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C H A P T E R
The Digestive System23
Overview of the Digestive System
• Organs are divided into two groups• The alimentary canal
• Mouth, pharynx, and esophagus• Stomach, small intestine, and large intestine (colon)
• Accessory digestive organs• Teeth and tongue• Gallbladder, salivary glands, liver, and pancreas
Figure 23.1
The Alimentary Canal and Accessory Digestive Organs
Mouth (oral cavity)Tongue
Esophagus
LiverGallbladder
Anus
DuodenumJejunumIleum
Small intestine
Parotid glandSublingual glandSubmandibulargland
Salivary glands
Pharynx
StomachPancreas(Spleen)
Transverse colonDescending colonAscending colonCecumSigmoid colonRectumVermiform appendixAnal canal
Large intestine
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Digestive Processes
• Ingestion• Propulsion• Mechanical digestion• Chemical digestion• Absorption• Defecation
Peristalsis
• Major means of propulsion
• Adjacent segments of the alimentary canal relax and contract
Figure 23.3a
Segmentation
• Rhythmic local contractions of the intestine
• Mixes food with digestive juices
Figure 23.3b
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The Peritoneal Cavity and Peritoneum• Peritoneum – a serous membrane
• Visceral peritoneum – surrounds digestive organs• Parietal peritoneum – lines the body wall
• Peritoneal cavity – a slit-like potential space
Figure 23.5
Parietalperitoneum
Wall ofbody trunk
Kidney(retroperitoneal)
Peritonealcavity (withserous fluid)Stomach
Posterior
Anterior Visceral peritoneum
Liver
Falciformligament
Mesenteries
• Lesser omentum attaches to lesser curvature of stomach
Figure 23.6b(b)
Liver
Lesser omentumGallbladder
StomachDuodenum
Transverse colon
Small intestine
Cecum
Urinary bladder
Mesenteries
Figure 23.6c
Transverse colon
Greater omentum
Descending colon
JejunumMesentery
Transversemesocolon
Sigmoidmesocolon
Sigmoid colon
Ileum
(c)
• Greater omentum—a “fatty apron” of peritoneum
• Greater omentum and transverse colon reflected
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Pancreas
LiverLesser omentum
StomachDuodenumTransverse mesocolon
Greater omentumMesentery
Jejunum
Visceral peritoneum
Urinary bladder
Transverse colon
Ileum
Parietal peritoneum
Rectum
(d)
Mesenteries
• Sagittal section through the abdominopelvic cavity
• Mesenteries attach to posterior abdominal wall
Figure 23.6d
Histology of the Alimentary Canal Wall• Same four layers from esophagus to anus
• The mucosa – innermost layer• Consists of:
• Epithelium• Lamina propria• Muscularis mucosae
• The submucosa – external to the mucosa• Contains blood and lymphatic vessels, nerve fibers
• The muscularis externa – external to the submucosa• Two layers
• Circular muscularis• Longitudinal muscularis
• The serosa – the outermost layer• The visceral peritoneum
• Nerve plexuses• Myenteric nerve plexus • Submucosal nerve plexus
Histology of the Alimentary Canal
Figure 23.7a
Glands in submucosa
Submucosa
LumenMucosa-associatedlymphoid tissue
Duct of gland outside alimentary canal
Gland in mucosaNerve
ArteryVein
Lymphatic vesselMesentery
Intrinsic nerve plexuses
Mucosa
Muscularis externa
Serosa
(a) Longitudinal and cross-sectional views through the small intestine
Myenteric nerve plexusSubmucosal nerve plexus
EpitheliumLamina propriaMuscularis mucosae
Longitudinal muscleCircular muscle
EpitheliumConnective tissue
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Histology of the Alimentary Canal
Figure 23.7b(b) Light micrograph cross section through the small intestine (30×)
Submucosa
Mucosa
Muscularis externa
Serosa
Smooth Muscle
Figure 23.8a, b
Small intestine
(a) Location and plane of section shown in (b)
(b) Cross section of the intestine showing the smooth muscle layers (one circular and the other longitudinal) running at right angles to each other
Mucosa
Longitudinal layer of smooth muscle (shows smooth muscle fibers in cross section, 145×)
Circular layer of smooth muscle (shows longitudinal views of smooth muscle fibers, 145×)
The Mouth and Associated Organs
• The mouth – oral cavity• Mucosal layer
• Stratified squamous epithelium• Lamina propria
• The lips and cheeks• Formed from orbicularis oris and buccinator muscles,
respectively
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Anatomy of the Mouth
Figure 23.11a
UvulaSoft palate Palatoglossal arch
Palatinetonsil
Hard palateOral cavity
Tongue
Lingual tonsilOropharynx
Epiglottis
Hyoid bone
Laryngopharynx
Esophagus
Trachea
(a) Sagittal section of the oral cavity and pharynx
UvulaPalatinetonsil
Sublingualfold withopenings ofsublingual ducts
Tongue
Upper lip
Lower lip
VestibuleGingivae (gums)
Gingivae (gums)
Hard palate
Soft palate
Lingual frenulumOpening ofsubmandibularduct
Palatineraphe
Inferior labialfrenulum
Posterior wallof oropharynx
Palatopharyngealarch
Superior labialfrenulum
Palatoglossalarch
(b) Anterior view
Figure 23.11b
Anatomy of the Mouth
• The labial frenulum• Connects lips to gum
• The palate• Forms the roof
of the mouth
The Tongue
• Interlacing fascicles of skeletal muscle• Grips food and repositions it• Helps form some consonants • Intrinsic muscles – within the tongue• Extrinsic muscles – external to the tongue
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The Superior Surface of the Tongue• Tongue papillae
• Filiform papillae• Fungiform papillae• Vallate papillae
Figure 23.12
The Teeth
• Dentition and dental formula• Deciduous teeth – 20 teeth
• First appear at 6 months of age• Permanent teeth – 32 teeth
• Most erupt by the end of adolescence• Dental formula – shorthand
• Way to indicate number and position of teeth
The Teeth
Figure 23.13a
IncisorsCentral (6–8 mo)
IncisorsCentral (7 yr)
Canine (eyetooth)(16–20 mo)
Canine (eyetooth)(11 yr)Premolars(bicuspids)First premolar(11 yr)
MolarsFirst molar(10–15 mo)
MolarsFirst molar (6–7 yr)
Lateral (8–10 mo)
Lateral (8 yr)
Second molar(about 2 yr)
Second molar(12–13 yr)Third molar(wisdom tooth)(17–25 yr)
(a)
Permanentteeth
Deciduous(milk) teeth
Second premolar(12–13 yr)
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The Teeth
Figure 23.13b
Deciduous teeth Permanent teeth(b)
Crown
Neck
Root
Enamel
Dentin
Dentinal tubules
Pulp cavity(containsblood vesselsand nerves)
Gingiva(gum)
Cementum
Root canal
Periodontalligament
Apicalforamen
Bone
Tooth Structure
• Longitudinal section of tooth in alveolus
Figure 23.14
The Salivary Glands
Figure 23.15
Teeth
Ducts ofsublingualgland
Sublingualgland
Submandibularduct
Posterior belly ofdigastric muscle
Parotid ductMasseter muscleBody of mandible(cut)
Parotid gland
Tongue
Submandibulargland
(a)
Frenulumof tongue
Mylohyoidmuscle (cut)Anterior belly ofdigastric muscle Mucous cells
(b)
Serous cellsforming demilunes
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The Pharynx• Oropharynx and laryngopharynx – passages for air and food
• Lined with stratified squamous epithelium• External muscle layer
• Consists of superior, middle, and inferior pharyngeal constrictors
The Esophagus
• Gross anatomy – muscular tube• Begins as a continuation of the pharynx• Joins the stomach inferior to the diaphragm
• Microscopic anatomy• 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• Most external layer – adventitia
Microscopic Structure of the Esophagus
Figure 23.16
Mucosa(contains a stratifiedsquamous epithelium)Submucosa (areolarconnective tissue)Lumen
Muscularis externa
Adventitia (fibrousconnective tissue)
(a) Cross section through esophagus (5×) (b) Gastroesophageal junction, longitudinal section (85×)
Circular layerLongitudinal layer
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The Stomach
• Site where food is churned into chyme • Protein digestion begins• Secretes pepsin
• Functions under acidic conditions
The Stomach
Figure 23.17a
Cardia
Esophagus
Pyloric sphincter(valve) at pylorus
Pyloriccanal
Pyloricantrum
Rugae ofmucosa
Body
Lumen
Serosa
Fundus
Lessercurvature
Greatercurvature
Muscularis externa
(a)
Duodenum
Longitudinal layerCircular layerOblique layer
The Stomach—Microscopic Anatomy
Figure 23.18
Mucosa
Surfaceepithelium
LaminapropriaMuscularismucosae
Oblique layerCircular layerLongitudinallayer
Serosa(a) Layers of the stomach wall, longitudinal section
Stomach wall
Muscularis externa(contains myentericplexus)
Submucosa (containssubmucosal plexus)
(b) Enlarged view of gastric pits and gastric glands
Mucous neck cells
Parietal cell
Surface epithelium (mucous cells)
Gastric pits
Chief cell
Enteroendocrine cell
Gastricpit
Gastricgland
(c) Location of the HCl-producing parietal cells and pepsin-secreting chief cells in a gastric gland
Pepsinogen
Mitochondria
PepsinHCl
Chief cell
Enteroendocrine cell
Parietal cell
Gastric pits
Gastric gland
Mucus-secretingcells
Surface mucous cell
Mucus neck cells
HCl secreting parietal cells
Enzyme secreting chief cells
Muscularismucosae
(d) Micrograph of the stomach mucosa, view similar to part (b) (115×)
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The Small Intestine – Gross Anatomy
• Longest portion of the alimentary canal• Site of most enzymatic digestion and absorption• Three subdivisions
• Duodenum• Brunner’s glands
• Jejunum• Ileum
• Peyer’s patches
The Small Intestine—Structural Features
Figure 23.20
Vein carryingblood tohepatic portalvessel
MusclelayersCircularfolds
Villi
(a)
Lumen
(b)
Absorptivecells
Lacteal
Intestinalcrypt
Mucosaassociatedlymphoidtissue
MuscularismucosaeDuodenalgland Submucosa
EnteroendocrinecellsVenuleLymphatic vessel
Microvilli(brush border)
GobletcellBloodcapillaries Vilus
(c) Intestinal crypt
Absorptive cells
Villi
Gobletcells
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 Duodenum and Related Organs
Figure 23.19
Jejunum
Mucosawith folds
Cystic duct
Duodenum
Hepatopancreaticampulla and sphincter
Gallbladder
Right and left hepatic ducts of liver
Bile duct and sphincter
Main pancreatic duct and sphincter
PancreasTail of pancreas
Head of pancreas
Common hepatic duct
Major duodenalpapilla
Accessory pancreatic duct
The Large Intestine• Digested residue contains few nutrients• Small amount of digestion by bacteria • Main functions – absorb water and electrolytes• Mass peristaltic movements force feces toward the rectum
Gross Anatomy of Large Intestine
• Subdivided into:• Cecum, vermiform appendix, colon, rectum, anal canal
• Special features of large intestine• Teniae coli, haustra, and epiploic appendages
• Cecum – a blind pouch• Vermiform appendix – contains lymphoid tissue
• Neutralizes pathogens• Colon – divided into distinct segments
• Ascending, transverse, descending, and sigmoid colon• Rectum – descends along the inferior half of the sacrum• Anal Canal – the last subdivision of the large intestine
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Gross Anatomy of Large Intestine
Figure 23.21a
(a)
Right colic(hepatic) flexure
Transverse colon
Superiormesenteric arteryHaustrum
Ascending colonIIeum
IIeocecal valve
Vermiform appendix
Cecum
Left colic(splenic) flexureTransverse mesocolonEpiploic appendages
Descending colon
Teniae coli
Sigmoid colon
Cut edge of mesentery
External anal sphincter
Rectum
Anal canal
Gross Anatomy of Large Intestine
Figure 23.21b(b)
Rectal valve
Rectum
Anal canal
Levator ani muscle
Anus
Anal sinuses
Anal columnsAnal valves
Internal analsphincter
External analsphincter
Hemorrhoidalveins
Pectinate line
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 Liver
• Largest gland in the body• Performs over 500 functions• Digestive function – bile production• Performs many metabolic functions
Visceral Surface of the Liver
Figure 23.25b
(b)
Hepatic portal vein Hepatic veinsInferior venacava
Bare area
Right hepaticartery
Right hepatic duct
Cystic duct
Gallbladder
Right lobe
Quadrate lobeLigamentum teresFalciform ligament
Fissure
Common hepaticduct
Left hepatic duct
Left hepatic artery
Porta hepatis
Ligamentumvenosum infissure
Left lobe
Caudatelobe
Microscopic Anatomy of Liver
Figure 23.26
(a) (b)Lobule Central vein Connectivetissue septum
(c)
Interlobular veins(to hepatic vein)
Central vein
Sinusoids
Portal triad
Plates ofhepatocytes
Portal vein
Bile canaliculi
Fenestrated lining (endothelial cells) of sinusoids
Bile duct (receivesbile from bile canaliculi)
Bile ductPortal arteriolePortal venuleHepatic
macrophages in sinusoid walls
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The Gallbladder
• Stores and concentrates bile• Expels bile into duodenum
• Bile emulsifies fats• Cholecystokinin—released from enteroendocrine
cells in response to fatty chyme
The Gallbladder
Figure 23.19
Jejunum
Mucosawith folds
Cystic duct
Duodenum
Hepatopancreaticampulla and sphincter
Gallbladder
Right and left hepatic ducts of liver
Bile duct and sphincter
Main pancreatic duct and sphincter
PancreasTail of pancreas
Head of pancreas
Common hepatic duct
Major duodenalpapilla
Accessory pancreatic duct
The Pancreas• Pancreatic duct penetrates duodenal wall• Endocrine functions
• Insulin (beta cells) and glucagons (alpha cells)• Exocrine functions – Acinar cells make, store, and secrete pancreatic
enzymes• Majority of pancreatic secretions• Pancreatic juice secreted into small intestine
• Carbohydrases• Lipases• Nucleases• Proteolytic enzymes
Figure 23.27
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The Pancreas
Figure 23.27
Disorders of the Digestive System
• Intestinal obstruction • Mechanical obstructions
• Adhesions, tumors, or foreign objects• Nonmechanical obstruction
• Halt in peristalsis • Trauma• Intestines touched during surgery
Disorders of the Digestive System
• Inflammatory bowel disease • Inflammation of intestinal wall
• Crohn’s disease• Ulcerative colitis
• Viral hepatitis – jaundice and flu-like symptoms • Major types – A, B, C, and G
• Cystic Fibrosis and the Pancreas
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The Digestive System in Later Life
• Middle age – gallstones and ulcers • Old age – activity of digestive organs decline
• Fewer digestive juices and enzymes produced• Absorption is less efficient• Dehydration of fecal mass leads to constipation• Diverticulosis and cancer of digestive organs
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