Chapter 24 The Digestive System

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Chapter 24 The Digestive System

• BIO 211 Lab
• Instructor Dr. Gollwitzer

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• Today in class we will
• Discuss the gastrointestinal tract and identify
  its components
• Identify the accessory organs associated with
  the digestive tract
• The layers that make up the digestive tract wall
  and each layers characteristics
• Mucosa
• Submucosa
• Muscularis externa
• Serosa
• Begin our discussion of the gastrointestinal
  tract components in more detail
• Oral cavity
• Pharynx
• Esophagus

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Digestive System

• Gastrointestinal (GI) tract continuous,
  muscular tube from mouth to anus
• Oral cavity (mouth)
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anus

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Digestive System

• Accessory digestive organs any digestive organ
  attached to the GI tract
• Teeth
• Tongue
• Glandular organs
• Salivary glands
• Pancreas
• Liver
• Gall Bladder

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Figure 24-1
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Histological Organization

• 4 Major layers to wall
• Mucosa
• Submucosa
• Muscularis externa
• Serosa

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Figure 24-3
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Mucosa

• Inner lining of digestive tract
• A mucous membrane
• Consists of
• Epithelium - moistened by glandular secretions
• Lamina propria areolar CT filler

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Mucosa Digestive Epithelia

• Depend on location, function, stresses
• Simple or stratified
• Columnar or squamous

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Mucosa Digestive Epithelia

• Nonkeratinized stratified squamous epithelium
• Where mechanical stress most severe
• In oral cavity, pharynx, esophagus, rectum
• Life span 2-3 days
• Simple columnar epithelium
• Where absorption and secretion occur
• (with villi and goblet cells)
• Stomach, small intestine, most of large intestine
• Life span 6 days

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Mucosa Lamina Propria

• Layer of areolar (loose connective) tissue
• Contains
• Blood vessels, sensory nerve endings, lymphatic
  vessels, lymphoid tissue
• Mucosal glands and glandular secretions
• Muscularis mucosae/interna
• Band of smooth muscle and elastic fibers
• Smooth muscle arranged in 2 concentric layers
• Inner, circular layer (around lumen)
• Outer, longitudinal layer
• Contractions alter shape of lumen and move
  epithelial folds

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Mucosa Glandular Structures

• Have secretory function
• Associated with simple columnar cells and/or
  mucous-secreting cells
• Goblet cells (exocrine) ? mucus
• Enteroendocrine cells
• ? hormones (e.g., G cells ? gastrin)
• Coordinate activities of digestive tract and
  accessory glands
• e.g., chief cells, G cells, parietal cells

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Submucosa

• Layer of dense, irregular connective tissue
• Around muscularis mucosae
• Contains
• Large blood vessels and lymphatic vessels
• Exocrine glands ? buffers and enzymes into lumen
• Submucosal plexus contains nerve fibers and
  neurons

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Muscularis Externa

• Dominated by smooth muscle cells
• Inner, circular layer
• Outer, longitudinal layer
• Important role in mechanical processing and
  movement of materials along tract
• Also contains
• Lymphoid nodules (Peyers patches)
• Masses of lymphoid tissue have lymphocytes that
  protect small intestine from bacteria that are
  normal inhabitants of large intestine
• Myenteric plexus/plexus of Auerbach
• Network of neurons located between circular and
  longitudinal muscle layers
• Movements coordinated by enteric nervous system
  (part of ANS)

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Serosa

• A serous membrane lines sealed, internal
  subdivisions of ventral body cavity
• Covers muscularis externa of most of digestive
  tract
• except oral cavity, pharynx, esophagus, and
  rectum, which have adventitia (a dense collagen
  fibrous sheath)

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Figure 24-3
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Figure 24-3
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Oral (Buccal) Cavity

• Space within mouth
• Lined by oral mucosa (nonkeratinized stratified
  squamous epithelium)
• Vestibule space between cheeks or lips and
  teeth
• Labial frenulum fold of mucosa connects lip to
  gum
• Cheeks lateral walls of oral cavity
• Mucosae supported by fat pads and buccinator
  muscles
• Labia (lips)
• Gingivae (gums)
• Ridges that surround base of each tooth
• On alveolar processes of maxillary bones and
  mandible

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Oral (Buccal) Cavity

• Palates
• Hard palate formed by maxillary bones
  (anterior) and palatine bones (posterior)
• Soft palate fleshy part posterior to hard
  palate
• Formed from skeletal muscle
• Posterior margin supports uvula dangling
  process that helps prevent food from entering
  pharynx prematurely
• Tongue
• Muscular organ attached to floor of oral cavity
• Dorsum (superior) surface covered with lingual
  papillae (location of taste buds)
• Lingual frenulum thin fold of mucous membrane
  that attaches tongue to floor of oral cavity

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Figure 24-6
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Pharynx

• Throat area posterior to nasal and oral cavities
• Shared by respiratory and digestive systems
• Extends between internal nares and entrances to
  larynx and esophagus
• Nasopharynx
• Posterior portion or nasal cavity
• Separated from oral cavity by soft palate
• Contains
• Pharyngeal tonsils and opening to auditory
  (eustachian) tube
• Oropharynx
• Posterior portion of oral cavity
• Between soft palate and base of tongue
• Fauces opening between oral cavity and
  oropharynx
• Laryngopharynx
• Inferior part
• Between hyoid bone and entrances to esophagus and
  larynx

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Esophagus

• Hollow, muscular tube
• From posterior laryngopharynx to stomach
• Descends through thoracic cavity posterior to
  trachea
• Enters abdominopelvic cavity through esophageal
  hiatus in diaphragm

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Esophageal Histology

• Muscularis externa transitions from
• Skeletal muscle fibers (superior third) to
• Smooth muscle fibers (inferior third)
• Adventitia
• CT outside muscularis externa (no serosa)
• Anchors esophagus against dorsal body wall

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Esophageal Sphincters

• Circular smooth muscles that control
  entrance/exit to esophagus
• Upper esophageal sphincter
• In superior 1 in.
• Prevents air from entering
• Lower esophageal sphincter
• At inferior end
• Prevents backflow from stomach

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• Today in class we will
• Complete our more detailed discussion of the
  gastrointestinal tract components
• Stomach
• Small intestine
• Large intestine
• Anus
• Begin our discussion of the 4 accessory
  digestive organs
• Teeth
• Tongue
• Glandular organs
• Salivary glands

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Stomach

• Muscular tube with extra layers of smooth muscle
  cells
• Strengthen stomach wall
• Assist in mixing/churning activities required to
  form chyme
• Muscularis mucosae/interna has extra outer,
  circular layer
• Muscularis externa has oblique layer

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Figure 24-13
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Stomach

• Has expanded J shape
• Lesser curvature shorter, inner, medial surface
• Greater curvature longer, outer, lateral surface

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Figure 24-12b
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Stomach

• Has 4 regions
• Cardia
• Smallest region where esophagus enters (at
  gastroesophageal/cardiac sphincter)
• Fundus
• Superior hump to left of cardia that contacts the
  diaphragm
• Body (corpus)
• Main (middle) region of the stomach between
  fundus and curve
• Pylorus
• Last part of stomach that enters the duodenum
  through pyloric sphincter

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Stomach

• Rugae
• Folds of gastric mucosa
• Temporary features
• Let gastric lumen expand (almost disappear when
  stomach full)

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Stomach

• Gastric pits
• Depressions that open onto gastric surface
• Each communicates with several gastric glands
• Mucous cells line neck
• Gastric glands
• In fundus and body of stomach
• 2 types of secretory cells
• Parietal cells
• Chief cells

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Figure 24-13b
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Small Intestine

• 3 Regions
• Duodenum (10 in.)
• First part connects to pylorus
• Mixing bowl
• Receives chyme from stomach and secretions from
  pancreas and liver
• Curves in a C that encloses head of the pancreas
• Jejunum (8 feet)
• Middle part
• Site for most of chemical digestion and nutrient
  absorption
• Ileum (12 feet)
• Last part
• Connects to large intestine via ileocecal valve

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Small Intestine

• Plicae circulares
• inner folds in intestinal lining
• Permanent feature -- do not disappear when small
  intestine fills
• Wall
• Mucosa
• Intestinal epithelium
• Simple columnar epithelium covers villi (pl
  singular villus)
• Villus capillaries/villus capillary network
• Lacteals
• Intestinal glands (goblet cells, enteroendocrine
  cells)
• Muscularis mucosae/interna
• Submucosa, muscularis externa, and serosa as
  described previously

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Figure 24-16
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Figure 24-17
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Large Intestine (Bowel)

• 3 Main sections
• Cecum
• Colon (4.5 ft)
• Rectum (6 in.)

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Large Intestine (Bowel)

• Cecum
• Pouch-like structure that connects to ileum (at
  ileocecal valve)
• Vermiform appendix small, worm-like appendage
  of lymphoid tissue attached to cecum

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Large Intestine (Bowel)

• Colon
• Ascending colon along right side takes right
  turn at superior end right colic/hepatic
  flexure
• Transverse colon crosses from R to L takes
  right turn downward at left end left
  colic/splenic flexure
• Descending colon along left side curves
  medially at sigmoid flexure
• Sigmoid colon S-shaped empties into rectum
• Haustra
• Pouches caused by longitudinal bands of
  muscularis externa (taeniae coli)
• Permit expansion and elongation of colon

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Large Intestine (Bowel)

• Rectum
• Anal canal last portion
• Contain small longitudinal folds (anal columns)
• Anus/anal orifice exit of anal canal
• Contains 2 sphincters
• Internal anal sphincter smooth muscle,
  involuntary control
• External anal sphincter skeletal muscle,
  voluntary control

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Figure 24-24a
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Teeth

• Located in alveoli (bony sockets) on alveolar
  processes of maxillary bones and mandible
• Number
• Primary (deciduous)
• 5/side/jaw 20
• Secondary
• 3 molars/side/jaw 32
• Replace primary teeth

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Figure 24-9a, b
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Tooth Type No./ Jaw Location Shape Function No. Roots
Incisors 4 Center front Blade Clip/cut 1
Cuspids/ Canines 2 Lateral to incisors Conical w/sharp ridge, pointed tip Tear/ slash/ shear 1
Bicuspids/ Premolars 4 Posterior to cuspids Flattened crown, prominent ridges Crush/ mash/ grind 1 or 2
Molars (wisdom teeth) 6 Posterior to bicuspids Large, flattened crown, prominent ridges Crush/ grind 3 or more
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Teeth

• Parts
• Crown above gum line
• Neck boundary between crown and root
• Root below gum line sits in alveolus

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Teeth

• Layers/Regions
• Outer
• Enamel
• Thin, white layer that covers crown
• Hardest biologically manufactured substance
• Cementum
• Thin layer that covers root protects and helps
  anchor tooth
• Inner
• Dentin
• Makes up bulk of tooth
• Mineralized matrix (CaPO4 crystals) similar to
  bone, except acellular
• Pulp cavity
• Central region
• Receives blood vessels and nerves from root canal
• Root canal
• Narrow tunnel at root/base of tooth
• Blood vessels and nerves enter through apical
  foramen

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Figure 24-8a, b
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Salivary Glands

• Outside oral cavity, but secrete into oral cavity
• 3 pairs
• Parotid glands
• Extend from mastoid process of temporal bone
  across outer surface of masseter muscle
• Parotid (Stensons) duct from parotid through
  buccinator to oral cavity
• Sublingual glands
• Under the floor of the mouth
• Many small sublingual (Rivinus) ducts open along
  side of lingual frenulum
• Submandibular glands
• Along inner surfaces of mandible
• Submandibular (Whartons) ducts open into mouth
  on either side of lingual frenulum immediately
  posterior to teeth

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Figure 24-7a, b
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• Today in class we will
• Complete our discussion of the 4 accessory
  digestive organs
• Glandular organs
• Pancreas
• Liver
• Histology
• Bile duct system
• Gall bladder
• Trace the path of food through the digestive
  system

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Pancreas

• Lies posterior to stomach, extends laterally from
  duodenum toward spleen
• Covered by thin, CT capsule
• 3 regions
• Head burrowed in loop of duodenum
• Tail tapered end against spleen
• Body slender middle region
• Ducts deliver digestive enzymes and buffers to
  duodenum
• Main pancreatic duct (of Wirsung) large, main
  duct ? major duodenal papilla with common bile
  duct ? duodenal ampulla (chamber)
• Accessory pancreatic duct (of Santorini) may
  branch from pancreatic duct ? minor duodenal
  papilla ? duodenal ampulla

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Fig. 24-18, p. 888
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Gallbladder

• Hollow, pear-shaped, muscular sac
• Located in recess on posterior surface of livers
  right lobe
• 3 regions
• Fundus bottom of pouch
• Body main region
• Neck narrow end where cystic duct exits

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Figure 24-19
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Liver

• Largest visceral organ
• Covered by tough, fibrous capsule
• Divided into 4 lobes
• Right lobe largest, in right hypochondriac
  region
• Left lobe narrow part extending into left
  hypochondriac region
• Caudate lobe on inferior side superior, near
  IVC
• Quadrate lobe inferior to caudate, near
  gallbladder
• Falciform ligament on anterior surface between
  R and L lobes

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Liver Histology

• Hepatocytes liver cells simple cuboidal cells
• Each liver lobe divided into lobules
• Liver lobule basic functional unit of liver
• Has hexagonal shape (6 corners)
• Hepatic triad at each corner contains
• Branch of hepatic portal vein
• Branch of hepatic artery
• Branch of bile duct
• Separated by interlobular septa
• Each has a central vein
• Hepatocytes
• Form 1-cell wide plates arranged like spokes
  around a wheel around central vein
• Plates are separated by sinusoids modified
  blood vessels for large solutes can pass out
  of/into blood stream

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Liver Histology

• Branches of hepatic portal vein (from intestine
  to liver) and hepatic artery (to liver from
  systemic circulation) ? sinusoids
• Hepatocytes absorb solutes and secrete materials
  (plasma proteins) into sinusoids
• Blood leaves sinusoids and enters central vein of
  lobule
• Central veins ? ? hepatic veins ? IVC ? heart
• Kupffer cells
• Phagocytic cells (lymphatic cells)
• Functions
• Engulf pathogens, cell debris and damaged blood
  vessels
• Store iron, lipids, heavy metals absorbed by
  digestive tract

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Figure 24-20
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Bile Duct System

• Hepatocytes ? bile ? bile canaliculi ? bile
  ductules ? R and L hepatic ducts ? common hepatic
  duct
• Common hepatic duct cystic duct (to/from
  gallbladder) ? common bile duct
• Common bile duct meets pancreatic duct at
  duodenal ampulla ? duodenal papilla
• Hepatopancreatic sphincter - encircles
• Common bile duct, pancreatic duct, and duodenal
  papilla

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Figure 24-21a, b