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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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