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PowerPoint® Lecture Slide Presentation by Vince Austin
Human Anatomy & Physiology
FIFTH EDITION
Elaine N. Marieb
Chapter 24
The Digestive System
Part A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Digestive System
• Takes in food
• Breaks food down into nutrient
molecules
• Absorbs these molecules into the
blood stream
• Rids the body of indigestible
remains
2
Digestive System: Overview
• The alimentary canal or gastrointestinal
(GI) tract digests and absorbs food
• Alimentary canal – mouth, pharynx,
esophagus, stomach, small intestine,
and large intestine: a long tube
• Accessory digestive organs – teeth,
tongue, gallbladder, salivary glands,
liver, and pancreas; not part of the
tube but help with digestion
3
4
Figure 22.1
Digestion
• Mechanism for nourishing the body
• Most nutrients in food require either
degradation ( break apart) or release prior to
absorption
• Digestion occurs in GI Tract
• Digestion
– Mechanical
• chewing and peristalsis
– Chemical
• enzymes, HCl
• The GI tract is a “disassembly” line
• Nutrients become more available to the body in each step
6
7
Digestive Process
• There are six essential activities:
• Ingestion
• Propulsion
• Mechanical digestion
• Chemical digestion
• Absorption
• Defecation
Figure 22.2
8
Essential Activities of Digestion
• Ingestion – taking food in
• Propulsion – swallowing and peristalsis
• Peristalsis – waves of contraction
and relaxation of muscles in organ
walls, esophagus and intestine
• Mechanical digestion – chewing;
mixing; churning food
9
Movement of digestive materials
• Visceral (organ) smooth muscle shows
rhythmic cycles of activity
• Pacemaker cells
• Peristalsis
• Waves that move a bolus (rounded
mass of food that is swallowed)
• Segmentation
• Churn and fragment a bolus
10
Figure 22.4 Peristalsis
11
Figure 22.4
Essential Activities of Digestion
bolus
12
Figure 22.3a, b
Essential Activities of Digestion
• Chemical digestion – catabolic
breakdown of food
• Absorption – movement of nutrients
from GI tract to blood or lymph
• Defecation – elimination of indigestible
solid wastes
13
14
Figure 22.2
Figure 22.2
GI Tract
• External environment for the digestive
process; open to outside at both ends
• Regulation of digestion involves:
• Mechanical and chemical stimuli
• Extrinsic control by CNS centers
• Intrinsic control by local centers
15
Receptors of the GI Tract
• Mechano- and chemoreceptors respond to:
• Stretch by food in lumen
• Osmolarity(solute concentration) and pH
• Presence of substrate or
• End products of digestion
• They initiate reflexes that:
• Activate or inhibit digestive glands
• Mix lumen contents and move them along
16
Control of the digestive system
• Movement of materials along the digestive
tract is controlled by:
• Neural mechanisms
• Parasympathetic and local reflexes
• Hormonal mechanisms
• Enhance or inhibit smooth muscle
contraction
• Local mechanisms
• Coordinate response to changes in pH or
chemical stimuli
17
Nervous Control of the GI Tract
• Intrinsic controls
• Nerve plexuses near the GI tract initiate short
reflexes
• Short reflexes are mediated by local enteric plexuses
(gut brain)
• Extrinsic controls
• Long reflexes arising within or outside the GI tract
• Involve CNS centers and extrinsic autonomic nerves
18
Nervous Control of the GI Tract
19
Figure 22.4
Figure 22.5 The Regulation of Digestive
Activities
20
Figure 22.5
Digestive System Organs and Peritoneum
• Peritoneum –
serous
membrane of the
abdominal cavity
• Visceral –
covers external
surface of
most digestive
organs
• Parietal – lines
the body wall
21
Figure 22.5a
Digestive System Organs and Peritoneum
• Peritoneal cavity
• Lubricates
digestive
organs
• Allows them
to slide across
one another
22
Figure 22.5a
Digestive System Organs and Peritoneum
• Mesentery – double layer of
peritoneum
• Vascular(blood) and nerve
supplies to the viscera
• A means to hold digestive
organs in place and store
fat
• Retroperitoneal organs –
organs outside the
peritoneum
• Peritoneal organs
(intraperitoneal) – organs
surrounded by peritoneum
23
Figure 22.5b
24
The digestive system organs and the
peritoneum
• Mesenteries
• Sheets of serous membranes that support
portions of the digestive tract
• Greater omentum lies anterior to abdominal
viscera
• Provides padding, protection, insulation, and
energy reserves
• Lesser omentum
25
26
Figure 24.2 Mesenteries
27
Figure 24.2b
Figure 24.2 Mesenteries
28
Figure 24.2c
Figure 24.2 Mesenteries
29
Figure 24.2d
PowerPoint® Lecture Slide Presentation by Vince Austin
Human Anatomy & Physiology
FIFTH EDITION
Elaine N. Marieb
Chapter 24
The Digestive System
Part B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Blood Supply: Splanchnic Circulation
• Arteries that branch off abdominal aorata and the
organs they serve include
• The hepatic: liver, splenic:spleen, and left gastric:
stomach
• Inferior and superior mesenteric: small and large
intestines
• Hepatic portal circulation:
• Collects nutrient-rich venous blood from the
digestive viscera (organs)
• Delivers it to the liver for metabolic processing and
storage
31
Histology of the Alimentary Canal
• From esophagus to the anal canal the walls of
the GI tract have the same four tunics
• From the lumen outward they are the:
• mucosa
• submucosa
• muscularis externa
• and serosa
• Note: serosa is technically not present on the
esophagus
• Each tunic has a predominant tissue type and
specific digestive function
32
Histology of the Alimentary Canal
33
Figure 24.6
Figure 24.3 The Structure of the Digestive Tract
34
Figure 24.3
Mucosa
• Moist epithelial layer -- lines the lumen of the
alimentary canal
• Its three major functions are:
• Secretion of mucus
• Absorption of the end products of digestion
• Protection against infectious disease
• Consists of three layers:
• lining epithelium
• lamina propria
• muscularis mucosae
35
Mucosa: Epithelial Lining
• Consists of simple columnar epithelium and
mucus-secreting goblet cells
• The mucus secretions:
• Protect digestive organs from digesting
themselves
• Ease food along tract
• Stomach and small intestine mucosa contain:
• Enzyme-secreting cells
• Hormone-secreting cells
36
Mucosa: Lamina Propria and Muscularis
Mucosae
• Lamina Propria
• Loose areolar and reticular connective tissue
• Nourishes the epithelium and absorbs nutrients
• Contains lymph nodes (part of MALT-Mucosa
Associated Lymphatic Tissue) important in
defense against bacteria
• Muscularis mucosae– smooth muscle cells that
produce local movements of mucosa
37
Mucosa: Other Sublayers
• Submucosa – dense connective tissue containing:
• elastic fibers
• blood and lymphatic vessels
• lymph nodes
• nerves
• Muscularis externa – responsible for segmentation
and peristalsis
• Serosa – the protective visceral peritoneum
• Replaced by the fibrous adventitia in the
esophagus
38
• Retroperitoneal organs have both an adventitia
and serosa
Enteric Nervous System
• Composed of two major intrinsic nerve plexuses
• Submucosal nerve plexus – regulates glands
and smooth muscle in the mucosa
• Myenteric nerve plexus:
• Major nerve supply that controls GI tract mobility
• Segmentation and peristalsis
• Linked to the CNS via long autonomic reflex arc
• Parasympatheitic enhances
• Sympathetic inhibits
39
Enteric nevous system or “gut brain” 100 million
neurons-more than the spinal cord
PowerPoint® Lecture Slide Presentation by Vince Austin
Human Anatomy & Physiology
FIFTH EDITION
Elaine N. Marieb
Chapter 24
The Digestive System
Part C
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Mouth
• Oral or buccal cavity:
• Is bounded by lips, cheeks, palate, and tongue
• Has the oral orifice as its anterior opening
• Is continuous with the oropharynx posteriorly
• To withstand abrasions:
• The mouth is lined with stratified squamous
epithelium
• The gums, hard palate, and dorsum of the
tongue are slightly keratinized
41
Mouth
42
Figure 24.7a
Lips and Cheeks
• Have a core of skeletal muscles
• Lips: orbicularis oris
• Cheeks: buccinators
• Vestibule – bounded by the lips and cheeks
externally and teeth and gums internally
• Oral cavity proper– area that lies within the
teeth and gums
• Labial frenulum – median fold that joins the
internal aspect of each lip to the gum
43
Figure 24.7b
Lips and Cheeks
44
Figure 24.7b
Palate
• Hard palate – underlain by palatine bones and
palatine processes of the maxillae
• Assists the tongue in chewing
• Slightly corrugated on either side of the raphe
(midline ridge)
• Soft palate – mobile fold formed mostly of
skeletal muscle
• Closes off the nasopharynx during swallowing
• Uvula projects downward from its free edge
• Palatoglossal and palatopharyngeal arches form
the borders of the fauces
45
Cleft palate
Tongue
• Occupies the floor of the mouth and fills the oral
cavity when mouth is closed
• Functions include:
• Gripping and repositioning food during chewing
• Mixing food with saliva and forming the bolus
• Initiation of swallowing, and speech
• Intrinsic muscles change the shape of the
tongue
• Extrinsic muscles alter the tongue’s position
• Lingual frenulum secures the tongue to the floor
of the mouth
47
Homeostatic Imbalance
• Ankyloglossia – congenital situation where the
lingual frenulum is extremely short
• Commonly referred to as being “tongue-tied”
• Corrected surgically by cutting the frenulum
48
Tongue
• Superior surface bears three types of papillae
• Filiform – give the tongue roughness and
provide friction
• Fungiform – scattered widely over the tongue
and give it a reddish hue
• Circumvallate – V-shaped row in back of tongue
• Sulcus terminalis – groove that separates the
tongue into two areas:
• Anterior 2/3 residing in the oral cavity
• Posterior third residing in the oropharynx
49
Tongue
50
Figure 24.8a
Taste and taste buds
• Most of the 10,000 or so taste buds are found
on the tongue
• Taste buds are found in papillae of the tongue
mucosa
• Fungiform and circumvallate papillae contain
taste buds
Five taste sensations
• There are five basic taste sensations
• Sweet – sugars, saccharin, alcohol, and some
amino acids
• Salt – metal ions
• Sour – hydrogen ions
• Bitter – alkaloids such as quinine and nicotine
• Umami – elicited by the amino acid glutamate
• Taste is 80% smell
Salivary Glands
• Produce and secrete saliva that:
• Cleanses the mouth
• Moistens and dissolves food chemicals
• Aids in bolus formation
• Contains enzymes that breakdown starch
• Three pairs of extrinsic glands –
• parotid
• submandibular
• sublingual
• Intrinsic salivary glands (buccal glands) –
scattered throughout the oral mucosa
54
Salivary Glands
• Parotid – lies anterior to the ear between the
masseter muscle and skin
• Parotid duct – opens into the vestibule next to
the second upper molar
• Submandibular – lies along the medial aspect of
the mandibular body
• Its ducts open at the base of the lingual
frenulum
• Sublingual – lies anterior to the submandibular
gland under the tongue
• It opens via 10-12 ducts into the floor of the
mouth
55
Salivary Glands II
56
Figure 24.9a
Homeostatic Imbalance
• Mumps – inflammation of the parotid glands
caused by myxovirus
• Signs and symptoms – moderate fever and pain
in swallowing acidic foods
• In adult males, mumps carries 25% risk that
testes may become infected, leading to sterility
57
58
Saliva: Source and Composition
• Secreted from serous and mucous cells of
salivary glands
• A 97-99.5% water, hypo-osmotic, slightly acidic
solution containing
• Electrolytes – Na+, K+, Cl–, PO42–, HCO3–
• Digestive enzyme – salivary amylase
• Proteins – mucin, lysozyme, defensins, and IgA
• Metabolic wastes – urea and uric acid
59
Control of Salivation
• Intrinsic glands keep the mouth moist
• Extrinsic salivary glands secrete serous, enzymerich saliva in response to:
• Ingested food which stimulates chemoreceptors
and pressoreceptors
• The thought of food
• Strong sympathetic stimulation inhibits salivation
and results in dry mouth
60
Teeth
• Primary and permanent dentitions have formed
by age 21
• Primary – 20 deciduous teeth that erupt at
intervals between 6 and 24 months
• Permanent – enlarge and develop causing the
root of deciduous teeth to be resorbed and fall
out between the ages of 6 and 12 years
• All but the third molars have erupted by the end
of adolescence
• There are usually 32 permanent teeth
61
Teeth
62
Figure 24.10.2
Classification of Teeth
• Teeth are classified according to their shape and
function
• Incisors – chisel-shaped teeth adapted for
cutting or nipping
• Canines – conical or fanglike teeth that tear or
pierce
• Premolars (bicuspids) and molars – have broad
crowns with rounded tips and are best suited
for grinding or crushing
• During chewing, upper and lower molars lock
together generating crushing force = 170N
63
Tooth Structure
Two main regions – crown and the root
• Crown – exposed part of the tooth above the
gingiva (gum) encapsulaed by enamel
• Enamel – acelluar, brittle material composed of
calcium salts and hydroxyapatite crystals;the
hardest substance in the body
• Root – portion of the tooth embedded in the
jawbone
64
Tooth Structure
• Neck – constriction where the crown and root
come together
• Cementum – calcified connective tissue
• Covers the root
• Attaches it to the periodontal ligament
• Periodontal ligament
• Anchors the tooth in the alveolus of the jaw
• Forms the fibrous joint called a gomaphosis
• Gingival sulcus – depression where the gingival
borders the tooth
65
Tooth Structure
• Dentin – bonelike material deep to the enamel
cap that forms the bulk of the tooth
• Pulp cavity – cavity surrounded by dentin that
contains pulp
• Pulp – connective tissue, blood vessels, and
nerves
• Root canal – portion of the pulp cavity that
extends into the root
• Apical foramen – proximal opening to the root
canal
• Odontoblasts – secrete and maintain dentin
throughout life
66
Tooth Structure
67
Figure 24.11
Homeostatic Imbalance
• Root canal therapy – blows to the teeth can
cause swelling and consequently pinch off the
blood supply to the tooth. The nerve dies and
may become infected with bacteria
• the cavity is sterilized and filled with an inert
material
• The tooth is capped
68
Tooth and Gum Disease
• Dental caries or cavities – gradual
demineralization of enamel and dentin by
bacterial action
• Dental plaque, a film of sugar, bacteria, and
mouth debris, adheres to teeth
• Acid produced by the bacteria in the plaque
dissolves calcium salts
• Without these salts, organic matter is digested
by proteolytic enzymes
• Daily flossing and brushing help prevent caries
by removing forming plaque
69
Dental Caries
70
Tooth and Gum Disease: Periodontitis
• Gingivitis –plaque accumulates, calcifies and
forms calculus, or tartar
• Accumulation of tartar:
• Disrupts the seal between the gingivae and the teeth
• Puts the gums at risk for infection
• Periodontitis – serious gum disease resulting
from an immune response
• Attack of the immune system against intruders:
• Also carves pockets around the teeth and
• Dissolves bone away
71
• Can cause heart problems including heart
attack!!
Periodontitis
72
Pharynx
• From the mouth, the oro- and laryngopharynx
allow passage of:
• Food and fluids to the esophagus
• Air to the trachea
• Lined with stratified squamous epithelium and
mucus glands
• Has two skeletal muscle layers
• Inner longitudinal
• Outer pharyngeal constrictors
73
Esophagus
• Muscular tube going from the laryngopharynx to
the stomach
• Travels through the mediastinum and pierces the
diaphragm
• Joins the stomach at the cardiac orifice
74
Homeostatic Imbalance
• Heartburn (gastroesophageal reflux disease or
GERD) – burning, radiating substernal pain
caused by acidic gastric juice regurgitated into
the esophagus
• Caused by excess eating or drinking, and
conditions that force abdominal contents
superiorly (e.g., extreme obesity, pregnancy,
and running)
• Hiatus hernia – structural abnormality in which
the superior part of the stomach protrudes
slightly above the diaphragm
• Prolonged episodes can lead to esophagitis,
ulcers, and cancer
75
Esophageal Characteristics
• Esophageal mucosa – nonkeratinized stratified
squamous epithelium
• The empty esophagus is folded longitudinally and
flattens when food is present
• Glands secrete mucus as a bolus moves through the
esophagus
• Muscularis changes from skeletal (superiorly) to
smooth muscle (inferiorly)
76
Digestive Processes in the Mouth
• Food is ingested
• Mechanical digestion begins (chewing) or mastication
• Propulsion is initiated by swallowing
• Salivary amylase begins chemical breakdown of
starch
• The pharynx and esophagus serve as conduits to pass
food from the mouth to the stomach
77
Deglutition (Swallowing)
• Involves the coordinated activity of the tongue, soft
palate, pharynx, esophagus and 22 separate muscle
groups
• Buccal phase – bolus is forced into the oropharynx
• Pharyngeal-esophageal phase – controlled by the
medulla and lower pons
• All routes except into the digestive tract are sealed
off
• Peristalsis moves food through the pharynx to the
esophagus
78
Deglutition (Swallowing)
79
Figure 24.13a-c
Deglutition (Swallowing)
80
Figure 24.13d, e
Stomach
81
Figure 24.14a
Digestion in the Stomach
• The stomach:
• Holds ingested food
• Degrades it both physically and chemically
• Chemical breakdown of proteins begins and food is
converted to chyme
• Delivers chyme to the small intestine
• Enzymatically digests proteins with pepsin
• Secretes intrinsic factor required for absorption of
vitamin B12
82
Microscopic Anatomy of the Stomach
• Muscularis – has an additional oblique layer that
• Allows the stomach to churn, mix and pummel food
physically
• Breaks down food into smaller fragments
• Epithelial lining is composed of:
• Goblet cells that produce a coat of alkaline mucus
• Gastric pits containing gastric glands that secrete:
• Gastric juice
• Mucus
• Gastrin
83
Glands of the Stomach Fundus and Body
• Gastric glands of the fundus and body have a variety
of secretory cells
• Mucous neck cells – secrete acid mucus
• Parietal (oxyntic) cells – secrete HCl and intrinsic
factor
• Chief (zymogenic) cells – produce pepsinogen
• Pepsinogen is activated to pepsin by:
– HCl in the stomach
– Pepsin itself by a positive feedback mechanism
• Enteroendocrine cells – secrete gastrin, histamine,
endorphins, serotonin, cholecystokinin (CCK), and
somatostatin into the lamina propria
84
85
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
86
Regulation of Gastric Secretion
• Neural and hormonal mechanisms regulate the release
of gastric juice
• Stimulatory and inhibitory events occur in three
phases
• Cephalic (reflex) phase: prior to food entry
• Gastric phase: once food enters the stomach
• Intestinal phase: as partially digested food enters the
duodenum
87
Cephalic Phase
• Excitatory events include:
• Sight or thought of food
• Stimulation of taste or smell receptors
• Inhibitory events include:
• Loss of appetite or depression
• Decrease in stimulation of the parasympathetic
division
88
Gastric Phase
• Excitatory events include:
• Stomach distension
• Activation of stretch receptors (neural activation)
• Activation of chemoreceptors by peptides, caffeine,
and rising pH
• Release of gastrin to the blood
• Inhibitory events include:
• A pH lower than 2
• Emotional upset which overrides the
parasympathetic division
89
Intestinal Phase
• Excitatory phase – low pH and partially digested food
enters the duodenum
• Inhibitory phase – distension of duodenum, presence
of fatty, acidic, or hypertonic chyme, and/or irritants
in the duodenum
• Closes the pyloric sphincter
• Releases enterogastrones that inhibit gastric secretion
90
Release of Gastric Juice
91
Figure 24.16
92
93
94
Regulation and Mechanism of HCl Secretion
• HCl secretion is stimulated by ACh, histamine, and
gastrin
• Release of hydrochloric acid:
• Is low if only one ligand binds to parietal cells
• Is prolific if all three ligands bind to parietal cells
• Antihistamines and cimetidine (Tagamet) block H2
receptors and decrease HCl release; proton pump
inhibitors like Prilosec interupt HCl production at
pump and stop production.
95
Regulation and Mechanism of HCl Secretion
96
Figure 24.17
Response of the Stomach to Filling
• Stomach pressure remains constant until about 1L of
food is ingested
• Relative unchanging pressure results from reflexmediated relaxation and plasticity
• Reflex-mediated events include:
• Receptive relaxation – as food travels in the
esophagus, stomach muscles relax
• Adaptive relaxation – the stomach dilates in response
to gastric filling
• Plasticity – intrinsic ability of smooth muscle to
exhibit the stress-relaxation response
97
Gastric Contractile Activity
• Peristaltic waves move toward the pylorus at the rate
of 3 per minute
• This basic electrical rhythm (BER) is initiated by
pacemaker cells (cells of Cajal)
• Most vigorous peristalsis and mixing occurs near the
pylorus
• Chyme is either:
• Delivered in small amounts to the duodenum or
• Forced backward into the stomach for further mixing
98
Gastric Contractile Activity
99
Figure 24.18
Regulation of Gastric Emptying
• Gastric emptying is regulated by:
• The neural enterogastric reflex
• Hormonal (enterogastrone) mechanisms
• These mechanisms inhibit gastric secretion and
duodenal filling
• Carbohydrate-rich chyme moves through the
duodenum quickly
• Fat-laden chyme is digested more slowly causing
food to remain in the stomach longer
100
1
Regulation of Gastric Emptying
Figure 24.19
Homeostatic Imbalance
• Vomiting (emesis) – the stomach empties via a
different route (oral)
• Causes include extreme stretching, irritants such as
bacterial toxins, excessive alcohol, spicy foods, and
certain drugs
• The emetic center of the medulla initiates a number of
motor responses
• Diaphragm and abdominal wall muscle contract
• Cardiac sphincter relaxes and soft palate closes off
the nasal passages
• Excessive vomiting can cause dehydration and upset
electrolyte and pH balance
102
Small Intestine: Gross Anatomy
• Runs from pyloric sphincter to the ileocecal valve
• Has three subdivisions: duodenum, jejunum, and
ileum
• The 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
103
104
Microscopic Anatomy of the Small Intestine
• Structural modifications of the small intestine wall
increase surface area
• Plicae circulares: deep circular folds of the mucosa
and submucosa
• Villi: fingerlike extensions of the mucosa
• Microvilli: tiny projections of absorptive mucosal
cells’ plasma membranes
105
Microscopic Anatomy of the Small Intestine
106
Figure 24.21a-c
Small Intestine: Histology of the Wall
• The epithelium of the mucosa is made up of:
• Absorptive cells and goblet cells
• Interspersed T cells (intraepithelial lymphocytes),
and
• Enteroendocrine cells
• Intestinal crypts cells secrete intestinal juice
• Peyer’s patches are found in the submucosa
• lymphoid tissue; fights infection causing bacteria
• Brunner’s glands in the duodenum secrete alkaline
mucus
107
Intestinal Juice
• Secreted by intestine glands in response to distension
or irritation of the mucosa
• It is slightly alkaline and isotonic with blood plasma
• Is largely water, enzyme-poor, but contains mucus
108
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:
• Is a remnant of the fetal umbilical vein
• Runs along the free edge of the falciform ligament
109
Liver: Associated Structures
• The lesser omentum anchors the liver to the stomach
• The hepatic blood vessels enter the liver at the porta
hepatis
• 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 fuses with the cystic duct
• These two ducts form the bile duct
110
Liver: Associated Structures
111
Figure 24.20
The Liver
112
p.286
Microscopic Anatomy of the Liver
• 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
• a bile duct and
• Hepatic artery – supplies oxygen-rich blood to the
liver
• Hepatic portal vein – carries venous blood with
nutrients from digestive viscera
113
Microscopic Anatomy of the Liver
114
Figure 24.24c, d
Microscopic Anatomy of the Liver
• Liver sinusoids – enlarged, leaky capillaries located
between hepatic plates
• Kupffer cells – hepatic macrophages found in liver
sinusoids
• Hepatocytes’(liver cells) functions include:
• Production of bile
• Processing blood borne nutrients
• Storage of fat-soluble vitamins
• Detoxification
• Secreted bile flows between hepatocytes toward the
bile ducts in the portal triads
115
Microscopic Anatomy of the Liver
Figure 24.24c, d
116
Homeostatic Imbalance
• Hepatitis – inflammation of the liver often due to viral
infection
• Viruses causing hepatitis are catalogued has HVA
through HVF
• HVA and HVE are transmitted enterically and cause
self-limiting infections
• Hepatitis B is transmitted via blood transfusions,
contaminated needles, and sexual contact, and
increases the risk of liver cancer
• Hepatitis C produces chronic liver infection
• Nonviral hepatitis is caused by drug toxicity and wild
mushroom poisoning
117
Homeostatic Imbalance
• Cirrhosis – diffuse and progressive chronic
inflammation of the liver
• Typically results from chronic alcoholism or severe
chronic hepatitis
• The liver becomes fatty and fibrous and its activity is
depressed
• Scar tissue obstructs blood flow in the hepatic portal
system causing portal hypertension
118
Composition of Bile
• A yellow-green, alkaline solution containing bile
salts, bile pigments, cholesterol, neutral fats,
phospholipids, and electrolytes
• Bile salts are cholesterol derivatives that:
• Emulsify fat
• Facilitate fat and cholesterol absorption
• Help solubilize cholesterol
• Enterohepatic circulation recycles bile salts
• The chief bile pigment is bilirubin, a waste product of
heme
119
The Gallbladder
• Thin-walled, green 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
120
Regulation of Bile Release
• Acidic, fatty chyme causes the duodenum to release:
• Cholecystokinin (CCK) and secretin into the
bloodstream
• Bile salts and secretin transported in blood stimulate
the liver to produce bile
• Vagal stimulation causes weak contractions of the
gallbladder
• Cholecystokinin causes:
• The gallbladder to contract
• The hepatopancreatic sphincter to relax
• As a result, bile enters the duodenum
121
Regulation of Bile Release
122
Figure 24.25
Homeostatic Imbalance
• Gallstones – crystallization of cholesterol which can
obstruct the flow of bile
• Current treatments include: dissolving the crystals
with drugs, pulverizing them with ultrasound,
vaporizing them with lasers, and surgical removal of
the gallbladder
• cholecystectomy : surgery to remove gall bladder
• Obstructive jaundice – yellowish skin caused by bile
pigments deposited in the skin
• Due to blocked bile ducts
123
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
• Secretes pancreatic juice which breaks down all
categories of foodstuff
• Acini (clusters of secretory cells) contain zymogen
granules with digestive enzymes
• The pancreas also has an endocrine function – release
of insulin and glucagon
124
Pancreas
125
Figure 24.26a
Composition and Function of Pancreatic Juice
• Water solution of enzymes and electrolytes (primarily
HCO3)
• Neutralizes acid chyme
• Provides optimal environment for pancreatic
enzymes
• Enzymes are released in inactive form and activated
in the duodenum
126
Composition and Function of Pancreatic Juice
• Examples include
• Trypsinogen is activated to trypsin
• Procarboxypeptidase is activated to
carboxypeptidase
• Active enzymes secreted
• Amylase, lipases, and nucleases
• These enzymes require ions or bile for optimal
activity
127
Regulation of Pancreatic Secretion
• Secretin and CCK are released when fatty or acidic
chyme enters the duodenum
• CCK and secretin enter the bloodstream
• Upon reaching the pancreas:
• CCK induces the secretion of enzyme-rich pancreatic
juice
• Secretin causes secretion of bicarbonate-rich
pancreatic juice
• Vagal stimulation also causes release of pancreatic
juice
128
Regulation of Pancreatic Secretion
129
Figure 24.28
Digestion in the Small Intestine
• As chyme enters the duodenum
• Carbohydrates and proteins are only partially
digested
• No fat digestion has taken place
• Digestion continues in the small intestine
• Chyme is released slowly into the duodenum
• Because it is hypertonic and has low pH, mixing is
required for proper digestion
• Required substances needed are supplied by the liver
• Virtually all nutrient absorption takes place in the
small intestine
130
• Incorporated in the plasma membrane of the microvilli are a
number of enzymes that complete digestion:
• aminopeptidases attack the amino terminal (N-terminal) of
peptides producing amino acids.
• disaccharidases These enzymes convert disaccharides into their
monosaccharide subunits.
• maltase hydrolyzes maltose into glucose.
• sucrase hydrolyzes sucrose (common table sugar) into glucose
and fructose.
• lactase hydrolyzes lactose (milk sugar) into glucose and
galactose.
• Fructose simply diffuses into the villi, but both glucose and
galactose are absorbed by active transport.
• Fatty acids and monoglycerides. These become resynthesized into
fats as they enter the cells of the villus. The resulting small droplets
of fat are then discharged by exocytosis into the lymph vessels,
called lacteals, draining the villi.
131
Motility of the Small Intestine
• The most common motion of the small intestine is
segmentation
• It is initiated by intrinsic pacemaker cells (Cajal
cells)
• Moves contents steadily toward the ileocecal valve
• After nutrients have been absorbed:
• Peristalsis begins with each wave starting distal to
the previous
• Meal remnants, bacteria, mucosal cells, and debris
are moved into the large intestine
132
Control of Motility
• Local enteric neurons of the GI tract coordinate
intestinal motility
• Cholinergic neurons cause:
• Contraction and shortening of the circular muscle
layer
• Shortening of longitudinal muscle
• Distension of the intestine
• Other impulses relax the circular muscle
• The gastroileal reflex and gastrin:
• Relax the ileocecal sphincter
• Allow chyme to pass into the large intestine
133
Large Intestine
• 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
• Is subdivided into the cecum, appendix, colon,
rectum, and anal canal
• The saclike cecum:
• Lies below the ileocecal valve in the right iliac fossa
• Contains a wormlike vermiform appendix
134
Large Intestine
135
Figure 24.29a
The Large Intestine
136
Silv. p.686
Homeostatic Imbalance
• Appendicitis – inflammation of the appendix resulting
from blockage that traps infectious bacteria in its
lumen
• If the appendix ruptures, feces containing bacteria
spray over the abdominal contents causing peritonitis
• Treatment is surgical removal of the appendix
137
PowerPoint® Lecture Slide Presentation by Vince Austin
Human Anatomy & Physiology
FIFTH EDITION
Elaine N. Marieb
Chapter 24
The Digestive System
Part F
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Colon
• Has distinct regions: ascending colon, hepatic
flexure, transverse colon, splenic flexure, descending
colon, and sigmoid colon
• The transverse and sigmoid portions are anchored via
mesenteries called mesocolons
• The sigmoid colon joins the rectum
• The anal canal, the last segment of the large intestine,
opens to the exterior at the anus
139
Valves and Sphincters of the Rectum and Anus
• Three valves of the rectum stop feces from being
passed with gas
• The anus has two sphincters:
• Internal anal sphincter composed of smooth muscle
• External anal sphincter composed of skeletal muscle
• These sphincters are closed except during defecation
140
Mesenteries of Digestive Organs
141
Figure 24.30b
Mesenteries of Digestive Organs
142
Figure 24.30c
Mesenteries of Digestive Organs
143
Figure 24.30d
Large Intestine: Microscopic Anatomy
• Colon mucosa is simple columnar epithelium except
in the anal canal
• Has numerous deep crypts lined with goblet cells
• Anal canal mucosa is stratified squamous epithelium
• Anal sinuses exude mucus and compress feces
• Superficial venous plexuses are associated with the
anal canal
• Inflammation of these veins results in itchy
varicosities called hemorrhoids
144
Mechanical Digestion
• Haustral contractions
• propel and mix from pouch to pouch
145
146
147
Large Intestine: Microscopic Anatomy
148
Figure 24.29b
Bacterial Flora
• The bacterial flora of the large intestine consist of:
• Bacteria surviving the small intestine that enter the
cecum and
• Those entering via the anus
• These bacteria:
• Colonize the colon
• Ferment indigestible carbohydrates
• Release irritating acids and gases (flatus)
• Synthesize B complex vitamins and vitamin K
149
Functions of the Large Intestine
• Other than digestion of enteric bacteria, no further
digestion takes place
• Vitamins, water, and electrolytes are reclaimed
• Its major function is propulsion of fecal material
toward the anus
• Though essential for comfort, the colon is not
essential for life
150
Motility of the Large Intestine
• Haustral contractions
• Slow segmenting movements that move the contents
of the colon
• Haustra sequentially contract as they are stimulated
by distension
• Presence of food in the stomach:
• Activates the gastrocolic reflex
• Initiates peristalsis that forces contents toward the
rectum
151
Homeostatic Imbalance
• Diverticulosis – small herniation (diverticula) of the
mucosa of the colon walls caused by lack of bulk in
the colon
• Most common in the sigmoid colon in people over 70
• Diverticulitis – inflamed diverticula that can be life
threatening if the diverticula rupture
152
Figure 24.23 The Large Intestine
153
Figure 24.23b, c
Defecation
• Distension of rectal
walls caused by
feces
• Stimulates
contraction of
the rectal walls
• Relaxes the
internal anal
sphincter
• Voluntary signals
stimulate
relaxation of the
external anal
sphincter and
154
defecation occurs
Figure 24.32
The
Defecation
Reflex
155
Figure 24.25
Figure 24.27 Digestive Secretion and
Absorption of Water
156
Figure 24.27
Homeostatic Imbalance
• Diarrhea – watery stool resulting from any condition
that rushes food residue through the large intestine too
quickly
• This causes insufficient time for water absorption
• Prolonged diarrhea may result in dehydration and
electrolyte imbalance
• Constipation – hard stool that is difficult to pass
resulting from residues staying in the intestine too
long
• May result from lack of fiber in the diet
157
Food Poisoning: Salmonella
• Salmonella is spread by:
• Contaminated eggs and egg products
• Infected food handlers with feces-contaminated
hands
• Salmonella can cause:
• Bacteremia 4 to 7 days after infection
• Endocarditis, thrombi, bone infections, arthritis, and
meningitis
• Diagnosis is by positive stool samples
• Salmonellosis is treated symptomatically
158
Chemical Digestion: Carbohydrates
• Absorption: via cotransport with Na+, and facilitated
diffusion
• Enter the capillary bed in the villi
• Transported to the liver via the hepatic portal vein
• Enzymes used: salivary amylase, pancreatic amylase,
and brush border enzymes
159
Chemical Digestion: Proteins
• Absorption: similar to carbohydrates
• Enzymes used: pepsin in the stomach
• Enzymes acting in the small intestine
• Pancreatic enzymes – trypsin, chymotrypsin, and
carboxypeptidase
• Brush border enzymes – aminopeptidases,
carboxypeptidases, and dipeptidases
160
Chemical Digestion: Proteins
161
Figure 24.34
Chemical Digestion: Fats
• Absorption: Diffusion into intestinal cells where they
• Combine with proteins and extrude chylomicrons
• Enter lacteals and are transported to systemic
circulation via lymph
• Glycerol and short chain fatty acids
• are absorbed into the capillary blood in villi
• transported via the hepatic portal vein
• Enzymes/chemicals used: bile salts and pancreatic
lipase
162
Chemical Digestion: Fats
163
Figure 24.35
Fatty Acid Absorption
• Fatty acids and monoglycerides enter intestinal cells
via diffusion
• They are combined with proteins within the cells
• Resulting chylomicrons are extruded
• They enter lacteals and are transported to the
circulation via lymph
164
Fatty Acid Absorption
165
Figure 24.36
Chemical Digestion: Nucleic Acids
• Absorption: active transport via membrane carriers
• Absorbed in villi and transported to liver via hepatic
portal vein
• Enzymes used: pancreatic ribonucleases and
deoxyribonuclease in the small intestines
166
Electrolyte Absorption
• Most ions are actively absorbed along the length of
small intestine
• Na+ is coupled with absorption of glucose and amino
acids
• Ionic iron is transported into mucosal cells where it
binds to ferritin
• Anions passively follow the electrical potential
established by Na+
167
Electrolyte Absorption
• K+ diffuses across the intestinal mucosa in response to
osmotic gradients
• Ca2+ absorption:
• Is related to blood levels of ionic calcium
• Is regulated by Vitamin D and parathyroid hormone
(PTH)
168
Water Absorption
• 95% of water is absorbed in the small intestines by
osmosis
• Water moves in both directions across intestinal
mucosa
• Net osmosis occurs whenever a concentration
gradient is established by active transport of solutes
into the mucosal cells
• Water uptake is coupled with solute uptake, and as
water moves into mucosal cells, substances follow
along their concentration gradients
169
Malabsorption of Nutrients
• Results from anything that interferes with delivery of
bile or pancreatic juice
• Factors that damage the intestinal mucosa (e.g.,
bacterial infection)
• Gluten enteropathy (adult celiac disease) – gluten
damages the intestinal villi and reduces the length of
microvilli
• Treated by eliminating gluten from the diet (all
grains but rice and corn)
170
Embryonic Development of the Digestive System
• 3rd week – endoderm has folded and foregut and
hindgut have formed
• The midgut is open and continuous with the yolk sac
• Mouth and anal openings are nearly formed
• 8th week – accessory organs are budding from
endoderm
171
Embryonic Development of the Digestive System
172
Figure 24.37
Homeostatic Imbalance
• Peritonitis – inflammation of the peritoneum caused
by a piercing wound, perforating ulcer, or burst
appendix
• Often, infected membranes tend to stick together
localizing the infection
• Generalized or widespread peritonitis is dangerous
and often lethal
• Treatment includes removing infectious debris and
massive doses of antibiotics
173
Homeostatic Imbalance
• Cleft palate – palatine bones, palatine process of the
maxillae (or both) fail to fuse
• Tracheoesophageal fistula – opening between the
esophagus and trachea
• Cystic fibrosis – impairs pancreatic activity
174
Developmental Aspects
• During fetal life, nutrition is via the placenta, but the
GI tract is stimulated toward maturity by amniotic
fluid swallowed in utero
• At birth, feeding is an infant’s most important
function and is enhanced by
• Rooting reflex (helps infant find the nipple) and
sucking reflex (aid in swallowing)
• Digestive system has few problems until the onset of
old age
• During old age the GI tract activity declines,
absorption is less efficient, and peristalsis is slowed
175
176
Overview of GI Tract
• Upper GI Tract
– Mouth and pharnyx provide entryway
– Lead to Esophagus
• Esophagus wall consists of same layers found in remainder
of GI tract
– epithelium (mucosa), submucosa (submucosal plexus), muscularis externa
(circular and longitudinal muscles + myenteric plexus); serosa
– Secretory glands found throughout GI except colon
• exocrine glands (ducted) and endocrine (ductless) glands
Upper GI Tract Continued
• Sphincters
– circular muscles located throughout digestive tract
• Lower Esophageal Sphincter
– allows passage of food into stomach
• Pyloric Sphincter
– Controls release of chyme from stomach to duodenum
• Ileocecal sphincter
• Stomach
– J shaped organ
– volume ranges from 50 to 1500 mL (2 -52 oz)
Lower GI tract and Accessory
Organs
• Small Intestine
– Duodenum (< 1 ft), jejunum and ileum (~9 ft)
– main site for nutrient digestion and absorption
– duodenum receives secretions from liver, gallbladder
and pancreas
• Liver
– hepatocytes make bile from cholesterol
– right and left hepatic bile ducts join to form common
hepatic duct
• unites with cystic duct leading to gallbladder
Lower GI tract and Accessory
Organs
• Gallbladder
– capacity to hold 40-50 mL of bile
– functions to concentrate and store bile
– Common hepatic bile duct goes to duodenum
(Sphincter of Oddi)
• Pancreas
– 2 types of active tissue
• Acini or ducted exocrine tissue
– produces digestive enzymes
• secreted into small ducts within pancreas which leads
to common hepatic bile duct
• empties into duodenum via Sphincter of Oddi
• Ductless endocrine tissue
– secretes hormones, glucagon and insulin into blood
Structure of SI
• Epithelial surface (mucosa) structured to maximize
surface area
– Large folds of mucosa (folds of Kerckring)
– Villi
• projections lined with 100s of absorptive cells
• contain blood capillaries and central lacteal
– Microvilla
•
•
•
•
extensions of plasma membrane of absorptive cells
possess surface coat of glycocalyx
forms brush border
most digestive enzymes produced by SI found here.
Structure of SI
• Crypts of Lieberkuhn
– pits located between villi
– epithelial cells in these crypt migrate upward and out
of crypts toward tips of villi
– turnover every 3-5 days
– Cells include
• paneth cells secrete proteins with unknown function
• globlet cells secrete mucus
• Enterochromaffin cells with endocrine functions
Structure of SI
• Gut Associated Lymphoid Tissue (GALT)
– mucosa and submucosa
– Leukocytes (white blood cells)
• Found between intestinal absorptive cells
– T-lymphocytes, Natural Killer cells, Microfold cells
• Peyer’s Patches
– Aggregates of lymphoid tissue underneath M cells
– T-lymphocytes, B-lymphocytes and macrophages
– provide defense against bacteria and foreign bodies
• Ileocecal valve
– allows unabsorbed materials to leave ileum and enter cecum
Colon
• Cecum
• Ascending, transverse and descending
sections
• Sigmoid sections
• Haustra or pouches
– created by contraction of 1 or 3 muscular strips
(tenaie coli) along with contraction of circular
muscles
• Absorbs water and lytes
Regulation Of Digestive
Process
• Regulatory Peptides (GI hormones and neuropeptides)
– Gastrin (hormone)
• produced mainly by gastric (G) cells in stomach (pyloric gland)
• released upon entry of food into stomach
– stimulates HCl release
– gastric and intestinal motility
– pepsinogen release
– Cholecystokinin (CCK, hormone)
• produced by SI cells
• released upon entry of chyme into stomach
– amino acids, fat
• targets pancreas and gallbladder
Regulatory Peptides
• Secretin (hormone)
–
–
–
–
produced by cells of SI
secreted in response to chyme (acid) into duodenum
targets pancreas (HCO-3)
may inhibit GI motility
• Gastric Inhibitory Polypeptide (GIP; hormone)
– produced by SI cells
– Inhibits gastric secretions and motility
– Simulates intestinal and insulin secretions
Regulatory Peptides
• Somatostatin
– Produced by pancreatic and SI cells
– Paracrine
• released by endocrine cells but diffuse through
extracellular space to gastric juice
– Inhibits gastrin secretion
– Increases release of GIP, secretin, motilin
– Inhibits gastric acid release, gastric motility, pancreatic
exocrine secretions and gall bladder secretions
Regulatory Peptides
• Motilin
– Small intestine
– Contraction of intestinal smooth muscle
• Vasoactive intestinal peptide (VIP; Neurocrine)
– Neurons within gut
– Stimulates intestinal secretions, relaxes most GI sphincters,
inhibits gastric acid secretion and stimulates HCO3-
• Gastrin-releasing pepetide (GRP; Neurocrine)
– Released from nerves, stimulates release of HCL, gastrin and
CCK
188
Neural Regulation
• Enteric Nervous System
– located in wall of GI tract
– begins in esophagus and ends at anus
– controls peristaltic activity/GI motility (myenteric plexus)
• affected by parasympathetic and sympathetic nervous
systems
– Acetylcholine increase GI motility
– Norepinephrine and epinephrine inhibit GI activity
– Also influences GI secretions (submucosal plexus)
Process of Digestion
• Oral Cavity and Salivary Glands
– mastication of food
– mixed with salivary gland secretions
• parotid glands
– water, lytes and enzymes
• submandibular and sublingual glands
– water, lytes, enzymes and mucus
– Enzymes
• alpha amylase and lingual lipase
Process of Digestion
• Esophagus
– Entry of food results in peristalsis
• acetylcholine
– LES
• LES pressure decreases upon swallowing
• Neural and hormonal regulation
• functions to prevent gastric reflux
Process of Digestion
• Stomach
– Body extends from LES to angular notch
• includes fundus
• serves as reservoir as well as producer of gastric juice
– Antrum (Pyloric Portion)
• extends from angular notch to duodenum
• grinds and mixes food with digestive juice (chyme)
• provides strong peristalsis
Stomach
• Gastric Juices
– produced by 3 different gastric glands
• Cardiac glands (fundus)
– mucus cells (HCO3- and mucus) and endocrine cells
– no parietal (oxyntic cells)
• Parietal glands (body)
– mucus cells, oxyntic cells (secrete HCl and IF), chief cells
(secrete pepsinogen upon stimulation by acetylcholine)
• Pyloric glands (antrum)
– both mucus and oxyntic cells
– G cells that produce gastrin
– Acetylcholine stimulates chief, oxyntic and
other gastric cells
Gastric Juices
• HCl
–
–
–
–
activates inactive zymogen pepsinogen to active pepsin
denatures proteins
serves as bactericide
nutrient release
• Mucus
– lubricates and protects gastric lining
• Intrinsic Factor (IF)
– necessary for vitamin B12 absorption
HCl
• Release from oxyntic cells stimulated by
– acetylcholine, gastrin, CCK, histamine
– Histamine
• secreted from mast cells
• paracrine that binds to H2 receptors on parietal cells to stimulate HCl release
• mechanism used in drug therapy to lower acid
– Drugs (tagamet) prevent histamine from binding to H2 receptors
– decreases acid release
– Parietal cells contain
• Potassium chloride transport system
• Hydrogen (proton) potassium ATPase exchange system
– Proton pump
• Result: secretion of hydrogen and chloride
• Proton pump targetted/exploited in drug therapy to lower acid
– Prilosec binds to proton pump
Gastric Emptying
• GI hormones and neuropeptides affect a
pacemaker (between fudnus and body of stomach)
– determines frequency and rate of contractions
– 1-5 mL (<1 tsp) chyme allowed to enter duodenum
every 30 seconds
– Fat appears to slow gastric emptying
• Inhibitors
– Secretin, GIP, Somatostatin, CCK
Small Intestine
• Chyme has low pH
– SI protected by pancreatic secretions and secretions
from Brunner’s glands (located in mucosa and submucosa of the first few
centimeters of the duodenum)
– Secretin and CCK
• Digestive juices
– Glands within crypts of Lieberkuhn
– Pancreas
• Responsible for enzymes that digest 50% CHO, 50% protein,
90% lipids
Digestive Juice
• Pancreatic Proteases
– trypsinogen, chymotrypsinogen,
procarboxypeptidases, proelastase, collagenase
– secreted in vesicles
– must be activated
• trypinsogen converted to trypsin by
enteropeptidase (enterokinase) and by free trypsin
– hydrolyze peptide bonds either internally or from
ends
– mono, di and some tri can be absorbed
• Brush border aminopeptidases to hydrolyze
further
Digestive Juice
• CHO
– Pancreatic alpha amylase
• hydrolyzes alpha 1-4 bonds
– Alpha dextrinase to hydrolyze 1-6 branches
– Brush border enzymes (isomaltase, maltase, sucrase,
lactase)
• Lipids
– Pancreatic lipase hydrolyzes TG to yield MG, FFA and glycerol
– Phospholipase and cholesterol esterase
– Bile required and sometimes pancreatic colipase (binds TG and pancreatic
lipase displacing bile)
Bile Synthesis
• Made in liver cells from cholesterol
– cholesterol oxidized
• chenodeoxycholic acid and cholic acid
– conjugated to glycine or taurine
• glycocholic acid and taurocholic acid
– conjugation of bile acids with amino acids
improves its ability to form micelles
– cholesterol and PLs secreted into bile
• bile acid dependent frx
– Water, Lytes and bilirubin secreted in bile
Bile Storage
• Gallbladder
– Concentrates bile by removing 90% of water
– Stores bile
– Stimulated to release bile by CCK
• CCK secreted in response to amino acids and lipids
– Bile is secreted into duodenum
Function of Bile
• Digestion
– Bile acids are amphipathic
• Contain polar (hypdrophic) and non-polar (hydrophobic) areas
– Decrease surface tension of fat
• Hydrophobic part surrounds and dissolves into the ingested fat molecule
and helps break apart
– Permits emulsification of fat
• Increases exposed surface area of lipids
– Allow digestive enzymes (lipase) to get close to fat
• Absorption
– Micelle Formation
Micelles
• Contain ~ 40 bile salt molecules
– Hydrophobic centers, hydrophilic periphery
• Fatty acids, MAGs, cholesterol, fat soluble
vitamins enter micelles
– Mixed Micelles
• Micelles travel to brush border
– Contents diffuse through unstirred water layer and into
enterocytes
– Bile acids released back to lumen for reuse
Enterohepatic Circulation
• > 90% of bile acids secreted in duodenum are reabsorbed
into ileum
• Enters portal vein for transport via blood back to liver
• Reabsorbed bile acids are secreted in bile along with
newly made bile acids
• New bile mixed with recirculated bile is sent via cystic
duct for storage in gallbladder
• Pool of bile (2-4 g) may recycle 1-2 times/meal
Secondary Bile Acids
• Bile acids not reabsorbed in ileum may be deconjugated
by bacteria in colon
– deconjugated bile acids form secondary bile acids
• cholic acid is converted to deoxycholic acid which may be reabsorbed
• chenodeoxycholic acid is converted to lithocholic acid which is excreted
in feces
– ~ 0.5 g of bile salts are lost in feces daily
– Some fibers bind bile salts and acids and prevent conversion to
secondary bile acids (possible risk factor in colon cancer?)
Decreasing Blood Cholesterol
• Only route of cholesterol excretion is in feces
• By increasing excretion of bile in feces, one can
lower blood cholesterol levels
– necessitates use of body cholesterol for synthesis of
new bile acids
– drugs (powdered resins) bind bile and enhance
excretion from body (cholestyramine)
– soluble fiber behaves like resins
Role of Intestinal Brush Border
in Digestion
• CHO Digestion
– Isomaltase
• 1-6 bonds in oligosaccharides and dextrins
– Sucrase
• alpha 1-4 bonds in sucrose and maltose
– Glucoamylase, glucosidase, maltase
• alpha 1-4 bonds in oligosaccharides, maltotriose and maltose
– Maltase, Lactase and Sucrase
Role of Intestinal Brush Border
in Digestion
• Protein Digestion
– Aminopeptidases hydrolyze N terminal amino acids
from oligopeptides, tripeptides and dipeptides
– Tripeptidases = 1 free aa + dipeptide
– Dipeptidases = 2 free aa
Absorption
• Begins in duodenum
• Continues throughout jejunum and ileum
• May be accomplished by
– simple diffusion
– facilitated diffusion
– active transport
– pinocytosis or endocytosis
– paracellular absorption
• Mechanism depends on
– nutrient solubility; electrical gradient; size
Colon
• Greater absorption of sodium, chloride and water
– 90-95% of water and sodium reabsorbed
• Secretion of bicarbonate into colon
– neutralize acids produced by colonic anaerobic
bacteria via action on CHO
• Progressive dehydration of unabsorbed materials
– 1 L of chyme reduced to < 200 mL of defecated
material
Colon
• Bacteria
– synthesize small amounts of vitamin K, biotin and folate
– CHO digestion (fiber)
• Produce acids in colon include short chain fatty acids
– acetate, butyrate (preferred energy source for colon cells; regulate cell growth),
propionate
– may be absorbed by colonocyte
– Protein digestion
• Ammonia (must be controlled in those with liver disease)
• Probiotics and Prebiotics
– Increase populations with no adverse health effects and decrease
populations with adverse health effects (pathogenic strains)
211
212
The Large Intestine
Silv. p.686
213
214
NaCl Absorption by the Colon
Silv. p.687
215
216
Digestive Secretions:
(7 L / Day From Tissues into Lumen)
•
•
•
•
•
•
•
•
Salivary glands
Pancreas
Water
Enzymes
Mucus
Ions: H+, K+, Na+
HCO3-, ClMass Balance (H2O)
217
Enzymatic digestion in the human digestive system
218
Motility: Smooth Muscle
Contractions
• Tonic – support
• Phasic – move
products
– Parastalsis – moves

– Segmentation –
mixes
Figure 21-4: Contractions in the GI tract
219
Digestion
of fats
220
Regulating Digestion: CNS and
Enteric Nervous System (ENS)
221
Gastric Phase: The Stomach
Figure 21-15: The mucus-bicarbonate barrier of the gastric mucosa
222
PowerPoint® Lecture Slide Presentation by Vince Austin
Human Anatomy & Physiology
FIFTH EDITION
Elaine N. Marieb
Chapter 24
Pathology
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Digestive Health: Protection &
Problems
• Immune defense: M-cells, Peyer's patches,
lymphocytes
• Irritable bowel disease – chronic inflammation
• Diarrhea: leads to dehydration (4 million
deaths/yr)
– Osmotic-solutes prevent H2O reabsorption
– Secretory- bacterial toxins ("flush out' pathogens)
• Vomiting (emesis) can lead to alkalosis
• Ulcers- Helicobacter pylori
• "heart-burn"  acid reflux disease (GERD)
224
Peptic Ulcer
•Lesions in the wall of the stomach
or duodenum.
•Primarily caused by bacteria.
•Treated with antacids AND
antibiotics.
225
Appendicitis
•Usually caused by fecal obstruction
or anatomical “kinking” of the
appendix.
•A rupture leads to peritonitis.
226
Peritonitis
(Inflammation of the Peritoneum)
•Usually results from an infection caused by a
external or internal penetrating wound.
•Bacteria enter the sterile areas of the body
surrounding the digestive system.
•May become lethal if not treated with high
doses of antibiotics.
227
Hepatitis
(Inflammation of the liver)
•Caused by drugs, chemicals, viruses,
alcohol, etc.
•Viral “Hepatitis A” is usually caused
by the ingestion of food.
•Viral “Hepatitis B” and “Hepatitis C”
are “blood-borne” pathogens.
228
Homeostatic Imbalance
• Hepatitis – inflammation of the liver often due to
viral infection
• Viruses causing hepatitis are catalogued has
HVA through HVF
• HVA and HVE are transmitted enterically and
cause self-limiting infections
• Hepatitis B is transmitted via blood transfusions,
contaminated needles, and sexual contact, and
increases the risk of liver cancer
• Hepatitis C produces chronic liver infection
• Nonviral hepatitis is caused by drug toxicity and
wild mushroom poisoning
229
Diverticulitis
•Small herniations of the mucosa in the
large intestine.
•These areas can become inflamed and
possibly rupture.
•Prevention is the treatment of choice. A
diet high in fiber will help prevent
diverticulitis.
230
231
Diverticulosis
232
Emesis
(Vomiting)
•Can be caused by microbes, allergies,
gluttony, poisons, etc.
233
Diarrhea
•Movement of fecal material through
the GI Tract too rapidly.
•May be caused by microbes, spicy
foods, stress, etc.
234
Constipation
•Infrequent defecation of fecal
material.
•Usually caused by a diet low in
fiber and water.
235
Cirrhosis of the Liver
•Condition where liver cells are
destroyed and replaced by fibrous
connective tissue.
•Alcoholism is a common cause of
cirrhosis.
236
Gall Stones
(Biliary Calculi)
•Crystalization of cholesterol and bile
salts.
•Blocks the bile duct or fills the gall
bladder.
237
Jaundice
•Build-up of bile in the skin and
conjunctiva causing a yellowing of the skin.
•May be caused by damage to the liver,
gall bladder, or any of the ducts that
service these organs.
238
Jaundice
239
Bulemia Nervosa
•Condition where a patient binges on
food and then purges with either
laxatives or vomiting.
•Considered a psychological disorder
where the patient has a fear of gaining
weight.
•Treated with psychotherapy.
240
Anorexia Nervosa
•Psychological disorder where the patient
has a false perception of their own weight.
•Patient denies their own appetite.
•Patients are usually 15 - 20 % below normal
body weight.
•Extreme cases are lethal.
•Closely associated with bulimia nervosa.
241
Flatulence
•Excessive intestinal gas resulting
from bacteria in the intestines, diet,
or swallowing air.
242
Cystic Fibrosis
•Genetic disorder where excess mucous
is produced.
•Causes a blocking of the pancreatic duct,
therefore enzymes cannot enter the
duodenum from the pancreas.
•Treated by giving digestive enzymes
orally.
243
NaCl Secretion by the Colon
Silv. p.688
244
Cancer
• Stomach and colon cancers rarely have early signs or
symptoms
• Metastasized colon cancers frequently cause
secondary liver cancer
• Prevention is by regular dental and medical
examinations
• Colon cancer is the 2nd largest cause of cancer deaths
in males (lung cancer is 1st)
• Forms from benign mucosal tumors called polyps
whose formation increases with age
• Regular colon examination should be done for all
those over 50
245
Malabsorption of Nutrients
• Results from anything that interferes with delivery of
bile or pancreatic juice
• Factors that damage the intestinal mucosa (e.g.,
bacterial infection)
• Gluten enteropathy (adult celiac disease) – gluten
damages the intestinal villi and reduces the length of
microvilli
• Treated by eliminating gluten from the diet (all
grains but rice and corn)
246
Food Poisoning: Salmonella
• Salmonella is spread by:
• Contaminated eggs and egg products
• Infected food handlers with feces-contaminated
hands
• Salmonella can cause:
• Bacteremia 4 to 7 days after infection
• Endocarditis, thrombi, bone infections, arthritis, and
meningitis
• Diagnosis is by positive stool samples
• Salmonellosis is treated symptomatically
247
Homeostatic Imbalance
• Hiatal hernia
• structural abnormality in which the superior part of
the stomach protrudes slightly above the diaphragm
• Prolonged episodes can lead to esophagitis, ulcers,
and cancer
248
Age related changes in the digestive system
include:
• Thinner, more fragile epithelium
• Reduction in epithelial stem cells
• Weaker peristaltic contractions
• Effects of cumulative damage
• Increased cancer rates
249
Digestive Disorders
• Anorexia Nervosa
• Emesis
• Appendicitis
• Gall Stones
• Barrett’s esophagus
• GERD
• Buliemia Nervosa
• Gingivitis
• Cancer
• Hepatitis
• Cavities
• Hiatal Hernia
• Celiac Disease (gluten enteropathy)
• Jaundice
• Cirrhosis of the Liver
• Lactose Intolerance
• Constipation
• Mumps
• Crohn’s Disorder
• Peptic Ulcer
• Cystic Fibrosis
• Periodontitis
• Diarrhea
• Peritonitis
• Diverticulitis
250