cl-, intrisic factor
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Chief cells -pepsinogen and lipases
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Enteroendocrine cells -histamine, serotonin,
somatostatin, gastrin
Homeostatic Imbalances
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Gastric Ulcers
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90% caused by Helicobacter pylori
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10-20% of infected show symptoms
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Inhibit or lessen HCl production
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Damage stomach epithelium
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Disrupt cell junctions
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Promote chronic inflammation
Digestive Processes in the Stomach
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Protein digestion begins
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Alcohol and aspirin absorb into blood stream
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Secretion of intrinsic factor
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Required for absorbtion of vitamin B12
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B12 is needed for RBC production
Regulation of Gastric Secretion
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Neural mechanisms
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Hormonal mechanism
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Stimuli act on brain, stomach, intestine
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Three phases: cephalic, gastric, intestinal
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Not mutually exclusive phases
Cephalic (Reflex) Phase
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Short, lasting only minutes
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Prepares stomach to receive food
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Sight and smell of food are the trigger
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Stimulates mucous, chief, parietal and G cells
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Conditional response
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Special senses → hypothalamus → medulla → vagus → enteric plexus → stomach glands
Gastric Phase
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3-4 hours
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Neural and hormonal response
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Triggers
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Distension of stomach
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Increase in pH (decrease in acidity)
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Presence of undigested materials (proteins)
Gastric Phase
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Neural response
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Produces mixing waves
Stretch and chemoreceptors trigger short
(myenteric) reflexes
Ach from parsympathetic neurons stimulates
parietal and chief cells

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Increase in HCl and Pepsinogen
Proteins, alcohol, caffine stimulate chemoreceptors
Gastric Phase
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Hormonal response
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Increasing gastrin levels stimulates parietal and
chief cells
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Gastrin enters blood stream circulates back to stomach
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Increase in HCl and Pepsinogen
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Decrease in pH (acidity)
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Increase in gastric motility
Alkaline tide
Intestinal Phase
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Very long, lasting hours
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Controls gastric emptying
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Neural and hormonal responses
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Neural Responses
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Chyme leaving stomach relieves distension
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No-longer stimulating stretch receptors
Distension of duodenum by chyme stimulates intestinal stretch receptors
=enterogastric reflex
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Inhibits medulla (gastrin secretions) and local reflexes (gastric contractions)
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Stimulates contraction of pyloric sphincter
Mucous production stimulated in duodenum to protect
intestine
Intestinal Phase
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Hormonal response
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Triggered by the arrival of chyme in duodenum
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Arrival of lipids and carbohydrates
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Stimulates cholesystokinin (CCK) and gastric inhibitory peptide (GIP)
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CCK inhibits gastric secretion of acids and enzymes
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GIP inhibits gastric secretion and rate of contraction
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Meals low in fat enter intestine quicker than those high in fat
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Decrease in ph stimulates enteroendocrine cells to release secretin
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Secretin inhibits parietal and chief cells
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Stimulates pancreas to produce bicarbonate
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Stimulates liver to secrete bile
Partially digested proteins in duodenum stimulate G cells in duodenal wall.
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Allows more time for lipids to be broken down by intestine
G cells produce gastrin which circulates back to stomach increasing acid production
Gastric processing meets digestive requirements of specific meals
Gastric Motility and Emptying
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Response to distension of stomach
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Gastric Contractile Activity
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Smooth muscle pacemaker cells
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Interstitial cells of Cajal set basic electrical rhythm
(BER)
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Set maximum rate of contraction
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Do not initiate contractions
Regulation of Gastric Emptying
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Stomach and duodenum work in conjunction