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The Digestive System: Part B
Pharynx
• Oropharynx and laryngopharynx
• Allow passage of food, fluids, and air
• Stratified squamous epithelium lining
• Skeletal muscle layers: inner longitudinal, outer pharyngeal
constrictors
Esophagus
• Flat muscular tube from laryngopharynx to stomach
• Pierces diaphragm at esophageal hiatus
• Joins stomach at the cardiac orifice
Esophagus
• Esophageal mucosa contains stratified squamous epithelium
• Changes to simple columnar at the stomach
• Esophageal glands in submucosa secrete mucus to aid in bolus
movement
• Muscularis: skeletal superiorly; smooth inferiorly
• Adventitia instead of serosa
Digestive Processes: Mouth
• Ingestion
• Mechanical digestion
• Mastication is partly voluntary, partly reflexive
• Chemical digestion (salivary amylase and lingual lipase)
• Propulsion
• Deglutition (swallowing)
Deglutition
• Involves the tongue, soft palate, pharynx, esophagus, and 22
muscle groups
• Buccal phase
• Voluntary contraction of the tongue
• Pharyngeal-esophageal phase
• Involuntary
• Control center in the medulla and lower pons
Stomach: Gross Anatomy
• Cardiac region (cardia)
• Surrounds the cardiac orifice
• Fundus
• Dome-shaped region beneath the diaphragm
• Body
• Midportion
Stomach: Gross Anatomy
• Pyloric region: antrum, pyloric canal, and pylorus
• Pylorus is continuous with the duodenum through the pyloric
valve (sphincter)
• Greater curvature
• Convex lateral surface
• Lesser curvature
• Concave medial surface
Stomach: Gross Anatomy
• Lesser omentum
• From the liver to the lesser curvature
• Greater omentum
• Drapes from greater curvature
• Anterior to the small intestine
Stomach: Gross Anatomy
• ANS nerve supply
• Sympathetic via splanchnic nerves and celiac plexus
• Parasympathetic via vagus nerve
• Blood supply
• Celiac trunk
• Veins of the hepatic portal system
Stomach: Microscopic Anatomy
• Four tunics
• Muscularis and mucosa are modified
• Muscularis externa
• Three layers of smooth muscle
• Inner oblique layer allows stomach to churn, mix, move, and
physically break down food
Stomach: Microscopic Anatomy
• Mucosa
• Simple columnar epithelium composed of mucous cells
• Layer of mucus traps bicarbonate-rich fluid beneath it
• Gastric pits lead into gastric glands
Gastric Glands
• Cell types
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•
•
•
Mucous neck cells (secrete thin, acidic mucus)
Parietal cells
Chief cells
Enteroendocrine cells
Gastric Gland Secretions
• Glands in the fundus and body produce most of the gastric juice
• Parietal cell secretions
• HCl
•  pH 1.5–3.5 denatures protein in food, activates pepsin, and kills many
bacteria
• Intrinsic factor
• Glycoprotein required for absorption of vitamin B12 in small intestine
Gastric Gland Secretions
• Chief cell secretions
• Inactive enzyme pepsinogen
• Activated to pepsin by HCl and by pepsin itself (a positive
feedback mechanism)
Gastric Gland Secretions
• Enteroendocrine cells
• Secrete chemical messengers into the lamina propria
• Paracrines
• Serotonin and histamine
• Hormones
• Somatostatin and gastrin
Mucosal Barrier
• Layer of bicarbonate-rich mucus
• Tight junctions between epithelial cells
• Damaged epithelial cells are quickly replaced by division of
stem cells
Homeostatic Imbalance
• Gastritis: inflammation caused by anything that breaches the
mucosal barrier
• Peptic or gastric ulcers: erosion of the stomach wall
• Most are caused by Helicobacter pylori bacteria
Digestive Processes in the Stomach
• Physical digestion
• Denaturation of proteins
• Enzymatic digestion of proteins by pepsin (and rennin in
infants)
• Secretes intrinsic factor required for absorption of vitamin B12
• Lack of intrinsic factor  pernicious anemia
• Delivers chyme to the small intestine
Regulation of Gastric Secretion
• Neural and hormonal mechanisms
• Stimulatory and inhibitory events occur in three phases:
1. Cephalic (reflex) phase: few minutes prior to food entry
2. Gastric phase: 3–4 hours after food enters the stomach
Regulation of Gastric Secretion
3. Intestinal phase: brief stimulatory effect as partially digested food
enters the duodenum, followed by inhibitory effects (enterogastric
reflex and enterogastrones)
Regulation and Mechanism of HCl Secretion
• Three chemicals (ACh, histamine, and gastrin) stimulate
parietal cells through second-messenger systems
• All three are necessary for maximum HCl secretion
• Antihistamines block H2 receptors and decrease HCl release
Response of the Stomach to Filling
• Stretches to accommodate incoming food
• Reflex-mediated receptive relaxation
• Coordinated by the swallowing center of the brain stem
• Gastric accommodation
• Plasticity (stress-relaxation response) of smooth muscle
Gastric Contractile Activity
• Peristaltic waves move toward the pylorus at the rate of 3 per
minute
• Basic electrical rhythm (BER) initiated by pacemaker cells (cells
of Cajal)
• Distension and gastrin increase force of contraction
Gastric Contractile Activity
• Most vigorous near the pylorus
• Chyme is either
• Delivered in ~ 3 ml spurts to the duodenum, or
• Forced backward into the stomach
Regulation of Gastric Emptying
• As chyme enters the duodenum
• Receptors respond to stretch and chemical signals
• Enterogastric reflex and enterogastrones inhibit gastric secretion
and duodenal filling
• Carbohydrate-rich chyme moves quickly through the duodenum
• Fatty chyme remains in the duodenum 6 hours or more
Small Intestine: Gross Anatomy
• Major organ of digestion and absorption
• 2–4 m long; from pyloric sphincter to ileocecal valve
• Subdivisions
1. Duodenum (retroperitoneal)
2. Jejunum (attached posteriorly by mesentery)
3. Ileum (attached posteriorly by mesentery)
Duodenum
• The bile duct and main pancreatic duct
• Join at the hepatopancreatic ampulla
• Enter the duodenum at the major duodenal papilla
• Are controlled by the hepatopancreatic sphincter
Structural Modifications
• Increase surface area of proximal part for nutrient absorption
• Circular folds (plicae circulares)
• Villi
• Microvilli
Structural Modifications
• Circular folds
• Permanent (~1 cm deep)
• Force chyme to slowly spiral through lumen
Structural Modifications
• Villi
• Motile fingerlike extensions (~1 mm high) of the mucosa
• Villus epithelium
• Simple columnar absorptive cells (enterocytes)
• Goblet cells
Structural Modifications
• Microvilli
• Projections (brush border) of absorptive cells
• Bear brush border enzymes
Intestinal Crypts
• Intestinal crypt epithelium
• Secretory cells that produce intestinal juice
• Enteroendocrine cells
• Intraepithelial lymphocytes (IELs)
• Release cytokines that kill infected cells
• Paneth cells
• Secrete antimicrobial agents (defensins and lysozyme)
• Stem cells
Submucosa
• Peyer’s patches protect distal part against bacteria
• Duodenal (Brunner’s) glands of the duodenum secrete alkaline
mucus
Intestinal Juice
• Secreted in response to distension or irritation of the mucosa
• Slightly alkaline and isotonic with blood plasma
• Largely water, enzyme-poor, but contains mucus
• Facilitates transport and absorption of nutrients
Liver
• Largest gland in the body
• Four lobes—right, left, caudate, and quadrate
Liver
• Falciform ligament
• Separates the (larger) right and (smaller) left lobes
• Suspends liver from the diaphragm and anterior abdominal wall
• Round ligament (ligamentum teres)
• Remnant of fetal umbilical vein along free edge of falciform
ligament
Liver: Associated Structures
• Lesser omentum anchors liver to stomach
• Hepatic artery and vein at the porta hepatis
• Bile ducts
• Common hepatic duct leaves the liver
• Cystic duct connects to gallbladder
• Bile duct formed by the union of the above two ducts
Liver: Microscopic Anatomy
• Liver lobules
• Hexagonal structural and functional units
• Filter and process nutrient-rich blood
• Composed of plates of hepatocytes (liver cells)
• Longitudinal central vein
Liver: Microscopic Anatomy
• Portal triad at each corner of lobule
• Bile duct receives bile from bile canaliculi
• Portal arteriole is a branch of the hepatic artery
• Hepatic venule is a branch of the hepatic portal vein
• Liver sinusoids are leaky capillaries between hepatic plates
• Kupffer cells (hepatic macrophages) in liver sinusoids
Liver: Microscopic Anatomy
• Hepatocyte functions
• Process bloodborne nutrients
• Store fat-soluble vitamins
• Perform detoxification
• Produce ~900 ml bile per day
Bile
• Yellow-green, alkaline solution containing
• Bile salts: cholesterol derivatives that function in fat emulsification
and absorption
• Bilirubin: pigment formed from heme
• Cholesterol, neutral fats, phospholipids, and electrolytes
Bile
• Enterohepatic circulation
• Recycles bile salts
• Bile salts  duodenum  reabsorbed from ileum  hepatic
portal blood  liver  secreted into bile
The Gallbladder
• Thin-walled muscular sac on the ventral surface of the liver
• Stores and concentrates bile by absorbing its water and ions
• Releases bile via the cystic duct, which flows into the bile duct