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Chapter 38
Structure and Function of the
Digestive System
Mosby items and derived items © 2010, 2006 by Mosby, Inc., an affiliate of Elsevier Inc.
Gastrointestinal Tract
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Mouth
Esophagus
Stomach
Small intestine
Large intestine
Rectum
Anus
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Gastrointestinal Tract
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Gastrointestinal Tract Functions:
Overview
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Breaks down ingested food
Prepares food for uptake by the body’s cells
Provides body water
Eliminates wastes
Controlled by hormones and the autonomic
nervous system (except for chewing, swallowing,
and defecation)
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Gastrointestinal Tract
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Ingestion of food
Propulsion of food and wastes from the
mouth to the anus
Secretion of mucus, water, and enzymes
Mechanical digestion of food particles
Chemical digestion of food particles
Absorption of digested food
Elimination of waste by defecation
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Gastrointestinal Tract
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Histology
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Mucosa
Submucosa
Muscularis
Serosa or adventitia
Enteric plexus
Submucosal plexus
Myenteric plexus
Subserosal plexus
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Gastrointestinal Tract
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Mouth
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Reservoir for chewing and mixing of food with
saliva
Taste buds
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Salty, sour, bitter, and sweet
Olfactory nerves
Teeth
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32 permanent teeth
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Salivary Glands
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Three pairs
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Submandibular
Sublingual
Parotid
Saliva
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Water with mucus, sodium, bicarbonate, chloride,
potassium, and salivary α-amylase (carbohydrate
digestion)
Controlled by sympathetic and parasympathetic
fibers
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Swallowing
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Complex event mediated by the swallowing
center, located in the reticular formation of the
brainstem; also involves other brain regions,
including the insula/claustrum and cerebellum
The swallowing center and respiratory center
provide the coordinating innervation
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Superior constrictor muscle of the pharynx contracts,
prevents movement of food into the nasopharynx
Respiration inhibited and epiglottis slides downward
to prevent bolus from entering larynx and trachea
Movements of tongue and pharyngeal constrictors
propel the food into the esophagus in a series of
coordinated events taking 1 or 2 seconds
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Swallowing
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Esophagus
Peristalsis
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Primary and secondary
Upper esophageal sphincter
Lower esophageal sphincter
Phases
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Oral and pharyngeal phases
Esophageal phase
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Stomach
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A hollow, muscular organ that stores food,
secretes digestive juices, mixes food with the
juices, and propels partially digested food
(chyme)
Muscle layers
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Longitudinal
 Circular
 Oblique
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Stomach
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Boundaries
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Cardiac orifice
Pyloric sphincter
Pylorus
Functional areas
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Fundus
Body
Antrum
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Stomach
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Gastric Motility
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Swallowing
Gastrin
Cholecystokinin
Motilin
Secretin
Gastric emptying
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Gastric Secretion
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The stomach secretes large volumes of
gastric juices
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Mucus
 Acid
 Enzymes
 Hormones
 Intrinsic factor
 Gastroferrin
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Gastric Secretion
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Gastric pits
Gastric glands
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Parietal cells
• Hydrochloric acid and intrinsic factor
 Chief cells
• Pepsinogen
 G cells
• Gastrin
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Gastric Secretion
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Gastric glands
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Enterochromaffin-like cells
• Histamine
D cells
• Somatostatin
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Gastric Pits and Gastric Glands
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Gastric Secretion
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Phases of gastric secretion
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Cephalic phase
Gastric phase
Intestinal phase
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Small Intestine
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5 to 6 meters long
Three segments
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Duodenum
Jejunum
Ileum
• Ileocecal valve
Peritoneum
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Peritoneal cavity
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Small Intestine
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Muscle layers
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Mucosal folds (plica)
Villi
Microvilli
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Outer—longitudinal
Inner—circular
Brush border
Lamina propria
Lacteal
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Small Intestine
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Intestinal Digestion and
Absorption
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Hydrochloric acid
Pepsin
Pancreatic enzymes
Intestinal enzymes
Bile salts
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Intestinal Digestion and
Absorption
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Carbohydrates
Proteins
Fats
Water
Electrolytes
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Intestinal Digestion and Absorption
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Intestinal Digestion and Absorption
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Intestinal Digestion and Absorption
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Intestinal Motility
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Movements of the small intestine facilitate
digestion and absorption
Segmentation
Peristalsis
Ileogastric reflex
Intestinointestinal reflex
Gastroileal reflex
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Large Intestine
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Cecum
Appendix
Colon
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Ascending
Transverse
Descending
Sigmoid
Rectum
Anus
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Large Intestine
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Large Intestine
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Ileocecal valve
O’Beirne sphincter
Internal anal sphincter
External anal sphincter
Taenia coli
Haustra
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Gastrointestinal Absorption
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Intestinal Bacteria
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Stomach environment is relatively sterile
because of secreted stomach acid
Bile acid secretion, motility, and antibody
production keep bacterial numbers in the
duodenum to a minimum
A low concentration of aerobes in the jejunum
Anaerobic bacteria distal to the ileocecal
valve
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Anaerobes are 95% of the fecal flora in the colon
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Accessory Organs of Digestion
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Accessory Organs of Digestion
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Liver
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Lobes
• Separated and attached to the anterior abdominal wall by
the falciform ligament
• Right lobe

Caudate and quadrate lobes
• Left lobe
 Glisson capsule
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Liver
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Hepatic Portal Circulation
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Liver
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Liver lobules
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Hepatocytes
 Lipocytes
 Sinusoids
 Bile canaliculi
 Common bile duct
 Major duodenal papilla (sphincter of Oddi)
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Liver Lobules
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Liver
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Kupffer cells
Stellate cells
Pit cells
Disse space
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Secretion of Bile
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Alkaline, bitter-tasting, yellowish green fluid
that contains bile salts, cholesterol, bilirubin,
electrolytes, and water
Formed by hepatocytes and secreted into the
bile canaliculi
Enterohepatic circulation
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Recycling of bile salts
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Enterohepatic Circulation
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Metabolism of Bilirubin
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A byproduct of the destruction of aged red
blood cells
Gives bile a greenish black color and
produces the yellow tinge of jaundice
Unconjugated bilirubin
Conjugated bilirubin
Urobilinogen
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Metabolism of Bilirubin
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Vascular and Hematologic
Liver Functions
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Blood storage
Bacterial and foreign particle removal
Synthesizes clotting factors
Produces bile to absorb fat-soluble vitamins
Metabolizes fats
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Vascular and Hematologic
Liver Functions
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Metabolizes proteins
Metabolizes carbohydrates
Metabolic detoxification
Storage of minerals and vitamins
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Gallbladder
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A sac-like organ that lies on the inferior
surface of the liver
Function is to store and concentrate bile
between meals
Gallbladder holds about 90 ml of bile
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Exocrine Pancreas
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Composed of acini and networks of ducts that
secrete enzymes and alkaline fluids to assist
in digestion
Pancreatic duct
Ampulla of Vater
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Exocrine Pancreas
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Exocrine Pancreas
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Secretions
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Potassium, sodium, bicarbonate, magnesium,
calcium, and chloride
Enzymes
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Trypsinogen, chymotrypsinogen, and
procarboxypeptidase
 Trypsin inhibitor
 Pancreatic α-amylase
 Pancreatic lipase
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Aging and the Digestive Function
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Tooth enamel and dentin wear down
Teeth are lost
Periodontal disease
Gum recession
Osteoporotic bone changes
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Aging and the Digestive Function
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Number of taste buds decline
Sense of smell diminishes
Salivary secretion decreases
Decreased esophageal and gastric motility
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