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23 PART 1
The Digestive System
Overview of the Digestive System
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Organs are divided into two groups
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Alimentary canal
• Mouth, pharynx, and esophagus
• Stomach, small intestine, and large intestine
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Accessory digestive organs
• Teeth and tongue
• Gallbladder, salivary glands, liver, and pancreas
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Accessory organs are connected to the alimentary canal by ducts
• Secretions contribute to breakdown of foodstuffs
Abdominal Regions
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Four lines divide abdominal wall into nine regions
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Midclavicular lines—vertical lines of grid
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Subcostal plane—superior horizontal line
• Connects inferior points of costal margin
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Transtubercular plane—inferior horizontal line
• Connects tubercles of iliac crests
Abdominal Quadrants
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A simpler method of sectioning the anterior abdominal wall
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Right upper quadrant
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Left upper quadrant
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Right lower quadrant
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Left lower quadrant
The Peritoneal Cavity and Peritoneum
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Peritoneum—a serous membrane
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Visceral peritoneum—surrounds digestive organs
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Parietal peritoneum—lines the body wall
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Peritoneal cavity—a slitlike potential space
The Peritoneal Cavity and Peritoneum
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Mesentery—a double layer of peritoneum
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Holds organs in place
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Sites of fat storage
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Provides a route for circulatory vessels and nerves
The Peritoneal Cavity and Peritoneum
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Ventral mesenteries
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Falciform ligament
• Binds anterior part of liver to anterior abdominal wall
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Lesser omentum
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Attaches liver to the lesser curvature of stomach
The Peritoneal Cavity and Peritoneum
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Dorsal mesenteries
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Greater omentum
• Connects greater curvature of stomach to posterior abdominal wall
• A “fatty apron”
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Transverse mesocolon
• Holds transverse colon in place
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Sigmoid mesocolon
• Connects the sigmoid colon to posterior pelvic wall
The Peritoneal Cavity and Peritoneum
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Retroperitoneal organs
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Behind the peritoneum
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Peritoneal organs
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Digestive organs that keep their mesentery
Secondarily Retroperitoneal Organs
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Initially formed within peritoneum
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Become retroperitoneal
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Fuse to posterior abdominal wall
Digestive Processes
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Ingestion—occurs in the mouth
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Propulsion—movement of food
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Peristalsis—major means of propulsion
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Mechanical breakdown—prepares food for chemical digestion
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Chewing, churning food in stomach, segmentation
• Segmentation is rhythmic local constrictions of intestine
Digestive Processes
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Chemical digestion—complex molecules broken down to chemical components
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Mouth
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Stomach
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Small intestine
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Absorption—transport of digested nutrients
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Defecation—elimination of indigestible substances as feces
Peristalsis
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Major means of propulsion
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Adjacent segments of the alimentary canal relax and contract
Segmentation
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Rhythmic local contractions of the intestine
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Mixes food with digestive juices
Histology of the Alimentary Canal Wall
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Same four layers from esophagus to anus
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The mucosa—innermost layer
• Consists of
• Epithelium
• Lamina propria
• Muscularis mucosae
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The submucosa—external to the mucosa
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Contains blood and lymphatic vessels, nerve fibers
Histology of the Alimentary Canal Wall
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The muscularis externa—external to the submucosa
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Two layers
• Circular muscularis—inner layer
• Longitudinal muscularis—outer layer
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The serosa—the outermost layer
• Is the visceral peritoneum
Smooth Muscle
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Primarily found in walls of viscera
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Fibers elongated
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Have one centrally located nucleus
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Grouped into sheets
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Longitudinal layer—parallel to long axis of organ
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Circular layer—deeper layer, fibers run around circumference of organ
Smooth Muscle Contraction
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Mechanism of contraction
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Myofilaments operate by interaction with cytoskeleton
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Dense bodies—correspond to Z-discs of skeletal muscle
• Sliding myofilaments shorten the muscle cell by pulling on cytoskeleton
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Entry of Ca2+ into sarcoplasm stimulates contraction
Smooth Muscle Contraction
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Contraction is slow and sustained
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Takes 30x longer to contract and relax
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Resistant to fatigue
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Smooth muscle of arteries and visceral organs must sustain contraction over long
periods
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Energy requirements are low
• Mitochondria are not abundant
Innervation of Smooth Muscle
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Innervated by ANS
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Few fibers per sheet innervated
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Sheet of smooth muscle contracts as a unit
• Called single-unit innervation
Multiunit innervation
• Each smooth muscle cell innervated
• Iris of eye and arrector pili muscles
Nerve Plexuses
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Myenteric nerve plexus
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Lies between circular and longitudinal muscularis
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Controls peristalsis and segmentation
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Submucosal nerve plexus
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Lies in submucosa
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Signals glands to secrete
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Innervation
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Sympathetic and parasympathetic motor fibers
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Visceral sensory fibers
23 PART 2
The Digestive System
The Mouth and Associated Organs
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Mouth (oral cavity)
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Mucosa-lined cavity
• Boundaries are
• Lips anteriorly
• Cheeks laterally
• Palate superiorly
• Tongue inferiorly
• Fauces of oropharynx posteriorly
The Mouth and Associated Organs
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The mouth—oral cavity
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Mucosal layer
• Stratified squamous epithelium
• Lamina propria
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The lips and cheeks
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Formed from orbicularis oris and buccinator muscles, respectively
Anatomy of the Mouth
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The labial frenulum
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Connects lips to gum
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The palate
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Forms the roof of the mouth
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Boundaries of the fauces
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Palatoglossal arches
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Palatopharyngeal arches
The Tongue
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Interlacing fascicles of skeletal muscle
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Grips food and repositions it
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Helps form some consonants
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Intrinsic muscles—within the tongue
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Extrinsic muscles—external to the tongue
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Lingual frenulum
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Secures tongue to floor of mouth
The Superior Surface of the Tongue
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Tongue papillae
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Filiform papillae—no taste buds
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Fungiform papillae
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Vallate papillae
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Sulcus terminalis
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Marks border between mouth and pharynx
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Posterior one-third of tongue lies in oropharynx
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Lined with lingual tonsil
The Teeth
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Deciduous teeth—20 teeth
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First appear at 6 months of age
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Permanent teeth—32 teeth
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Most erupt by the end of adolescence
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Dental formula—shorthand
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Formula for adult dentition indicates number and position of teeth
• 2I, 1C, 2P, 3M
Vessels and Nerves to the Teeth
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Upper teeth
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Innervation is superior alveolar nerves branching from maxillary division of CN V
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Lower teeth
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Innervation is inferior alveolar nerves branching from mandibular branch of CN V
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Arterial supply to teeth
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Superior and inferior alveolar arteries (branching from maxillary arteries)
Tooth Structure
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Crown—exposed surface
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Root—in tooth socket
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Outer layer is enamel
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Dentin—underlies enamel
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Pulp cavity—center of tooth
The Salivary Glands
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Produce saliva
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Compound tubuloalveolar glands
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Parotid glands
• Parotid duct—parallel to zygomatic arch
• Contain only serous cells
Submandibular glands
• Lie along medial surface of mandible
Sublingual glands
• Lie in floor of oral cavity
• Contain primarily mucous cells
The Pharynx
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Oropharynx and laryngopharynx
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Passages for air and food
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Lined with stratified squamous epithelium
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External muscle layer
• Consists of superior, middle, and inferior pharyngeal constrictors
The Esophagus
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Gross anatomy—muscular tube
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Begins as a continuation of the pharynx
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Joins the stomach inferior to the diaphragm
• Cardiac sphincter—closes lumen to prevent stomach acid from entering
esophagus
The Esophagus
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Microscopic anatomy
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Epithelium is stratified squamous epithelium
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When empty, mucosa and submucosa are in longitudinal folds
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Mucous glands—primarily compound tubuloalveolar glands
The Esophagus
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Muscularis externa
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Skeletal muscle
• First one-third of length
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Mixture of skeletal and smooth muscle
• Middle one-third of length
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Smooth muscle
• Inferior one-third of length
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Adventitia
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Most external layer of esophagus
The Stomach
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Site where food is churned into chyme
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Secretion of pepsin begins protein digestion
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Functions under acidic conditions
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Food remains in stomach approximately 4 hours
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Regions of the stomach
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Cardial part
Fundus
Body
Pyloric part
Microscopic Anatomy of the Stomach
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Muscularis has three layers
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Circular and longitudinal layers and oblique layer
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Epithelium is simple columnar epithelium
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Mucosa dotted with gastric pits
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Gastric glands—deep to gastric pits
Microscopic Anatomy of the Stomach
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Gastric glands of fundus and body
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Mucous neck cells
• Secrete a special mucus
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Parietal (oxyntic) cells
• Secrete hydrochloric acid and gastric intrinsic factor
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Chief (zymogenic) cells
• Secrete pepsinogen
• Pepsinogen is activated to pepsin when it encounters acid in the gastric glands
The Small Intestine—Gross Anatomy
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Longest portion of the alimentary canal
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Site of most enzymatic digestion and absorption
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Three subdivisions
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Duodenum
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Jejunum
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Ileum
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Innervation
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Parasympathetic fibers from vagus nerve
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Sympathetic from thoracic splanchnic nerves
The Duodenum
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Receives digestive enzymes and bile
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Main pancreatic duct and common bile duct enter duodenum
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Sphincters control entry of bile and pancreatic juices
The Small Intestine—Microscopic Anatomy
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Modifications for absorption
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Circular folds
• Transverse ridges of mucosa and submucosa
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Villi
• Fingerlike projections of the mucosa
• Covered with simple columnar epithelium
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Microvilli
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Further increase surface area for absorption
Histology of the Intestinal Wall
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Absorptive enterocytes
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Uptake digested nutrients
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Goblet cells
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Secrete mucus that lubricates chyme
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Enteroendocrine cells
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Secrete hormones
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Intestinal crypts
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Epithelial cells secrete intestinal juice
23 PART 3
The Digestive System
The Large Intestine
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Digested residue contains few nutrients
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Small amount of digestion by bacteria
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Main functions
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Absorb water and electrolytes
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Mass peristaltic movements force feces toward the rectum
Gross Anatomy of Large Intestine
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Subdivided into
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Cecum, appendix, colon, rectum, anal canal
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Special features of large intestine
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Teniae coli
• Thickening of longitudinal muscularis
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Haustra
• Puckering created by teniae coli
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Epiploic appendages
• Fat-filled pouches of visceral peritoneum
Gross Anatomy of Large Intestine
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Cecum
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Blind pouch
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Beginning of large intestine
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Appendix
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Contains lymphoid tissue
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Neutralizes pathogens
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Colon
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Divided into distinct segments
• Ascending, transverse, descending, and sigmoid colon
Gross Anatomy of Large Intestine
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Rectum
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Descends along the inferior half of the sacrum
Anal canal
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The last subdivision of the large intestine
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Lined with stratified squamous epithelium
Vessels and Nerves of the Large Intestine
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First half of large intestine
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Arterial supply—superior mesenteric artery
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Innervation
• Sympathetic innervation—superior mesenteric and celiac ganglia
• Parasympathetic innervation—vagus nerve
Vessels and Nerves of the Large Intestine
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Distal half of large intestine
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Arterial supply—inferior mesenteric artery
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Innervation
• Sympathetic innervation—inferior mesenteric and hypogastric plexuses
• Parasympathetic innervation—pelvic splanchnic nerves
Microscopic Anatomy of Large Intestine
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Villi are absent
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Contains numerous goblet cells
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Intestinal crypts—simple tubular glands
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Lined with simple columnar epithelial tissue
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Epithelium changes at anal canal
• Becomes stratified squamous epithelium
The Liver
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Largest gland in the body
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Performs over 500 functions
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Digestive function
• Bile production
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Performs many metabolic functions
Microscopic Anatomy of the Liver
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Hepatocyte—functional cells of the liver
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Portal triad composed of
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Bile duct tributary
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Branch of hepatic portal vein
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Branch of hepatic artery
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Hepatic macrophages—destroy bacteria
Microscopic Anatomy of the Liver
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Some functions of hepatocytes
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Rough ER manufactures blood proteins
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Smooth ER produces bile salts, detoxifies poisons
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Peroxisomes detoxify poisons (alcohol)
Golgi apparatus packages secretory products
Mitochondria provide energy for liver processes
Glycosomes store sugar
Great capacity for regeneration
The Gallbladder
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Stores and concentrates bile
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Expels bile into duodenum
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Bile emulsifies fats
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Cholecystokinin—released from enteroendocrine cells in response to fatty chyme
23 PART 4
The Digestive System
The Pancreas
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Has both exocrine and endocrine functions
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Endocrine function
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Produces insulin and glucagon
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Regulates blood sugar
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Exocrine function
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Produces most enzymes that digest food in the small intestine
The Pancreas
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Is secondarily retroperitoneal
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Main pancreatic duct
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Extends the length of the pancreas
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Joins bile duct to form the hepatopancreatic ampulla
• Empties into duodenum
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Arterial supply is from hepatic, splenic, and superior mesenteric arteries
The Pancreas
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Exocrine function
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Acinar cells make, store, and secrete pancreatic enzymes
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Enzymes are activated in the duodenum
Peptic Ulcers
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Are erosions of the mucosa of a region of the alimentary canal
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Gastric ulcers
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Occur in pyloric region of the stomach
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Duodenal ulcers
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Occur in duodenum of the small intestine
Peptic Ulcers
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Caused by Helicobacter pylori
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H. pylori
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Acid-resistant
Binds to gastric epithelium
• Induces oversecretion of acid and inflammation
Disorders of the Digestive System
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Intestinal obstruction
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Mechanical obstructions
• Adhesions, tumors, or foreign objects
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Nonmechanical obstruction
• Halt in peristalsis
• Trauma
• Intestines touched during surgery
Disorders of the Digestive System
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Inflammatory bowel disease
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Inflammation of intestinal wall
• Crohn’s disease
• Ulcerative colitis
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Viral hepatitis
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Jaundice and flulike symptoms
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Major types—A, B, C, and G
Disorders of the Digestive System
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Cystic fibrosis and the pancreas
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Pancreatic ducts become blocked with mucus
• Clogged ducts prevent pancreatic juices from entering small intestine
• Leads to malabsorption of fats and other nutrients
Embryonic Development of the Digestive System
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Alimentary canal is formed in week 3
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Encloses tubular portion of yolk sac
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Vitelline duct
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Landmark dividing into three regions
• Foregut
• Midgut
• Hindgut
The Digestive System in Later Life
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Middle age
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Gallstones and ulcers
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Old age
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Activity of digestive organs declines
• Fewer digestive juices and enzymes are produced
• Absorption is less efficient
• Dehydration of fecal mass leads to constipation
• Diverticulosis and cancer of digestive organs