2009 Cengage-Wadsworth
Chapter 2
The Digestive System:Mechanism for Nourishing the
Body
2009 Cengage-Wadsworth
The Structures of the Digestive Tract & the Digestive Process
• Main structures: – Oral cavity– Esophagus– Stomach– Small & large
intestines
• Accessory organs: – Pancreas– Liver– Gallbladder
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The Structures of the Digestive Tract & the Digestive Process
• Lumen has 4 main layers:– Mucosa
• Epithelium, lamina propria, muscularis mucosa
– Submucosa• Connective tissue, lymphoid tissue, submucosal
plexus
– Muscularis externa• Circular & longitudinal smooth muscle, myenteric
plexus
– Serosa or adventitia• Connective tissue, visceral peritoneum
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The Oral Cavity
• Mouth & pharynx• Salivary glands• Enzymes in saliva
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The Esophagus
• Moves bolus from oral cavity to stomach
• Swallowing– Voluntary, pharyngeal, esophageal
• Peristalsis• Gastroesophageal sphincter• Selected diseases & conditions of
the esophagus - GERD
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The Stomach
• 4 main regions:– Cardia region– Fundus– Body– Antrum or distal pyloric region
• Grinds food & mixes with gastric juices to form chyme
• Strong peristalsis for gastric emptying
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The Stomach
• Gastric glands:– Cardiac glands– Oxyntic glands– Pyloric glands
• Cells in glands:– Neck (mucus)
cells– Parietal (oxyntic)
cells– Chief (peptic or
zymogenic) cells– Enteroendocrine
cells
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The Stomach
• Gastric juice– HCl
• Converts pepsinogen to pepsin• Denatures proteins• Releases nutrients from organic complexes• Acts as bacteriocide
– Enzymes - pepsin, amylase, lipase– Mucus– Intrinsic factor
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The Stomach
• Regulation of gastric secretions– Hormones & peptides that inhibit
gastric secretions– Hormones & neuropeptides that
stimulate gastric secretions – Other factors– HCl release stimulated by gastrin,
acetylcholine, histamine
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The Stomach
• Selected diseases & conditions of the stomach - peptic ulcers
• Regulation of gut motility & gastric emptying
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The Small Intestine
• Structural aspects of the small intestine– Folds of Kerckring - large circular
folds of mucosa– Villi - finger-like projections– Microvilli - hair-like extensions of the
cells’ plasma membranes– Brush border– Crypts of Lieberkühn
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The Small Intestine
• Regulation of intestinal secretions & motility
• Immune system protection of the gastrointestinal tract– Mucosa-associated lymphoid tissue
(MALT)– Gut-associated lymphoid tissue
(GALT)
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The Accessory Organs
• The pancreas– Digestive enzymes produced by
acinar exocrine cells– Pancreatic juice contains:
• Bicarbonate• Electrolytes• Pancreatic digestive enzymes
– Regulation of pancreatic secretions– Pancreatic digestive enzymes
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The Accessory Organs
• The liver– Consists of lobes made of lobules– Portal circulation– Bile canaliculi
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The Accessory Organs
• The gallbladder– Bile synthesis– Selected conditions/diseases of the
gallbladder– Bile storage– The function of bile– The recirculation & excretion of bile– Bile circulation & hypercholesterolemia
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The Digestive & Absorptive Processes
• Digestion– Breaking nutrients down– Occurs in lumen & at brush border
• Most digestion & absorption occur in small intestine
• Absorption may be by diffusion, facilitated diffusion, active transport, or pinocytosis/ endocytosis
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The Digestive & Absorptive Processes
• Mechanism of absorption depends on:– Solubility (fat vs. water)– Concentration or electrical gradient– Size of molecule
• Unabsorbed materials continue to colon
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The Colon or Large Intestine
• Contracts to mix materials• Proximal colonic epithelia absorb
Na, Cl, H2O
• Materials dehydrated• Intestinal bacteria populations of
interest
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Intestinal Bacteria (Microflora), Pre- and Probiotics, and Disease
• Fermentation - breakdown of CHO & protein anaerobically
• Generate lactate & short-chain fatty acids, gases
• Probiotics - foods containing live bacterial cultures
• Prebiotics - food ingredients that promote bacterial growth
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Intestinal Bacteria (Microflora), Pre- and Probiotics, and Disease
• Probiotics are thought to:– Enhance immunity– Prevent colonization by pathogens– Lower pH of the colon– Transform/promote excretion of toxic
substances– Enhance fecal bulk
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Coordination & Regulation of the Digestive Process
• Neural regulation– Myenteric plexus - peristalsis, motility– Submucosal plexus - secretions, local
blood flow
• Regulatory peptides– Hormones– Paracrines
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Regulatory Peptides
• Gastrin• Cholecystokinin
(CCK)• Secretin• Motilin• Glucose-
dependent insulinotropic peptide (GIP)
• Peptide YY• Enteroglucagon• Amyline
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Regulatory Peptides
• Paracrine-acting substances– Somatostatin– Glucagon-like
peptides– Insulin-like growth
factors
• Neurocrine peptides– Vasoactive
intestinal polypeptide (VIP)
– Gastrin-releasing peptide (GRP)
– Neurotensin– Substance powder
(P)
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Perspective 2
An Overview of Selected Digestive System Disorders
with Implications for Nourishing the Body
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Selected Disorders
• Disorder 1: Gastroesophageal reflux cisease
• Disorder 2: Inflammatory bowel diseases
• Disorder 3: Celiac disease• Disorder 4: Chronic pancreatitis
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