AP 150

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AP 150

• Chapter 24
• The Digestive System

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Components of the Digestive System
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Functions of the Digestive System

• Ingestion
• Mechanical processing
• Digestion
• Secretion
• Absorption
• Excretion

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The Digestive System Organs and the Peritoneum

• Peritoneum
• Visceral peritoneum Covers organs
• Parietal Covers interior surface of body wall
• Retroperitoneal Certain organs covered by
  peritoneum on only one surface and are considered
  behind the peritoneum e.g., kidneys, pancreas
• Mesenteries
• Sheets of serous membranes that support portions
  of the digestive tract
• Two layers of peritoneum with thin layer of loose
  C.T. between
• Routes by which vessels and nerves pass from body
  wall to organs
• Greater omentum
• Lies anterior to abdominal viscera
• Provides padding, protection, insulation, and
  energy reserves
• Connects greater curvature of the stomach to the
  transverse colon.
• Lesser omentum
• Connects lesser curvature of the stomach and the
  proximal part of the duodenum to the liver and
  diaphragm.

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Mesenteries
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Peritoneum and Peritoneal Cavity
Figure 23.5a
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Peritoneum and Peritoneal Cavity
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Histology of the Digestive Tract

• Mucosa. Innermost layer that lines the lumen
• - Mucous epithelium
• Stratified squamous in mouth, oropharynx,
  esophagus and anal canal)
• Simple columnar epithelium in the rest of the
  tract.
• Highly folded apical membranes forming microvilli
• - Loose connective tissue lamina propria
  contain MALT
• Muscularis mucosae smooth muscle
• Often folded into villi
• Submucosa.
• Layer of dense irregular connective tissue
• Nerves, blood vessels, lynmph nodules, and small
  glands located here
• Most innervated by nerves of the ANS submucosal
  plexus.

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Digestive Tract Histology The Tunics

• Muscularis
• Smooth muscle arranged in circular and longitu-
  dinal layers
• Myenteric plexus - network of ANS fibers that
  innervate muscles
• Serosa or adventitia
• Visceral peritoneum
• If organ not completely surrounded by serosa,
  then called an adventitia

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The Oral Cavity

• Functions include
• Analysis of material before swallowing
• Mechanical processing by teeth, tongue
• Lubrication
• Limited digestion
• Bounded by lips, cheeks, palate,and tongue
• Fauces (opening into pharynx) posteriorly
• Vestibule space between lip/cheeks and alveolar
  processes with teeth
• Oral cavity proper medial to alveolar processes
• Lined with moist stratified squamous epithelium

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Lips and Cheeks

• Both structures important in mastication and
  speech
• Lips (labia) orbicularis oris muscle within
• Keratinized stratified squamous exterior is thin
  and color of blood in dermis gives a red/pink
  color.
• Labial frenula (mucous folds) extend from
  alveolar processes of maxilla and mandible to the
  upper and lower lips, respectively.
• Many facial muscles act to move lips
• Cheeks lateral walls of oral cavity
• Buccinator muscle
• Buccal fat pad

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Palate and Palatine Tonsils

• Palate
• Hard palate anterior, supported by palatine
  process of maxilla and palatine bone
• Slightly corrugated on either side of midline
  raphe
• Soft palate posterior, consists of skeletal
  muscle and connective tissue
• Closes off nasopharynx during swallowing
• Uvula projects from posterior of soft palate
• Palatine tonsils lateral walls of fauces
• Housed within palatoglossal and palatopharyngeal
  arches

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Oral Cavity and Pharynx
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Tongue

• Functions to moves food in mouth, sensory
  analysis by taste buds, participates in speech
  and swallowing
• Muscular organ with free anterior surface and
  attached posterior surface.
• Covered with moist stratified squamous epithelium
• Intrinsic muscles change shape of the tongue
• Extrinsic muscles protrude or retract tongue,
  move side to side
• Innervated by the hypoglossal nerve
• Lingual frenulum attaches tongue inferiorly to
  floor of oral cavity

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Tongue

• Terminal sulcus groove that divides tongue into
  anterior 2/3 in oral cavity
  posterior 1/3 in oropharynx
• Anterior region in oral cavity has papillae,
  some of which have taste buds
• Posterior region in oropharynx no papillae
  abundant lymphoid tissue (lingual tonsils)

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Teeth

• Function in mastication
• Two sets
• Primary, deciduous, milk Childhood (20)
• Permanent or secondary Adult (32)
• Types
• Incisors (2) -cutting
• Canines (1)- piercing
• Premolars (2)- grinding
• Molars (3)-grinding
• Dental formula
• 2-1-2-3 / 2-1-2-3

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Teeth

• Anatomic crown enamel-covered part of tooth
  clinical crown is section of tooth above gum line
• Neck enameled part of tooth below gum line where
  crown and root come together
• Enamel outermost layer of anatomical crown.
  Layer of calcium salts and hydroxyapatite
  crystals. Hardest substance in body.
• Dentin living, cellular, calcified tissue. In
  the root, dentin is covered by cellular bone-like
  structure that helps hold tooth in the socket.
• Pulp cavity filled with blood vessels, nerves,
  and connective tissue
• Root canal portion of pulp cavity that extends
  into the root
• Cementum calcified c.t. that covers root and
  attaches it to periodontal ligament
• Periodontal ligaments hold tooth in socket.
  Forms fibrous joint called a gomphosis
• Apical foramen opening to root canal

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Salivary glands

• Exocrine glands
• Secrete salvia
• Watery solution
• Contains electrolytes, buffers, antibodiies, and
  salivary amylase (enzyme)
• Functions include
• Lubrication, moistening, and dissolving
• Initiation of digestion of complex carbohydrates
• Parasympathetic innervation stimulates salivary
  production
• Glands are either
• Serous
• Mucous
• Mixed

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Salivary Glands

• Three pairs of multicellular glands
• Parotid largest. Serous.
• Lies just anterior to the ear.
• Parotid duct crosses over masseter, penetrates
  buccinator, and enters the oral cavity adjacent
  to the 2nd upper molar
• Submandibular mixed, but more serous than
  mucous.
• Posterior, medial side of mandible.
• Duct enters oral cavity on either side of lingual
  frenulum
• Sublingual smallest. Mixed, but primarily
  mucous.
• Each has ducts that enter the floor of the oral
  cavity.
• Lingual glands. Small, coiled tubular glands on
  surface of tongue.

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Pharynx and Esophagus

• Pharynx
• Transports food and fluids to the esophagous and
  air to the trachea
• Posterior walls of oropharynx and laryngopharynx
  contains group of muscles called pharyngeal
  constrictors that contribute to swallowing

• Esophagus
• Transports food from pharynx to stomach
• Passes through esophageal hiatus (opening) of
  diaphragm and ends at stomach (at the cardiac
  orifice)
• Hiatal hernia widening of hiatus

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Histology of the Esophagus

• Esophageal mucosa nonkeratinized stratified
  squamous epithelium
• The empty esophagus is folded longitudinally and
  flattens when food is present
• Esophageal glands secrete mucus as a bolus moves
  through the esophagus
• Muscularis changes from skeletal (superiorly) to
  smooth muscle (inferiorly)
• Lacks a serosa
• Anchored by an adventitia

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Functions of the Stomach

• Bulk storage of undigested food
• Mechanical breakdown of food
• Disruption of chemical bonds via acids and
  enzymes
• Production of intrinsic factor
• Essential for absorption of Vit B-12
• Pernicious anemia develops when absent

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Stomach Anatomy

• Openings/Sphincters
• Gastroesophageal (cardiac) to esophagus
• Pyloric to duodenum
• Regions
• Cardiac - superior medial portion surrounds
  cardiac orifice
• Fundus - dome shaped region beneath the diaphragm
• Body - midportion of the stomach
• Pyloric antrum and pyloric canal adjacent to
  duodenum
• Greater curvature - convex lateral surface
  greater omentum attaches
• Lesser curvatures - concave medial surface
  lesser omentum

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Stomach

• Nerve supply sympathetic and parasympathetic
  fibers of the autonomic nervous system
• Blood supply from gastric artery of the celiac
  trunk, and corresponding veins (part of the
  hepatic portal system)

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The Stomach Lining
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Stomach Histology

• Layers
• Mucosa - contains gastric pits that open into
  gastric glands
• Submucosa - connective tissue
• Innervated by a submucosal plexus of ANS nerves
• Muscularis three layers
• Outer longitudinal
• Middle circular
• Inner oblique
• Innervated by a myenteric plexus of ANS nerves
• Serosa or visceral peritoneum outermost
• Rugae folds of the mucosa and submucosa when
  stomach is empty.

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Stomach Histology

• Gastric pits openings for gastric glands. Lined
  with simple columnar epithelium
• Cells of gastric pits and glands
• Surface mucus cells
• mucus that protects stomach lining from acid and
  digestive enzymes
• Mucous neck mucus
• Parietal cells hydrochloric acid and intrinsic
  factor
• Chief cells pepsinogen
• Enteroendocrine cells
• Enterochromaffin-like cells secrete histamine
  that stimulates acid secretion
• G-cells secrete gastrin
• D-cells secrete somatostatin
• inhibits gastrin and insulin secretion

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Stomach Lining

• The stomach is exposed to the harshest conditions
  in the digestive tract
• To keep from digesting itself, the stomach has a
  mucosal barrier with
• A thick coat of bicarbonate-rich mucus on the
  stomach wall
• Epithelial cells that are joined by tight
  junctions
• Gastric glands that have cells impermeable to HCl
• Damaged epithelial cells are quickly replaced

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Small Intestine Gross Anatomy

• Has important digestive and absorptive functions
• Receives secretions and buffers provided by
  pancreas, liver, and gall bladder
• Runs from pyloric sphincter to the ileocecal
  valve
• Has three subdivisions
• Duodenum
• Jejunum
• Ileum
• The common bile duct and main pancreatic duct
• Join the duodenum at the hepatopancreatic ampulla
  
• Are controlled by the sphincter of Oddi
• The jejunum extends from the duodenum to the
  ileum
• The ileum joins the large intestine at the
  ileocecal valve

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Modifications to Increase Surface Area

• Increase surface area 600 fold
• Plicae circulares - deep circular folds of the
  mucosa and submucosa
• Cause chyme to spiral thru
• Villi-extensions of mucosa that contain
  capillaries and lacteals.
• Microvilli folds of apical cell membranes of
  absorptive cells

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Mucosa and Submucosa of the Duodenum

• Cells and glands of the mucosa
• Absorptive cells simple columnar cells with
  microvilli
• produce digestive enzymes and absorb digested
  food
• Goblet cells produce protective mucus
• Enteroendocrine cells produce regulatory
  hormones
• Granular cells (paneth cells) may help protect
  from bacteria
• Intestinal glands (crypts of Lieberkühn)
• Tubular glands in mucosa at bases of villi
• Duodenal glands (Brunners glands)
• Tubular mucous glands of the submucosa

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Jejunum and Ileum

• The further one gets from the stomach there is a
  gradual decrease in
• Diameter
• Thickness of intestinal wall
• Number of circular folds
• Number of villi
• Major site of nutrient absorption
• Peyers patches lymphatic nodules numerous in
  mucosa and submucosa
• Ileocecal junction where ilium meets large
  intestine. Ileocecal sphincter and ileocecal valve

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Movement in Small Intestine

• Mixing and propulsion over short distances
• Segmental contractions mix
• Peristalsis propels
• Ileocecal sphincter remains slightly contracted
  until peristaltic waves reach it it relaxes,
  allowing chyme to move into cecum
• Cecal distention causes local reflex and
  ileocecal valve constricts
• Prevents more chyme from entering cecum
• Increases digestion and absorption in small
  intestine by slowing progress of chyme
• Prevents backflow

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Liver

• The largest gland in the body
• Superficially has four lobes right, left,
  caudate, and quadrate
• The falciform ligament
• Separates the right and left lobes anteriorly
• Suspends the liver from the diaphragm and
  anterior abdominal wall
• The ligamentum teres
• A remnant of the fetal umbilical vein
• Runs along the free edge of the falciform ligament

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Liver Associated Structures

• The lesser omentum anchors the liver to the
  stomach
• Porta on inferior surface. Vessels, ducts,
  nerves, exit/enter liver
• The gallbladder rests in a recess on the inferior
  surface of the right lobe
• Bile leaves the liver via
• Bile ducts, which fuse into the common hepatic
  duct
• The common hepatic duct, which fuses with the
  cystic duct
• These two ducts form the bile duct

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Liver
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Liver Microscopic Anatomy

• Hexagonal-shaped liver lobules are the structural
  and functional units of the liver
• Composed of hepatocyte (liver cell) plates
  radiating outward from a central vein
• Portal triads are found at each of the six
  corners of each liver lobule
• Portal triads consist of
• Bile duct - receives bile from bile canaliculi
• Hepatic artery supplies oxygen-rich blood to
  the liver cells
• Hepatic portal vein carries venous blood with
  nutrients from digestive viscera

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Liver Microscopic Anatomy

• Liver sinusoids enlarged, leaky capillaries
  located between hepatic plates
• Kupffer cells hepatic macrophages found in
  liver sinusoids

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Microscopic Anatomy of the Liver
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Blood and Bile Flow Through the Liver
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Functions of the Liver

• Bile production 600-1000 mL/day. Bile salts
  (bilirubin), cholesterol, fats, fat-soluble
  hormones, lecithin
• Neutralizes and dilutes stomach acid
• Bile salts emulsify fats. Most are reabsorbed in
  the ileum.
• Secretin (from the duodenum) stimulates bile
  secretions, increasing water and bicarbonate ion
  content of the bile
• Storage
• Glycogen, fat, vitamins, copper and iron. Hepatic
  portal blood comes to liver from small intestine.
• Nutrient interconversion
• Amino acids to energy producing compounds
• Hydroxylation of vitamin D. Vitamin D then
  travels to kidney where it is hydroxylated again
  into its active form
• Detoxification
• Hepatocytes remove ammonia and convert to urea
• Phagocytosis
• Kupffer cells phagocytize worn-out and dying red
  and white blood cells, some bacteria
• Synthesis
• Albumins, fibrinogen, globulins, heparin,
  clotting factors

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Gallbladder

• Thin-walled, green muscular sac on the ventral
  surface of the liver
• Lined with mucosa folded into rugae, inner
  muscularis, outer serosa
• Bile arrives constantly from liver is stored and
  concentrated
• Bile exits through cystic duct then into common
  bile duct
• Gallstones precipitated cholesterol
• Can block cystic duct

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Pancreas

• Location
• Lies deep to the greater curvature of the stomach
• The head is encircled by the duodenum and the
  tail abuts the spleen
• Exocrine function
• Acini - secretory units
• Secrete pancreatic juice that contains digestive
  enzymes
• Endocrine function
• Islets of Langerhans - composed of
• Alpha cells - secrete glucagon
• Beta cells - secrete insulin
• Delta cells - secrete somatostatin
• Pancreatic duct joins common bile duct and enters
  duodenum at the hepatopancreatic ampulla
  controlled by the hepatopancreatic sphincter
  (sphincter of Oddi)

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Liver, Gallbladder, Pancreas and Ducts
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Large Intestine

• Extends from ileocecal junction to anus
• Consists of cecum appendix, colon, rectum, anal
  canal
• Has three unique features
• Teniae coli three bands of longitudinal smooth
  muscle in its muscularis
• Haustra pocketlike sacs caused by the tone of
  the teniae coli
• Epiploic appendages fat-filled pouches of
  visceral peritoneum
• Movements sluggish (18-24 hours) chyme converted
  to feces.
• Absorption of water and salts, secretion of
  mucus, extensive action of microorganisms.
• 1500 mL chyme enter the cecum, 90 of volume
  reabsorbed yielding 80-150 mL of feces

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The Large Intestine
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Anatomy of Large Intestine

• Cecum
• Blind pouch, vermiform appendix attached
• Chyme passes out of ileum into cecum via the
  ileocecal valve
• Appendix walls contain numerous lymph nodules
• Connected to cecum and ileum via the mesoappendix
• Mucosa and submucosa dominated by large
  concentrations of lymphoid nodules
• Colon - divided into distinct segments
• Ascending, transverse, descending, and sigmoid
  colon
• Circular muscle layer complete
• Longitudinal incomplete (three teniae coli).
  Contractions of teniae form pouches called
  haustra. Small fat filled pouches called epiploic
  appendages

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Anatomy of Large Intestine

• Ascending colon
• Begins at superior border of cecum
• Ascends along lateral wall of peritoneum to
  inferior surface of liver
• Bends sharply to the left (hepatic flexure) and
  becomes the
• Transverse colon
• Crosses abdomen from right to left
• Supported by the transverse mesocolon
• Near the spleen it makes a 90 turn at splenic
  flexure and becomes the
• Descending colon
• Proceeds inferiorly on left side until reaching
  the iliac fossa
• Like the ascending colon, it lies
  retroperitoneally
• At the iliac fossa it curses at the sigmoid
  flexure and becomes the
• Sigmoid colon
• S-shaped segment ca. 15 cm (6 in.) long
• Lies posterior to the urinary bladder
• Suspended by the sigmoid mesocolon
• Empties into the rectum

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Anatomy of Large Intestine

• Rectum
• Descends along the inferior half of the sacrum
• Expandable for the temporary storage of feces
• Terminates at the anal canal
• Anal canal
• Contains small longitudinal folds called anal
  columns
• Transverse folds form the rectal valves that
  prevent movement of fecal material out of anal
  canal when flattulence occurs
• Lined by a stratified squamous epithelium
• Anus is the exit of the anal canal
• Circular smooth muscle forms the internal anal
  sphincter (smooth muscle)
• External anal sphincter formed by a ring of
  skeletal muscle
• Lamina propria and submucosa bear a network of
  veins which can become distended producing
  hemorrhoids

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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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Gross Anatomy of Rectum and Anal Canal
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Secretions of Large Intestine

• Mucus provides protection
• Parasympathetic stimulation increases rate of
  goblet cell secretion
• Pumps bacteria produce acid and the following
  remove acid from the epithelial cells that line
  the large intestine
• Exchange of bicarbonate ions for chloride ions
• Exchange of sodium ions for hydrogen ions
• Bacterial actions produce gases (flatus) from
  particular kinds of carbohydrates found in
  legumes and in artificial sugars like sorbitol
• Bacteria produce vitamin K which is then absorbed
• Feces consists of water, undigested food
  (cellulose), microorganisms, sloughed-off
  epithelial cells