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The Digestive System
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Overview of the Digestive System
• Organs are divided into two groups– Alimentary canal
• Mouth, pharynx, and esophagus
• Stomach, small intestine, and large intestine
– Accessory digestive organs
• Teeth and tongue• Gallbladder, salivary
glands, liver, and pancreas
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Digestive Processes
• Ingestion – occurs in the mouth• Propulsion – movement of food
– Peristalsis – major means of propulsion
• Mechanical digestion – prepares food for chemical digestion
– Chewing, churning of food in stomach, segmentation
• Chemical digestion – complex molecules broken down to chemical components
– Mouth– Stomach– Small intestine
• Absorption – transport of digested nutrients
• Defecation – elimination of indigestible substances as feces
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Digestive Tract Smooth Muscle Movement• Peristalsis
– Major means of propulsion– Adjacent segments of the
alimentary canal relax and contract
• Segmentation – Rhythmic local contractions of
the intestine– Mixes food with digestive juices
Figure 22.3a
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Histological Organization of Digestive Tract• Mucosa lines digestive tract (mucous epithelium)
– Epithelium lines the lumen, moistened by secretions of mucosal glands
– Lamina propria – loose areolar tissue
– Muscularis mucosa – layer of smooth that creates folds called rugae
• Submucosa - layer of dense irregular connective tissue, vascularized (blood and lymphatic), innervated – nerve plexus, submucosal glands
• Muscularis externa - smooth muscle arranged in circular and longitudinal layers• Adventitia or Serosa - serous membrane made of areolar connective tissue with
collagen and elastic fibers, covers most of the muscularis externa
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Digestive Tract Membranes• Peritoneum – a serous membrane
– Parietal peritoneum – lines the body wall– Visceral peritoneum – surrounds digestive organs
• Peritoneal cavity – a fluid filled space
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Digestive Tract Membranes• Mesentery – a double layer of peritoneum
– Holds organs in place– Sites of fat storage– Provides a route for circulatory vessels and nerves
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Mesenteries• Greater omentum – a “fatty apron” of peritoneum
– Attaches the greater curvature of the stomach to the dorsal body wall
– Covers the transverse colon and a large part of the small intestine
Figure 22.10c
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Mesenteries• Lesser omentum attaches to lesser curvature
of stomach
Figure 22.10b
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Oral cavity• Lined by oral mucosa = stratified
squamous but no serosa• Roof of cavity = hard and soft
palates• Uvula guards opening to
pharynx• Floor of cavity = tongue
– Mechanical processing– Assistance in chewing and
swallowing– Sensory analysis by touch,
temperature, and taste receptors
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The Oral Cavity• Its functions include:
– Analysis of material before swallowing
– Mechanical processing by the teeth, tongue, and palatal surfaces
– Lubrication
– Limited digestion
Figure 24.6a, b
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Salivary glands (three pairs)• Parotid, sublingual, and submandibular• Produce saliva - watery solution includes electrolytes, buffers,
glycoproteins, antibodies, enzymes • Functions include: lubrication, moistening, and dissolving, Initiation
of digestion of complex carbohydrates
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Teeth• Function in mastication of bolus• Contain three layers
– Enamel covering crown– Dentin forms basic structure– Root - periodontal ligaments hold teeth in alveoli
• 20 primary teeth AKA deciduous teeth• 32 teeth of secondary dentition
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The Pharynx• Oropharynx and laryngopharynx• Common passageway for food, liquids, and air• Lined with stratified squamous epithelium, no serosa• External muscle layer
– Consists of superior, middle, and inferior pharyngeal constrictors – Pharyngeal constrictor muscles assist in swallowing
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The Esophagus• Muscular tube - begins as a
continuation of the pharynx• Carries solids and liquids
from the pharynx to the stomach– Passes through esophageal
hiatus in diaphragm• Joins the stomach inferior to
the diaphragm– Cardiac sphincter – closes
lumen to prevent stomach acid from entering esophagus
• The wall of the esophagus contains mucosal (stratified squamous), submucosal, and muscularis layers
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The Esophagus– Epithelium is stratified squamous epithelium– When empty – mucosa and submucosa in longitudinal folds– Mucous glands – primarily compound tubuloalveolar glands– Muscularis externa
• Skeletal muscle first third of length
– Adventitia – most external layer
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The Stomach• Site where food is churned into chyme, Mechanical
breakdown of food • Breaking of chemical bonds via acids and enzymes,
decretion of pepsin and HCl begins protein digestion• Bulk storage of undigested food, food remains in
stomach approximately 4 hours
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Anatomy Of The Stomach• Cardia – superior, medial portion • Fundus – portion superior to stomach-esophageal junction• Body – area between the fundus and the curve of the J• Pylorus – antrum and pyloric canal adjacent to the duodenum• Rugae - ridges and folds in relaxed stomach
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Microscopic Anatomy of the Stomach• Muscularis has three layers
– Circular and longitudinal layers and oblique layer
• Epithelium is simple columnar epithelium• Mucosa dotted with gastric pits holds gastric glands
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Microscopic Anatomy of the Stomach• Gastric glands of fundus and body
– Mucous neck cells - secrete a special mucus– Parietal (oxyntic) cells - secrete hydrochloric acid and gastric
intrinsic factor– Chief (zymogenic) cells - secrete pepsinogen
• Glands produce 1500ml juice per day
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Small intestine
• Longest portion of the alimentary canal
• Site of most enzymatic digestion and absorption– Secretions and buffers
provided by pancreas, liver, gall bladder
• Three subdivisions:– Duodenum – Jejunum – Ileum
• Ileocecal sphincter - transition between small and large intestine
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The Duodenum• Receives digestive enzymes and bile• Main pancreatic duct and common bile duct enter
duodenum– Sphincters control entry of bile and pancreatic juices
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The Small Intestine – Microscopic Anatomy• Modifications for absorption
– Circular folds (plicae circulares) - transverse ridges of mucosa and submucosa
– Villi - finger-like projections of the mucosa, covered with simple columnar epithelium
– Microvilli - further increase surface area for absorption– Lacteals - terminal lymphatic in villus
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Histology of the Intestinal Wall• Absorptive cells - uptake digested nutrients• Goblet cells - secrete mucus that lubricates chyme • Enteroendocrine cells - secrete hormones• Intestinal crypts - epithelial cells secrete intestinal juice
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Large Intestine• Digested residue contains few nutrients• Small amount of digestion by bacteria • Main functions
– absorb water and electrolytes
– compact material into feces
– Absorb vitamins produced by bacteria
– Store fecal matter prior to defecation
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Gross Anatomy of Large Intestine• Four areas of the colon
– Ascending– Transverse– Descending– Sigmoid
• Special features of large intestine– Teniae coli - thickening of
longitudinal muscularis – Haustra - puckering
created by teniae coli– Cecum - blind pouch,
beginning of large intestine– Vermiform appendix -
Contains lymphoid tissue
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The Rectum• Rectum - descends along the
inferior half of the sacrum• Anal Canal - the last subdivision
of the large intestine– Lined with stratified squamous
epithelium
– Last portion of the digestive tract
– Terminates at the anal canal
– Internal and external anal sphincters
– Defecation reflex triggered by distention of rectal walls
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Microscopic Anatomy of Large Intestine
• Villi are absent • Contains numerous goblet
cells• Intestinal crypts – simple
tubular glands• Lined with simple columnar
epithelial tissue– Epithelium changes at anal
canal, becomes stratified squamous epithelium
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The pancreas• Pancreatic duct penetrates duodenal wall• Endocrine functions - insulin and glucagons• Exocrine functions - majority of pancreatic secretions, pancreatic
juice secreted into small intestine– Carbohydrases
– Lipases
– Nucleases
– Proteolytic enzymes
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The Pancreas• Exocrine function
– Acinar cells make, store, and secrete pancreatic enzymes
– Enzymes are activated in the duodenum
• Endocrine function– Produces insulin and glucagon
– Regulates blood sugar
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The liver• Performs metabolic and hematological regulation and produces bile• Four Lobes: left, right, caudate, quadrate• Histological organization
– Lobules containing single-cell thick plates of hepatocytes
– Lobules empty into bile ducts, bile ducts merge to left/right hepatic ducts
– L/R hepatic ducts merge to form common hepatic duct
– Common hepatic meets cystic duct to form common bile duct
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Liver lobule - basic functional unit• Hepatocytes form irregular plates arranged in
spoke-like fashion• Bile canaliculi carry bile to bile ductules• Bile ductules lead to bile ducts in portal areas
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Figure 24.21a, b
The Gallbladder
• Hollow, pear-shaped organ• Stores, modifies and concentrates bile• Bile – bile salts help buffer acids and acts as emulsifier
for lipid digestion
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