The Digestive System

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The Digestive System

• Chapter 19

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Gastrointestinal (GI) Tract

• Tube that includes mouth, Pharynx, Esophagus,
  Stomach, Small intestine, Large intestine
• Accessory organs teeth, tongue, salivary glands,
  liver, gallbladder, and pancreas

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Figure 19.1
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Overview- Operations

• Ingestion eating
• Secretion release of water, enzymes buffers
• Mixing propulsion movement along GI tract
• Digestion mechanical and chemical breakdown of
  foods
• Absorption getting it into the body
• Defecation dumping waste products defecation

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Wall Layers- Everywhere

• 4 layers
• Mucosa- epithelium, connective layer, glands,
  muscularis mucosae
• Submucosa- connective tissue, blood vessels,
  lymphatic vessels, enteric nervous system
• Muscularis- circular layer, longitudinal layer
• In mouth, pharynx upper esophagus skeletal
  muscle
• Also in external anal sphincter
• Serosa or Visceral peritoneum

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Figure 19.2
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Figure 19.3a
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Figure 19.3b
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Mouth

• Formed by cheeks, hard soft palate tongue
• Soft palate at back includes a hangy down part
  uvula
• During swallowing uvula prevents entry into nasal
  cavity
• Tongue- muscular accessory organ
• maneuvers food for chewing
• Adjusts shape for speech swallowing
• Lingual tonsils at base of tongue

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Salivary Glands

• 3 pairs of salivary glands
• Ducts empty into oral cavity
• Parotid-
• inferior anterior to ears
• Submandibular-
• in floor of mouth, medial inferior to mandible
• Sublingual
• Beneath tongue and superior to submandibular
• Saliva contains 99.5 water, salivary amylase,
  mucus and other solutes
• Dissolves food starts digestion of starches

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Figure 19.4
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Teeth

• Accessory organs in bony sockets of mandible
  maxilla
• 3 external regions
• Crown- above gums
• Root- 1 or more parts embedded in socket
• Neck between crown and root near gum line
• 3 layers of material
• Enamel- covers crown
• Dentin- majority of interior of tooth
• Pulp cavity - nerve, blood vessel lymphatics

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Figure 19.5
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Digestion in the Mouth

• Mechanical breakdown- chewing
• Mixed with saliva by tongue
• Salivary amylase chemically breaks down
  polysaccharides (starch)
• ?maltose and larger fragments
• Continues in the stomach until acidified
• Rounds up food into a soft bolus for swallowing

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Pharynx Esophagus

• On swallowing
• Bolus of food ? oropharynx?
• Laryngopharynx? esophagus
• Muscular contractions in pharynx help
• Upper esophageal sphincter (UES)
• Skeletal muscle controls entry to esophagus
• Lower esophageal sphincter (LES)
• Smooth muscle- regulates entry to stomach

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Figure 19.6a,b
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Swallowing

• Voluntary bolus forced into oropharynx
• Triggers oropharyngeal stage
• Involuntary breathing interrupted
• Soft palate move up-close nasopharynx
• Epiglottis seals off larynx
• Bolus moves into esophagus through UES
• Esophageal stage? peristalsis moves it toward
  stomach

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Figure 19.6c
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Stomach

• J- shaped enlargement of tract
• Serves as mixing chamber and holding reservoir
• Very elastic muscular
• 4 regions
• Cardia- surrounds upper opening
• Fundus- superior to left of cardia
• Body large central portion
• Pylorus- lower part leading to pyloric sphincter
  duodenum

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Figure 19.7
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Stomach Wall

• Mucosa
• Folds called rugae
• Epithelium- simple columnar mucous
• Form gastric glands lining gastric pits
• Secretory cells mucous neck cells
• Chief cells? inactive enzyme pepsinoge
• Parietal cells? HCl intrinsic factor
• Collectively gastric juice
• Muscularis- 3 Layers longitudinal, circular
  oblique

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Figure 19.8
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Figure 19.9
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Digestion Absorption

• Food entry? stretch rise in pH
• ?Nerve impulses ? secretion mixing waves? Food
  mixed with juice ?Chyme
• Small amount pushed through pyloric sphincter
• gastric emptying- Carb. foods fastest, lipids
  next proteins slowest
• Entry in duodenum? feedback inhibition of stomach
  activity
• Pepsin digests protein? peptides
• Little absorption- water, ions some drugs

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Pancreas

• Behind stomach-
• Produces pancreatic juice in acinar cells
• to duodenum via pancreatic duct
• NaHCO3 solution (pH 7.1-8.2) 1000ml/day
• Neutralize stomach acid and dilutes chyme
• Panceas- digestive enzymes
• Proteases chymotrypsinogen, trypsinogen, et. al.
• Activated by entreokinase from intestine
• Starch digesting- pancreatic amylase
• Pancreatic lipase
• Nucleotidases RNAase DNAase

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Liver Gall Bladder

• Largest organ after the skin
• On right below diaphragm
• Functional unit is lobule-
• Hepatocytes around central vein
• Open capillaries sinusoids
• Bile ? canaliculi? ducts? hepatic duct
• ?Gall bladder Pear-shaped organ on front (stores
  bile)
• ?cystic duct ?common bile duct

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Bile

• Bicarbonate, bile salts waste. 1000 ml/day
• Important for emulsifying fats
• Increases surface area for digestion
• Pigment is bilirubin- from broken-down heme
  during RBC recycling
• Digested to strecobilin- brown color
• Bile salts reabsorbed at end of small intestine-
  ileum
• recycle to liver in portal circulation

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Figure 19.10
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Figure 19.11a
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Figure 19.11b
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Liver Function

• Maintains blood glucose
• Stores as glycogen
• Uses absorbed sugars Converts amino acids?
  glucose
• Lipid metabolism
• Produces cholesterol triglycerides, makes bile
• Makes lipoproteins for lipid transport
• Excretion of bilirubin
• Processes drugs and other chemicals
• Store fat soluble vitamins
• Make active vitamin D

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Small Intestine

• 3 parts duodenum, jejunum, ileum
• Where most of the digestion occurs
• Essentially all of the nutrient absorption
• Ends in ileocecal sphincter

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Figure 19.12a
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Figure 19.12b
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Wall Structure

• Same 4 layers
• Epithelial- simple columnar
• Absorptive cells with microvilli
• Goblet cells- secrete mucus
• Intestinal glands- intestinal juice hormones
• Secretin, cholecystokinin (CCK),
  Glucose-dependent-insulinotrophic peptide (GIP)
• Lymphatic tissue- defense

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Wall Structure (Cont.)

• Duodenal glands- alkaline mucus
• Helps neutralize stomach acid
• Circular folds- increase surface area
• Villi- finger like projections of mucosa
• Increase surface area for absorption
• Include lacteals for lipid absorption

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Figure 19.13
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Motility Secretions

• Secretions alkaline, some enzymes
• Peptidases-breaks small peptides
• Disaccharidases attached to wall
• Water and salt to balance osmolality
• 2000 ml/day
• Segmentation activity- for mixing
• Peristalsis for movement after most absorption
  completed- slow waves

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Digestion Absorption

• Chyme enters with partially digested
  carbohydrates proteins
• Bile pancreatic juice intestinal juice
  completes the job
• Absorption is of monosaccharides amino acids
  phosphate sugar bases of DNA RNA fatty acids
  monoglycerides

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Carbohydrate Digestion

• Amylases
• Starch dextrin ? maltose
• Disaccharidases at surface
• Maltose maltose ? glucose
• Sucrase sucrose ? glucose fructose
• Lactase lactose? glucose galactose

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Protein Fat Digestion

• Trypsin, chymotrypsin, elastase, carboxypeptidase
  pepsin
• Proteins? small peptides
• Peptidases at surface
• Peptides? amino acids di- tri- peptides
• Lipase
• glycerides? fatty acids monoglycerides

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Absorption

• By diffusion, facilitated diffusion, osmosis
  active transport
• Carbohydrates ? monosaccharides
• Via portal system to liver
• Proteins (jejunum ileum)? amino acids
• Via portal system to liver
• Lipids? reformed to triglycerides
• Packaged in chlyomicrons with protein
• Via lacteals ? lymphatics

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Absorption (Cont.)

• Water salt
• Primarily osmotic movement along with other
  nutrients
• Vitamins
• Fat soluble absorbed with fat
• Water soluble with simple diffusion
• B12 combines with intrinsic factor absorbed by
  active transport in ileum

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Figure 19.14a
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Figure 19.14b
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Large Intestine

• Cecum, colon, rectum, anal canal
• Ileocecal canal? large intestine
• Below is cecum with appendix
• Colon- ascending, transverse, descending
  sigmoid
• ? rectum? anal canal
• Standard 4 layers with mucus secretion
• Few folds , little specialization for absorption
• Muscularis circular bands of longitudinal
  muscle

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Figure 19.15a
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Figure 19.15b
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Figure 19.16
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Digestion Absorption

• Slow emptying of ileum
• Slow peristalsis
• Mass peristalsis with food in stomach
• Moves from middle of colon ? rectum
• Bacterial digestion
• Produce some B-vitamins Vit. K
• Produce gases flatus
• Colon absorbs salt water

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Defecation Reflex

• Stretch of rectum wall? neural reflex
• ? contraction of longitudinal muscle
• Combined pressure parasympathetic activity?
  relaxing of internal anal sphincter
• External anal sphincter is voluntary
• Contraction of diaphragm abdominal wall muscles
  aid defecation

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Control

• Rule activate forward and inhibit behind
• three phases Cephalic, gastric, intestinal
• Cephalic- smell, sight, thought of food
• Neural signals stimulates salivary glands
  gastric glands
• Gastric- stretching, pH of stomach
• Gastrin activates stomach LES relaxes pyloric
  sphincter
• Neural signals gastrin signal satiety
  (fullness)

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Control (Cont.)

• Intestinal- responses to food entering duodenum
• neural endocrine
• CCK stimulated by AA fats
• Pancreatic enzyme release
• Gall bladder contraction
• Contraction of pyloric sphincter
• Acid stimulates secretin
• Stimulates HCO3- ions in pancreatic juice
• Inhibits gastrin action in stomach

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Aging

• Decreased secretion, motility, strength of
  responses
• loss of taste, periodontal disease, hiatal
  hernia, gastritis peptic ulcer disease
• Increased incidence of gall bladder problems,
  cirrhosis of liver, pancreatitis, constipation,
  hemorrhoids diverticulitis