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prepared by
Betsy C. Brantley
Valencia College
CHAPTER
16
The Digestive
System
© 2017 Pearson Education, Inc.
Digestive System Basics (16-1)
• Provides fuel for body cells’ functioning
• Digestive system includes:
• Digestive tract (gastrointestinal (GI) tract or alimentary
canal)
• Muscular tube
• Includes oral cavity (mouth), pharynx, esophagus,
stomach, small and large intestines, rectum, anus
• Accessory organs contribute to digestion
• Teeth, tongue, salivary glands, gallbladder, liver, and
pancreas
Figure 16-1 The Components of the Digestive System.
Accessory Organs of
the Digestive System
Teeth
Major Organs of
the Digestive Tract
Oral Cavity (Mouth)
Tongue
Pharynx
Salivary Glands
Esophagus
Liver
Stomach
Gallbladder
Small Intestine
Pancreas
Large Intestine
Anus
Six Functions of the Digestive System (16-1)
1. Ingestion
• Occurs when food and drink enter mouth
2. Mechanical processing
• Crushing of solid food to make it easier to move along
the digestive tract
• Increases surface area for enzymes to work
• Process begins in oral cavity with teeth and tongue
3. Digestion
• Chemical breakdown of food to absorbable size
Six Functions of Digestive System cont. (16-1)
4. Secretion
• Release of water, acids, enzymes, and buffers into
lumen of digestive tract
5. Absorption
• Movement of small organic molecules, electrolytes, and
water across digestive epithelium and into interstitial fluid
of digestive tract
6. Excretion
• Elimination of waste products from digestive tract
• Products are ejected as feces in process called
defecation
Figure 16-2 The Structure of the Digestive Tract.
Mesenteric
artery and vein
Circular fold
Mucosa
Mucosal epithelium
Mesentery
Circular
folds
Lamina propria
Villi
Mucosal glands
Submucosal gland
Muscularis
mucosae
Lymphatic vessel
Mucosa
Artery and vein
Submucosa
Submucosal
plexus
Muscularis
externa
Serosa
(visceral
peritoneum)
Circular muscle
layer
Myenteric plexus
Longitudinal
muscle layer
The Oral Cavity (16-2)
• Also called the buccal cavity
• Part of digestive tract that receives food
• Lined by oral mucosa
• Stratified squamous epithelium
• Contains tongue, teeth, and gingivae, or gums
• Gingivae are ridges of oral mucosae surrounding base of
teeth
Oral Cavity Functions (16-2)
• Senses food before swallowing
• Mechanically processes food
• Lubricates food with saliva and mucus
• Begins enzymatic digestion of carbohydrates
and lipids
Figure 16-4 The Oral Cavity.
Hard palate
Hard palate
Soft palate
Soft palate
Nasal cavity
Pharyngeal
tonsil
Entrance to
auditory tube
Palatal arches
Upper labium
(lip)
Uvula
Palatine
tonsil
Lingual
frenulum
Gingiva
Tongue
Nasopharynx
Cheek
Lower labium
(lip)
Gingiva
Uvula
Palatine tonsil
Tongue
Oropharynx
Vestibule
Vestibule
Palatal arch
Lingual tonsil
Epiglottis
Hyoid bone
Laryngopharynx
a An anterior view of the oral cavity,
as seen through the open mouth
© 2017 Pearson Education, Inc.
b Sagittal section of the oral cavity
Saliva Amount and Contents (16-2)
• About 1.0–1.5 liters produced each day
• During eating, production increases to about 7 mL
per minute
• Regulated by autonomic nervous system
• Composition
• 99.4 percent water
• Mucins, ions, buffers, waste products, metabolites, and
enzymes
• Mucins absorb water and form mucus
Saliva Functions (16-2)
• Water lubricates mouth and dissolves chemicals
• Mucus reduces friction and makes swallowing
easier
• Buffers keep pH near 7.0 and prevent buildup of
acids produced by bacteria
• Salivary antibodies (IgA) and lysozyme help control
bacterial levels
• Salivary amylase (an enzyme) begins chemical
digestion of starches (complex carbohydrates)
• Produced primarily by parotid salivary gland
The Pharynx (16-3)
• Commonly called the throat
• Serves as common passageway for food, liquid,
and air
• Food passes through oropharynx and
laryngopharynx to esophagus
• Mucosa is stratified squamous epithelium
• Lamina propria contains mucous glands and
tonsils
• Pharyngeal muscles cooperate with oral cavity and
esophageal muscles for swallowing
The Esophagus (16-3)
• Muscular tube that acts as passageway from
pharynx to stomach
• About 25 cm long and 2 cm wide
• Located posterior to trachea
• Enters abdominal cavity through esophageal hiatus
in diaphragm
• Diaphragmatic, or hiatal, hernia involves movement of
abdominal organs upward through the esophageal
hiatus
• Lined with stratified squamous epithelium
• Circular muscles at either end form upper and
lower esophageal sphincters
Swallowing (16-3)
• Also called deglutition
• Complex process
• Can be initiated voluntarily or involuntarily
• Proceeds automatically once begun
• Tongue forms food into bolus, or small mass
• Compression of bolus against hard palate initiates
swallowing process
The Stomach (16-4)
• Four primary functions
1. Temporary storage of ingested food
2. Mechanical breakdown of ingested food
3. Chemical digestion by acids and enzymes
4. Production of intrinsic factor needed for vitamin B12
absorption
• Chyme is mixture of food and gastric secretions
Regions of the Stomach (16-4)
• J-shaped organ with four main regions
1. Cardia
• Where the esophagus connects
2. Fundus
• Bulge of stomach superior to cardia
3. Body
• Large area between fundus and curve of the J
4. Pylorus
• Most distal portion
• Connects stomach to small intestine
• Pyloric sphincter regulates flow of chyme into small
intestine
Internal Features of the Stomach (16-4)
• Rugae
• Folds of mucosa
• Show prominently when stomach is empty
• Flatten out with stomach distention
• Stomach can expand to accommodate up to 1.0–1.5
liters
• Muscularis externa
• Has circular, longitudinal, and third oblique layer
• Extra layer strengthens stomach wall and assists in
mixing and churning chyme
Figure 16-8a The Anatomy of the Stomach.
Esophagus
Cardia
Diaphragm
Body
Fundus
Lesser curvature
(medial surface)
Lesser
omentum
Greater curvature
(lateral surface)
Greater
omentum
Pylorus
Rugae
a This anterior view of the stomach shows
important superficial landmarks.
The Gastric Wall (16-4)
• Lined by simple columnar epithelium with
numerous mucous cells
• Mucous epithelium secretes alkaline mucus that
protects epithelium
• Gastric pits
• Shallow depressions that open to gastric surface
• Mucous cells at base, or neck, undergo active mitosis,
replacing mucosal cells every three to seven days
• Gastric glands
• Located in fundus, body, and pylorus
• Connected to gastric pits
• Cells produce 1.5 liters/day of gastric juice
The Gastric Gland Cells (16-4)
• Parietal cells secrete:
• Intrinsic factor for vitamin B12 absorption
• Hydrochloric acid (HCl)
• Lowers pH of gastric juice to 1.5–2.0
• Kills microorganisms and activates enzymes
• Chief cells secrete:
• Pepsinogen, activated by HCl, which is converted into
proteolytic enzyme, pepsin
• In infants, also secrete rennin and gastric lipase that are
important in digestion of milk
Figure 16-8c,d The Anatomy of the Stomach.
Layers of the Stomach Wall
Lamina
propria
Mucosa
Gastric
pit
Gastric pit (opening
to gastric gland)
Mucous epithelium
Mucous
cells
Neck
Cells of
Gastric
Glands
Parietal
cells
Lamina propria
Muscularis mucosae
Gastric
gland
Submucosa
Artery
and
vein
Muscularis externa
Oblique muscle
Circular muscle
Lymphatic
vessel
Longitudinal muscle
Myenteric
plexus
Serosa
c This diagrammatic section shows the
organization of the stomach wall.
G cell
Chief
cells
Smooth
muscle
cell
d This diagram of a gastric
gland shows the sites of
parietal cells and chief cells.
Digestion in the Stomach (16-4)
• Pepsin initiates protein digestion to small peptides
• Salivary amylase will digest carbohydrates until pH
falls below 4.5
• Generally active one to two hours after a meal
• No nutrients are absorbed in the stomach
• Mucosa covered in alkaline mucus
• Epithelial cells lack transport mechanisms
• Gastric lining is impermeable to water
• Digestion is incomplete, nutrients are still complex
The Small Intestine (16-5)
• Major site of digestion and absorption
• 90% of nutrient absorption takes place here
• About 6 m long
• Divided into three segments
1. Duodenum
2. Jejunum
3. Ileum
Figure 16-10 The Segments of the Small Intestine.
Segments of the
Small Intestine
Duodenum
Circular folds
Jejunum
Ileum
Large
intestine
Gross anatomy of the jejunum
b A representative view of the jejunum
Rectum
a The positions of the duodenum, jejunum,
and ileum in the abdominopelvic cavity
The Duodenum (16-5)
• Segment closest to the stomach
• About 25 cm long
• “Mixing bowl” of the small intestine
• Receives chyme from stomach, plus liver and pancreatic
secretions
• C-shape curves around pancreas
• Mostly retroperitoneal, or behind peritoneum
The Jejunum and Ileum (16-5)
• Jejunum
• About 2.5 m long
• Most chemical digestion and nutrient absorption here
• Supported by mesentery and within peritoneum
• Ileum
• Final and longest segment (averages 3.5 m in length)
• Ends at ileocecal valve that controls flow into the cecum,
or first part of large intestine
The Intestinal Wall (16-5)
• Has three features that contribute to increased
surface area (from 3300 cm2 to about
2 million cm2)
1. Circular folds, or plicae circulares
• Permanent transverse ridges
2. Villi
• Fingerlike projections of mucosa covered with simple
columnar epithelium
3. Microvilli
• Modified apical surface of columnar cells
• Also called the brush border
Figure 16-11 The Intestinal Wall.
Capillaries
Circular fold
Villi
Mucous cells
Lacteal
Brush border
a A singular circular fold and multiple villi
Tip of villus
LM × 250
d A villus in
sectional view
Villi
Intestinal
gland
Lymphoid
nodule
Lacteal
Columnar
epithelial cell
Layers of the
Small Intestine
Mucous cell
Lacteal
Submucosal
artery and vein
Nerve
Mucosa
Muscularis
mucosae
Lymphatic
vessel
Submucosa
Muscularis
externa
Serosa
b The organization of the intestinal wall
Capillary
network
Lamina
propria
Submucosal
plexus
Circular layer
of smooth muscle
Lymphatic
vessel
Myenteric plexus
Longitudinal layer
of smooth muscle
Smooth
muscle
cell
Arteriole
Venule
c Internal structure in a single villus, showing
the capillary network and lacteal
Intestinal Villus (16-5)
• Each villus contains:
• Network of blood capillaries
• Transport absorbed nutrients to hepatic portal system for
delivery to the liver
• Lacteal, or lymphatic, capillary
• Transports absorbed fatty acids, packaged into
chylomicrons
• Intestinal glands at the villus base
• Secrete intestinal juice
• In duodenum, duodenal glands, or submucosal glands,
also secrete mucus, helping to buffer acidic chyme
Figure 16-11b&c The Intestinal Wall.
Villi
Intestinal
gland
Lymphoid
nodule
Lacteal
Columnar
epithelial cell
Layers of the
Small Intestine
Mucous cell
Lacteal
Submucosal
artery and vein
Nerve
Mucosa
Submucosa
Muscularis
mucosae
Lymphatic
vessel
Submucosal
plexus
Capillary
network
Lamina
propria
Circular layer
of smooth muscle
Muscularis
externa
Serosa
b The organization of the intestinal wall
Lymphatic
vessel
Myenteric plexus
Longitudinal layer
of smooth muscle
Smooth
muscle
cell
Arteriole
Venule
c Internal structure in a single villus, showing
the capillary network and lacteal
Intestinal Secretions (16-5)
• Intestinal juice produced at rate of about
1.8 liters/day
• Moistens intestinal contents
• Helps buffer acids
• Provides liquid environment for intestinal contents
• Composed mostly of water from mucosa
• Moves into lumen by osmosis
• Rest is secreted by intestinal glands
• Stimulated by touch and stretch receptors in intestinal wall
• Also respond to signals in cephalic phase
Intestinal Hormones (16-5)
• Gastrin released in response to incompletely
digested proteins
• Promotes stomach motility
• Stimulates production of acid and enzymes
• Secretin released in response to acidic chyme
• Increases secretion of bile and buffers by liver and
pancreas
• Cholecystokinin (CCK) released in response to
high-fat chyme
• In pancreas, increases enzyme production
• In gallbladder, causes the ejection of bile
Intestinal Hormones cont. (16-5)
• Gastric inhibitory peptide (GIP) released in
response to high-fat and high-glucose chyme
• Inhibits gastric activity
• Causes release of insulin
Digestion in the Small Intestine (16-5)
• Most important digestive processes are completed
in small intestine
• Final products of digestion absorbed here
• Most enzymes and buffers come from pancreas
and liver
• Small intestine enzymes produced by brush border cells
The Pancreas (16-6)
• Position and size
• Lies posterior to stomach
• Extends from duodenum toward spleen
• Retroperitoneal (only anterior surface covered with
peritoneum)
• About 15 cm long; weighing around 80 g
Histology of the Pancreas (16-6)
• Two distinct groups of cells
• Pancreatic islets (endocrine)
• Secrete insulin and glucagon
• Only about 1% of the pancreatic cells
• Pancreatic acinar cells (exocrine)
• Produce mixture of digestive enzymes, water, and buffers
as pancreatic juice
• Organized into pouches called pancreatic acini
• Duct networks converge into pancreatic duct
• Empties into duodenum along with the common bile duct
Figure 16-13 The Pancreas.
Pancreatic duct
Connective tissue septum
Exocrine cells in
pancreatic acini
Endocrine cells in
pancreatic islet
b Diagram of the cellular organization
of the pancreas.
Accessory
pancreatic
duct
Common bile
duct
Pancreatic
duct
Lobules
Tail of
pancreas
Duct
Body of
pancreas
Head of
pancreas
Duodenal
papilla
Duodenum
Pancreatic islet
(endocrine)
Pancreatic
acini
(exocrine)
Pancreas
a The gross anatomy of the pancreas The head of the pancreas
is tucked into a C-shaped curve of the duodenum that begins
at the pylorus of the stomach.
LM × 75
c Light micrograph of the cellular
organization of the pancreas.
Control of Pancreatic Secretions (16-6)
• About 1000 mL of pancreatic juice secreted each
day
• Regulated by hormones
• Secretin released from duodenum in response to
presence of acidic chyme entering from stomach
• Triggers pancreas to release watery, alkaline (pH 7.5–8.8)
fluid
• One of primary buffers in fluid is sodium bicarbonate
• Increases pH of chyme to optimal pH for digestive
enzymes
• Cholecystokinin (CCK) stimulates production and
release of pancreatic enzymes
Pancreatic Enzymes (16-6)
• Classified by substances they help break down
• Carbohydrases (general term) digest sugars and
starches
• Pancreatic amylase breaks down carbohydrates
• Pancreatic lipase breaks down lipids, or fats
• Nucleases break down nucleic acids
• Pancreatic proteases break down proteins
• Make up 70 percent of total pancreatic enzyme production
• Examples: trypsin, chymotrypsin, carboxypeptidase
• Secreted as inactive proenzymes and activated after
reaching small intestine
The Liver (16-6)
• Largest visceral organ
• Weighs about 1.5 kg (roughly 2.5 percent of total body
weight)
• Found in right hypochondriac and epigastric
abdominopelvic regions
• Wrapped in tough fibrous capsule and covered by visceral
peritoneum
• Divided into four lobes
• Large left and right lobes
• Smaller caudate and quadrate lobes
• Falciform ligament marks division between left and right
lobes on anterior surface
• Posterior margin of falciform ligament is the round ligament
Figure 16-14 The Surface Anatomy of the Liver.
Coronary ligament
Right lobe
Left lobe
Falciform ligament
Round ligament
Liver
Gallbladder
a Anterior surface of the liver
Left hepatic vein
Coronary ligament
Inferior vena cava
Lobes of Liver
Left lobe
Common bile duct
Caudate lobe
Hepatic portal vein
Right lobe
Hepatic artery
proper
b Posterior surface of the liver
Quadrate lobe
Gallbladder
Histology of the Liver (16-6)
• Lobes divided into about 100,000 liver lobules
• Basic functional unit of liver, each about 1 mm in
diameter
• Hepatocytes (liver cells) are covered in microvilli and
arranged in plates
• Specialized capillaries called sinusoids empty into
central vein
• Allow free exchange of water and solutes
• Contain phagocytic Kupffer cells in lining
• Engulf pathogens, debris, and damaged blood cells
Liver Functions (16-6)
• Over 200 known functions
• Can be categorized into three general roles
1. Metabolic regulation
2. Hematological regulation
3. Bile production
Metabolic Regulation by the Liver (16-6)
• Liver has primary role in regulating composition of
circulating blood
• Blood flows from absorptive areas of digestive tract
through liver where hepatocytes:
• Extract nutrients and toxins from blood
• Store and synthesize nutrient molecules
• Fat-soluble vitamins (A, D, E, and K) are stored
• Monitor and adjust circulating levels of organic nutrients
• High blood glucose triggers synthesis of glycogen
• Low blood glucose triggers breakdown of glycogen and
release of glucose
© 2017 Pearson Education, Inc.
Production and Role of Bile (16-6)
• Bile is synthesized by the liver
• Bile contains:
• Water and ions that dilute and buffer
• Bilirubin (pigment derived from hemoglobin) from
destroyed RBCs
• Cholesterol and bile salts
• Bile salts break apart large fat droplets into smaller ones
• Process called emulsification
• Increases surface area of lipids for enzymatic action
The Gallbladder (16-6)
• Hollow, pear-shaped organ located in recess on
posterior surface of liver’s right lobe
• Functions are bile storage and bile modification
• Releases bile into cystic duct, which flows into common
bile duct, then enters duodenum at duodenal papilla
• Hepatopancreatic sphincter surrounds shared
passageway of common bile duct and pancreatic duct
• Limits entry of secretions into duodenum until needed
• CCK triggers contractions of gallbladder and relaxation of
sphincter
• Bile becomes more concentrated as it is stored
• Too concentrated bile can form gallstones
Figure 16-16 The Gallbladder.
Cystic duct
Common
hepatic duct
Cut edge of
lesser omentum
Gallbladder
Hepatic
portal vein
Common bile
duct
Common hepatic
artery
Liver
Duodenum
Common
bile duct
Hepatopancreatic
sphincter
Pancreas
Pancreatic
duct
Duodenal
papilla
Intestinal lumen
Stomach
Pancreas
b This sectional view of part of the duodenum shows the duodenal
papilla opening and location of the hepatopancreatic sphincter.
a A view of the inferior surface of the liver,
showing the position of the gallbladder
and the ducts that transport bile from the
liver to the gallbladder and duodenum.
The Large Intestine (16-7)
• Also called the large bowel
• Begins at ileocecal valve, ends at anus
• Total length of about 1.5 m divided into three parts:
cecum, colon, rectum
• Horseshoe-shape frames the small intestine
• Main functions
• Reabsorbing water, compacting chyme into feces
• Absorbing vitamins freed by bacterial action
• Storing feces prior to defecation
The Cecum (16-7)
• An expanded pouch
• Chyme enters through ileocecal valve
• Functions to initiate compaction
• Appendix, or vermiform appendix
• Attached on posteromedial surface
• Contains lymphoid nodules for immune function
• Inflammation is called appendicitis
The Colon (16-7)
• Longest portion of the large intestine
• Has a large diameter, lacks villi, and has abundant
mucous cells
• External features
• Haustra: pouches that allow colon to expand and
elongate
• Teniae coli: three longitudinal bands of smooth muscle
• Divided into four segments
• Ascending colon, transverse colon, descending
colon, sigmoid colon
The Rectum (16-7)
• Forms last 15 cm of digestive tract
• Expandable organ for temporarily storing feces
• Anal canal is last portion
• Contains longitudinal anal columns
• Anus is the exit of the anal canal
• Lined with stratified squamous epithelium
• Internal anal sphincter made of smooth muscle of
muscularis externa, involuntary control
• External anal sphincter is voluntary, skeletal muscle
Figure 16-17 The Large Intestine.
Aorta
Splenic vein
Hepatic portal vein
Superior mesenteric artery
Inferior mesenteric vein
Superior mesenteric vein
Inferior vena cava
Greater
omentum (cut)
Transverse
colon
Descending
colon
Inferior
mesenteric
artery
Ascending
colon
Haustra
Ileum
Ileocecal valve
Cecum
Tenia coli
Appendix
Sigmoid colon
a The gross anatomy
and regions of the
large intestine
Rectum
Ileocecal
valve
Cecum
(cut open)
Rectum
Anal canal
Anal columns
Appendix
Internal anal sphincter
External anal sphincter
b The cecum and appendix
Anus
c The rectum and anus
Absorption in the Large Intestine (16-7)
• Major function of large intestine is reabsorption of
substances
• Reabsorbs water
• Of 1500 mL of chyme that enters cecum each day, 1300
mL are reabsorbed
• Reabsorbs bile salts
• Bile salts absorbed in the cecum
• Transported to the liver for secretion in bile
Absorption of Organic Wastes - Toxins (16-7)
• Bacterial action breaks down peptides in feces
• Generates ammonia, nitrogenous compounds, hydrogen
sulfide
• Odor of feces from these compounds
• Ammonia and other toxins absorbed into hepatic portal
circulation
• Liver converts them into nontoxic compounds that can be
excreted by kidneys
• Indigestible carbohydrates in colon
• Provide nutrient source for bacteria
• Metabolic activities of those bacteria create flatus, or gas
Defecation Process (16-7)
• Internal anal sphincter relaxes involuntarily
• Conscious relaxation of external sphincter required
for defecation
• Other conscious actions help force fecal material
into rectum
• Tensing abdominal muscles or elevating intra-abdominal
pressure
• Valsalva maneuver
• Attempting to forcibly exhale with a closed glottis
• Repeated bouts of straining to defecate permanently
distends veins in the anal canal, producing hemorrhoids
Water and Electrolyte Absorption (16-8)
• Total of about 9000 mL water and secretions
added into digestive tract each day
• All but 150 mL absorbed
• Intestinal epithelial cells constantly absorb
dissolved nutrients and ions
• Water “follows” through osmosis
• Absorption of Na+ and Cl– most important factor
promoting water reabsorption
• Other ions include K+, Mg2+, I–, HCO3–, Fe2+, and Ca2+
• Ca2+ absorption under control of parathyroid hormone
and calcitriol
Digestive Tract Lining (16-1)
• Protects surrounding tissues from:
• Corrosive effects of digestive acids and enzymes
• Physical abrasion
• Bacteria that are ingested or live in digestive tract
• 4 layers
1. Mucosa
2. Submucosa
3. Muscularis externa
4. Serosa
Mucosa (16-1)
• Mucous membrane that forms the inner lining of
the digestive tract
• Consists of:
• Mucosal epithelium
• Underlying layer of areolar tissue called the lamina
propria
• Muscle layer called the muscularis mucosae
• Smooth muscle that helps move mucosa
Mucosal Epithelia (16-1)
• Stratified squamous in high physical stress organs
• Oral cavity, pharynx, esophagus, anus
• Rest is simple columnar with surface modifications
• Ducts of secretory glands open to surface of
epithelium
• Circular folds and villi increase surface area for
absorption
Submucosa (16-1)
• Layer of dense irregular connective tissue
• Binds mucosa to muscularis externa
• Contains blood vessels and lymphatics
• Outer margin contains:
• Parasympathetic neurons and sensory neurons
• Submucosal plexus
• Neural network that can function without CNS
• Regulates secretion and motility
Muscularis Externa (16-1)
• Band of smooth muscle arranged in:
• Inner circular and outer longitudinal layer
• Function to mix and propel materials
• Myenteric plexus between layers of muscle
• Contains parasympathetic ganglia, sensory neurons,
interneurons, and sympathetic postganglionic fibers
• Parasympathetic stimulation increases activity
• Sympathetic stimulation decreases activity
Serosa and Adventitia (16-1)
• Serosa
• Serous membrane covering muscularis externa along GI
tract enclosed by peritoneum
• Also called the visceral peritoneum
• Continuous with the parietal peritoneum, which lines
inner surfaces of body wall
• Adventitia
• Layer covering muscularis externa of regions where
there is no serosa
• Examples: oral cavity, pharynx, esophagus, and rectum
• Attaches GI tract to adjacent structures
Mesenteries (16-1)
• Mesenteries
• Double sheets of serous membrane (parietal and
visceral peritoneum)
• Suspend portions of digestive tract
• Provide pathways for blood vessels, lymphatics, and
nerves
• Help organize and stabilize attached organs
Movement of Digestive Materials (16-1)
• Pacesetter cells in smooth muscle of digestive tract
trigger contraction
• Peristalsis
• Waves of contraction initiated by circular layer, followed
by longitudinal layer
• Propels bolus (food mass) down tract
• Segmentation
• A mixing action with no propulsion
4
Figure 16-3 Peristalsis.
Initial State
Longitudinal
Muscle
Circular
muscle
1
From
mouth
To
anus
Contraction of circular muscles behind bolus
Contraction
2
Contraction of longitudinal muscles ahead of bolus
Contraction
Contraction
3
A wave of contraction in circular muscle layer
forces bolus forward
The Tongue (16-2)
• Manipulates food within oral cavity
• Mechanically compresses, abrades, distorts material
• Assists in chewing and preparing food for swallowing
• Provides sensory analysis of touch, temperature, and
taste
• Lingual tonsils
• Paired lymphoid nodules at base of tongue
• Help resist infection
The Salivary Glands (16-2)
• Three pairs of glands that secrete into oral cavity
• Parotid salivary glands
• On each side of oral cavity between mandible and skin
• Parotid duct empties into vestibule at level of second upper
molar
• Sublingual salivary glands
• Under mucous membrane on floor of mouth
• Numerous sublingual ducts open on either side of lingual
frenulum
• Submandibular salivary glands
• In floor of the mouth along inner surfaces of mandible
• Ducts open into mouth behind teeth on either side of lingual
frenulum
Figure 16-5 The Salivary Glands.
Parotid duct
Openings of
sublingual
ducts
Lingual
frenulum
Opening of left
submandibular
duct
Submandibular
duct
Salivary Glands
Parotid salivary
gland
Sublingual
salivary gland
Submandibular
salivary gland
Figure 16-6a Teeth: Structural Components and
Pulp cavity
Dental Succession.
Crown
Enamel
Dentin
Gingiva
Neck
Cementum
Periodontal
ligament
Root
Root canal
Bone of
alveolus
a Diagrammatic section through a typical
adult tooth
Branches of blood
vessels and nerves
Centralincisors (7–8 yr)
Figure 16-6b,c Teeth: Structural Components
and Dental Succession.
Lateral incisor
(8–9 yr)
Cuspid
(11–12 yr)
1st Premolar
(10 –11 yr)
Upper
dental
arch
Central incisors (7.5 mo)
1st Molar
(6–7 yr)
UPPER JAW
Lateral incisor
(9 mo)
Cuspid (18 mo)
Hard palate
2nd Premolar
(10 –12 yr)
2nd Molar
(12–13 yr)
Hard palate
3rd Molar
(17–21 yr)
Primary 1st
molar (14 mo)
3rd Molar
(17–21 yr)
Primary 2nd
molar (24 mo)
2nd Molar
(11–13 yr)
1st Molar
(6–7 yr)
Primary 2nd
molar (20 mo)
Primay 1st
molar (12 mo)
2nd Premolar
(11–12 yr)
Lower
dental
arch
1st Premolar
(10 –12 yr)
Cuspid (9–10 yr)
Cuspid (16 mo)
Lateral incisor
(7 mo)
Central incisors (6 mo)
b Primary teeth, with the age
at eruption given in months
LOWER JAW
Lateral incisor (7–8 yr)
Central incisors (6–7 yr)
c Adult teeth, with the age at eruption
given in years
Phases of Swallowing (16-3)
1. Buccal phase
• Voluntary phase
• Movement of bolus into back of oral cavity and into
oropharynx
• Soft palate closes over nasopharynx
2. Pharyngeal phase
• Epiglottis folds over larynx
• Food and liquid are directed past closed glottis
• Uvula and soft palate block nasopharynx
3. Esophageal phase
• Bolus is pushed into esophagus and toward stomach
• Pharyngeal and esophageal phases are involuntary due to
swallowing reflex
Figure 16-7 The Swallowing Process.
1
Buccal Phase
Hard palate
Soft palate
Bolus
Tongue
Oropharynx
Epiglottis
Trachea
2
Pharyngeal Phase
Uvula
The pharyngeal phase
begins as the bolus
comes into contact with
the palatal arches and the
posterior pharyngeal wall.
Elevation of the larynx and
folding of the epiglottis
direct the bolus past the
closed glottis. At the
same time, the uvula and
soft palate block passage
back to the nasopharynx.
Tongue
Bolus
Epiglottis
Larynx
3
Esophageal Phase
Peristalsis in
esophagus
4
Trachea
Bolus Enters Stomach
Thoracic
cavity
Lower
esophageal
sphincter
Stomach
© 2017 Pearson Education, Inc.
The buccal phase begins
with the compression of
the bolus against the hard
palate. Retraction of the
tongue then forces the
bolus into the oropharynx
and assists in elevating
the soft palate, thereby
sealing off the
nasopharynx. Once
the bolus enters the
oropharynx, reflex
responses begin and the
bolus is moved toward
the stomach.
The esophageal phase
begins as the contraction
of pharyngeal muscles
forces the bolus through
the entrance to the
esophagus. Once in the
esophagus, the bolus is
pushed toward the
stomach by peristalsis.
The approach of the
bolus triggers the
opening of the lower
esophageal sphincter.
The bolus then continues
into the stomach.
Slide 4
Mesenteries Associated with the Stomach (16-4)
• Greater omentum
• Very large peritoneal pouch
• Extends from greater curvature of stomach down over
abdominal viscera
• Lesser omentum
• Smaller peritoneal pouch
• Extends from lesser curvature of stomach to liver
Figure 16-8b The Anatomy
of the Stomach.
Diaphragm
Liver
Pancreas
Lesser
omentum
Stomach
Mesentery
Duodenum
Mesentery
Rectum
Transverse
colon
Greater
omentum
Parietal
peritoneum
Small
intestine
Uterus
Bladder
b The stomach is surrounded by the peritoneal
cavity. Its position is maintained by the greater
and lesser omenta.
Regulation of Gastric Activity (16-4)
• Production of acid and enzymes
• Controlled by central nervous system
• Regulated by reflexes involving stomach wall
• Regulated by hormones of digestive tract
• Involves three overlapping phases
1. Cephalic phase
2. Gastric phase
3. Intestinal phase
Cephalic Phase (16-4)
• Triggered by sight, smell, taste, thought of food
• Prepares stomach to receive food
• Parasympathetic stimulation of gastric cells
increases production of gastric juice
• Rates up to 500 mL/hour
• Generally only lasts short period
Figure 16-9-1 Regulation of Gastric Activity
Food
1
CEPHALIC PHASE
The cephalic phase of gastric
secretion begins when you see,
smell, taste, or think of food.
This phase is directed by the
parasympathetic division of the
autonomic nervous system. It
prepares the stomach to
receive food. In response to
stimulation, the production of
gastric juice speeds up,
reaching rates of about 500
mL/h, or about 2 cups per hour.
This phase generally lasts only
minutes.
Sight, smell, taste,
or thoughts of food
Central nervous system
Vagus nerve (N X)
Submucosal
plexus
Gastrin
Mucous
cells
Mucus
Chief
cells
Pepsinogen
Parietal
cells
HCl
G cells
KEY
Neural
stimulation
Secretion
Gastric Phase (16-4)
• Begins when food enters stomach
• Stretch reflexes increase myenteric stimulation of
mixing waves
• Submucosal plexus
• Stimulates parietal and chief cells
• Stimulates G cells to produce gastrin
• Results in rapid increase in gastric juice production
• Phase may continue for several hours
Figure 16-9-2 Regulation of Gastric Activity
2
GASTRIC PHASE
Submucosal and
The gastric phase begins
myenteric plexuses
when food arrives in the
stomach. The stimulation of
stretch receptors in the
stomach wall and of
Mucous
carried by
chemoreceptors in the
cells
bloodstream
mucosa triggers local reflexes
Chief
in the submucosal and
cells
myenteric plexuses. This
Parietal
Gastrin
results in mixing waves from
cells
the muscularis externa, and
G cells
the secretion of mucus,
pepsinogen, and HCl from the
cells of the gastric glands.
Distension
Elevated pH
Stretch
receptors
Chemoreceptors
Mucus
Pepsinogen
HCI
Partly
digested
peptides
Mixing
waves
KEY
Neural
stimulation
Hormonal
stimulation
Intestinal Phase (16-4)
• Begins when chyme enters small intestine
• Mostly inhibitory controls, slowing gastric emptying
• Enterogastric reflex inhibits gastrin production
• Intestinal hormones secretin, cholecystokinin (CCK), and
gastric inhibitory peptide (GIP) reduce gastric activity
• Ensures efficient intestinal functions
• Secretion, digestion, and absorption
Figure 16-9-3 Regulation of Gastric Activity
3
INTESTINAL PHASE
The intestinal phase of gastric
secretion begins when chyme
first enters the duodenum of the
small intestine. The function of
the intestinal phase is to control
the rate of gastric emptying
to ensure that the secretory,
digestive, and absorptive
functions of the small intestine
can proceed efficiently.
Enterogastric
reflex
Myenteric
plexus
carried by bloodstream
Duodenal
stretch and
chemoreceptors
CCK
GIP
Chief
cells
Parietal
cells
Peristalsis
Presence of
lipids and
carbohydrates
KEY
Secretin
Decreased pH
Inhibition
Intestinal Movements (16-5)
• Weak peristaltic contractions move chyme toward
jejunum
• Local reflexes not under CNS control
• Gastroenteric reflex
• Initiated by distention of stomach
• Increases glandular secretion and peristaltic contractions
along length of small intestine
• Empties duodenum
• Gastroileal reflex
• Triggered by gastrin
• Relaxes ileocecal valve
• Material pushed from ileum into large intestine
Figure 16-12 The Activities of Major Digestive
Tract Hormones.
Ingested
food
Hormone
Action
Food in
stomach
Acid production by
parietal cells
KEY
stimulates
inhibits
Gastrin
Stimulation of gastric
motility; mixing waves
increase in intensity
GIP
Release of insulin
from pancreas
Pancreas
Release of pancreatic
enzymes and buffers
Chyme in
duodenum
Secretin
and CCK
Bile secretion and
ejection of bile
from gallbladder
facilitates
Material
arrives in
jejunum
facilitates
Nutrient absorption
facilitates
NUTRIENT
UTILIZATION
BY ALL TISSUES
Portal Areas of the Liver (16-6)
• Also called portal triad
• Found at each of six corners of lobule
• Includes three parts
1. Branch of hepatic portal vein
2. Branch of hepatic artery proper
3. Small branch of bile duct
Blood Flow through the Liver (16-6)
• Blood comes from hepatic artery and hepatic
portal vein
• As it flows through sinusoids, hepatocytes:
• Absorb nutrient molecules
• Secrete plasma proteins
• Blood flows from sinusoids into central vein
• Merge to form hepatic veins
• Drain into inferior vena cava
Bile Production in the Liver (16-6)
• Hepatocytes secrete bile into narrow channels
called bile canaliculi
• Located between adjacent liver cells
• Carry bile into bile duct in portal triad
• Bile flows into common hepatic duct, then to:
• Common bile duct into the duodenum or
• Cystic duct into the gallbladder
Figure 16-15 Liver Histology.
1 mm
a A diagrammatic view of
liver structure, showing
relationships among
lobules
Interlobular
septum
Bile
duct
Branch of
hepatic portal vein
Portal area
Bile
ductules
Portal Area
Bile
duct
Branch of
Branch of
hepatic portal vein hepatic artery
(containing blood)
proper
Hepatocytes
Sinusoids
Central
vein
Kupffer
cells
Bile
canaliculi
Portal Area
Bile duct
Portal area
Branch of hepatic
portal vein
LM × 320
c A sectional view showing the vessels
and ducts within a portal area
Branch of hepatic
artery proper
b A single liver lobule and its cellular components
Hematological Regulation by the Liver (16-6)
• Largest blood reservoir in the body
• Receives about 25 percent of cardiac output
• Phagocytic Kupffer cells remove old or damaged
RBCs, debris, and pathogens from blood
• Hepatocytes synthesize plasma proteins
• Determine osmotic pressure of plasma
• Function as nutrient transporters
• Key elements of clotting and complement cascades
Table 16.2
Movements of the Large Intestine (16-7)
• Gastroileal and gastroenteric reflexes move
material into cecum
• Transit time through large intestine very slow
• Allows for water reabsorption
• Mass movements
• Powerful peristaltic contractions
• Occur a few times per day
• Triggered by distention of stomach and duodenum
• Forces feces into rectum, producing urge to defecate
Processing and Absorption of Nutrients (16-8)
• Balanced diets include all ingredients needed to
maintain homeostasis
• Carbohydrates, proteins, lipids
• Broken down into absorbable forms by enzymes
• Process called hydrolysis
• Used by cells to generate ATP or build complex
carbohydrates, proteins, lipids
• Water, electrolytes (minerals), vitamins
• No processing, but require special transport mechanisms
Absorption of Vitamins (16-7)
• Vitamins are organic molecules essential to
metabolic reactions
• Bacteria in colon make three key vitamins
1. Vitamin K
• Needed for production of clotting factors
2. Biotin
• Essential for glucose metabolism
3. Vitamin B5 (pantothenic acid)
• Required for synthesis of neurotransmitters and steroid
hormones
Absorption of Organic Wastes - Bilirubin (16-7)
• Bacteria convert bilirubin into other products
• Some are absorbed into bloodstream
• Excreted in urine, giving yellow color
• Some remain in the colon
• Give feces brown color
Defecation Reflex (16-7)
• Involves two positive feedback loops
1. Shorter loop
• Stretch receptors in rectal walls stimulate local peristalsis
• Moves feces toward anus, distends rectum
2. Longer loop
• Stretch receptors stimulate parasympathetic reflex in
sacral spinal cord
• Stimulate increasing peristalsis and increased distention in
rectum
Absorption of Vitamins (16-8)
• Fat-soluble vitamins: A, D, E, and K
• Absorbed in micelles along with lipids
• Vitamin K also produced in colon by bacteria
• Water-soluble vitamins: B vitamins and C
• All but B12 are easily absorbed by digestive epithelium
• B12 requires intrinsic factor (protein secreted by parietal
cells of stomach) for absorption
• Bacteria in gut are also source of water-soluble vitamins
Carbohydrate Digestion and Absorption (16-8)
• Begins in mouth during mastication
• Salivary amylase breaks down complex carbohydrates
into di- or trisaccharides
• Pancreatic amylase continues process
• Brush border enzymes on intestinal microvilli
complete breakdown into monosaccharides
• Monosaccharides absorbed through facilitated
diffusion or cotransport
• Transported into capillaries by diffusion
Figure 16-18a ChemicalCARBOHYDRATES
Events in Digestion
REGION
Salivary
amylase
ORAL CAVITY
ESOPHAGUS
STOMACH
Disaccharides
Trisaccharides
SMALL INTESTINE
Pancreatic
alpha-amylase
Disaccharides
INTESTINAL
MUCOSA
Lactase
Trisaccharides
Maltase, Sucrase
Brush border
FACILITATED
DIFFUSION AND
COTRANSPORT
Monosaccharides
Cell body
FACILITATED
DIFFUSION
ROUTE TO BLOODSTREAM
Carbohydrates and amino acids
are absorbed and transported by
intestinal capillaries. Lipids form
chylomicrons that diffuse into
lacteals and are delivered to the left
subclavian vein by the thoracic duct.
Capillary
Monosaccharides
Lipid Digestion and Absorption (16-8)
• Involves lingual lipase from glands under tongue and
pancreatic lipase from pancreas
• Triglycerides enter duodenum in large fat droplets
• Bile salts emulsify droplets; pancreatic lipase breaks
apart triglycerides
• Triglycerides broken into fatty acids and monoglycerides
• These combine with bile salts to form micelles
• Micelles diffuse into epithelial cells and are converted
into triglycerides
• Coated with proteins to form chylomicrons
• Secreted by exocytosis into interstitial fluids
• Absorbed into lacteals and transported through lymphatic
system to left subclavian vein
LIPIDS in Digestion
Figure 16-18b Chemical Events
REGION
Lingual
lipase
ORAL CAVITY
ESOPHAGUS
STOMACH
SMALL INTESTINE
Bile salts
and
pancreatic
lipase
Monoglycerides,
Fatty acids in
micelles
INTESTINAL
MUCOSA
DIFFUSION
Brush border
Monoglycerides,
Fatty acids
Triglycerides
Cell body
Chylomicrons
EXOCYTOSIS
Lacteal
ROUTE TO BLOODSTREAM
Carbohydrates and amino acids
are absorbed and transported by
intestinal capillaries. Lipids form
Chylomicrons
chylomicrons that diffuse into
lacteals and are delivered to the left
subclavian vein by the thoracic duct.
Protein Digestion and Absorption (16-8)
• Complex and time-consuming process
• Initiated by mastication in mouth and exposure to HCl in stomach
• Pepsin (an enzyme in the stomach)
• Breaks proteins into large peptides
• Pancreatic proteolytic enzymes
• Break down polypeptide chains into short peptide chains and
amino acids
• Peptidases on brush border (microvilli in small intestine)
• Complete process by cleaving off individual amino acids
• Amino acids absorbed through facilitated diffusion and cotransport
• Transported into capillaries by diffusion
PROTEINS in Digestion
Figure 16-18c Chemical Events
REGION
ORAL CAVITY
ESOPHAGUS
STOMACH
Pepsin
Polypeptides
SMALL INTESTINE
Trypsin
Chymotrypsin
Elastase
Carboxypeptidase
Short peptides,
Amino acids
INTESTINAL
MUCOSA
Dipeptidases
Brush border
FACILITATED
DIFFUSION AND
COTRANSPORT
Cell body
Amino acids
FACILITATED
DIFFUSION AND
COTRANSPORT
Capillary
ROUTE TO BLOODSTREAM
Carbohydrates and amino acids
are absorbed and transported by
intestinal capillaries. Lipids form
Amino acids
chylomicrons that diffuse into
lacteals and are delivered to the left
subclavian vein by the thoracic duct.
Table 16.3
Digestive System Age-Related Changes (16-9)
• Division rate of epithelial stem cells declines
• More susceptible to damage by abrasion, acids, enzymes
• Peptic ulcers more likely
• Smooth muscle tone decreases
• Slows rate of peristalsis, leads to constipation
• Straining leads to hemorrhoids
• Weakening muscular sphincters can lead to more
frequent “heartburn”
• Cumulative damage becomes apparent
• Tooth loss due to dental caries (“cavities”) or gingivitis
• Cirrhosis or other liver disease
Digestive System Age-Related Changes
cont. (16-9)
• Increase in cancer rate
• Rates of colon and stomach cancers rise with age
• Oral, esophageal, pharyngeal cancers more common in
smokers
• Dehydration
• Osmoreceptor sensitivity declines
• Aging of other systems affects digestive tract
• Loss in bone mass and calcium can lead to tooth loss
• Loss of taste and olfactory sensations changes diets
Digestive System Integration with Other
Systems (16-10)
• Functionally linked to all other systems
• Anatomically connected to nervous,
cardiovascular, endocrine, and lymphatic systems
• Endocrine function producing hormones
• Extensive structural and functional connections to
cardiovascular system