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Digestive System
Part A
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
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
 Accessory digestive organs – teeth, tongue,
gallbladder, salivary glands, liver, and pancreas
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Digestive System: Overview
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Figure 23.1
Digestive Process
 The GI tract is a “disassembly” line
 Nutrients become more available to the body in
each step
 There are six essential activities:
 Ingestion, propulsion, and mechanical digestion
 Chemical digestion, absorption, and defecation
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Digestive Process
Figure 23.2
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Gastrointestinal Tract Activities
 Ingestion – taking food into the digestive tract
 Propulsion – swallowing and peristalsis
 Peristalsis – waves of contraction and relaxation of
muscles in the organ walls
 Mechanical digestion – chewing, mixing, and
churning food
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Peristalsis and Segmentation
Figure 23.3
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Gastrointestinal Tract Activities
 Chemical digestion – catabolic breakdown of food
 Absorption – movement of nutrients from the GI
tract to the blood or lymph
 Defecation – elimination of indigestible solid wastes
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GI Tract
 External environment for the digestive process
 Regulation of digestion involves:
 Mechanical and chemical stimuli – stretch
receptors, osmolarity, and presence of substrate in
the lumen
 Extrinsic control by CNS centers
 Intrinsic control by local centers
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Receptors of the GI Tract
 Mechano- and chemoreceptors respond to:
 Stretch, osmolarity, and pH
 Presence of substrate, and end products of digestion
 They initiate reflexes that:
 Activate or inhibit digestive glands
 Mix lumen contents and move them along
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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
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Nervous Control of the GI Tract
Figure 23.4
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Peritoneum and Peritoneal Cavity
 Peritoneum – serous membrane of the abdominal
cavity
 Visceral – covers external surface of most digestive
organs
 Parietal – lines the body wall
 Peritoneal cavity
 Lubricates digestive organs
 Allows them to slide across one another
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Peritoneum and Peritoneal Cavity
Figure 23.5a
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Peritoneum and Peritoneal Cavity
 Mesentery – double layer of peritoneum that
provides:
 Vascular 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
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Peritoneum and Peritoneal Cavity
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Figure 23.5b
Blood Supply: Splanchnic Circulation
 Arteries and the organs they serve include
 The hepatic, splenic, and left gastric: spleen, liver,
and stomach
 Inferior and superior mesenteric: small and large
intestines
 Hepatic portal circulation:
 Collects nutrient-rich venous blood from the
digestive viscera
 Delivers this blood to the liver for metabolic
processing and storage
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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
 Each tunic has a predominant tissue type and a
specific digestive function
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Histology of the Alimentary Canal
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Figure 23.6
Mucosa
 Moist epithelial layer that 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: a lining epithelium, lamina
propria, and muscularis mucosae
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Mucosa: Epithelial Lining
 Consists of simple columnar epithelium and mucussecreting goblet cells
 The mucus secretions:
 Protect digestive organs from digesting themselves
 Ease food along the tract
 Stomach and small intestine mucosa contain:
 Enzyme-secreting cells
 Hormone-secreting cells (making them endocrine
and digestive organs)
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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) important in
defense against bacteria
 Muscularis mucosae – smooth muscle cells that
produce local movements of mucosa
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Mucosa: Other Sublayers
 Submucosa – dense connective tissue containing
elastic fibers, blood and lymphatic vessels, lymph
nodes, and nerves
 Muscularis externa – responsible for segmentation
and peristalsis
 Serosa – the protective visceral peritoneum
 Replaced by the fibrous adventitia in the esophagus
 Retroperitoneal organs have both an adventitia and
serosa
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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 are largely automatic
involving local reflex arcs
 Linked to the CNS via long autonomic reflex arc
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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
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Anatomy of the Oral Cavity: Mouth
Figure 23.7a
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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
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Oral Cavity and Pharynx: Anterior View
Figure 23.7b
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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)
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Palate
 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
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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
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Tongue
 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
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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
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Tongue
Figure 23.8
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Salivary Glands
 Produce and secrete saliva that:
 Cleanses the mouth
 Moistens and dissolves food chemicals
 Aids in bolus formation
 Contains enzymes that break down starch
 Three pairs of extrinsic glands – parotid,
submandibular, and sublingual
 Intrinsic salivary glands (buccal glands) – scattered
throughout the oral mucosa
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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
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Salivary Glands
Figure 23.9a
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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
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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
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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
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Deciduous Teeth
Figure 23.10.1
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Permanent Teeth
Figure 23.10.2
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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
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Dental Formula: Permanent Teeth
 A shorthand way of indicating the number and
relative position of teeth
 Written as ratio of upper to lower teeth for the
mouth
 Primary: 2I (incisors), 1C (canine), 2M (molars)
 Permanent: 2I, 1C, 2PM (premolars), 3M
2I
2I
1C
1C
2PM
2PM
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3M
3M
X
2 (32 teeth)
Tooth Structure
 Two main regions – crown and the root
 Crown – exposed part of the tooth above the gingiva
(gum)
 Enamel – acellular, brittle material composed of
calcium salts and hydroxyapatite crystals is the
hardest substance in the body
 Encapsules the crown of the tooth
 Root – portion of the tooth embedded in the jawbone
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Tooth Structure
 Neck – constriction where the crown and root come
together
 Cementum – calcified connective tissue
 Covers the root
 Attaches it to the periodontal ligament
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Tooth Structure
 Periodontal ligament
 Anchors the tooth in the alveolus of the jaw
 Forms the fibrous joint called a gomaphosis
 Gingival sulcus – depression where the gingiva
borders the tooth
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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
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Tooth Structure
Figure 23.11
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Tooth and Gum Disease
 Dental caries – 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
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Tooth and Gum Disease: Periodontitis
 Gingivitis – as plaque accumulates, it calcifies and
forms calculus, or tartar
 Accumulation of calculus:
 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
 Immune system attacks intruders as well as body
tissues, carving pockets around the teeth and
dissolving bone
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Digestive System
Part B
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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
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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
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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)
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Digestive Processes in the Mouth
 Food is ingested
 Mechanical digestion begins (chewing)
 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
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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
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Deglutition (Swallowing)
Bolus of
food
Tongue
Uvula
Pharynx
Bolus
Epiglottis
Epiglottis
Glottis
Esophagus
Trachea
(a) Upper esophageal
sphincter contracted
Bolus
(c) Upper esophageal
sphincter contracted
(b) Upper esophageal
sphincter relaxed
Relaxed
muscles
Relaxed muscles
Circular muscles
contract, constricting
passageway and pushing
bolus down
Bolus of food
Gastroesophageal
sphincter open
Longitudinal muscles
contract, shortening
passageway ahead of bolus
Gastroesophageal
sphincter closed
Stomach
(d)
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(e)
Figure 23.13
Stomach
 Chemical breakdown of proteins begins and food is
converted to chyme
 Cardiac region – surrounds the cardiac orifice
 Fundus – dome-shaped region beneath the
diaphragm
 Body – midportion of the stomach
 Pyloric region – made up of the antrum and canal
which terminates at the pylorus
 The pylorus is continuous with the duodenum
through the pyloric sphincter
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Stomach
 Greater curvature – entire extent of the convex
lateral surface
 Lesser curvature – concave medial surface
 Lesser omentum – runs from the liver to the lesser
curvature
 Greater omentum – drapes inferiorly from the
greater curvature to the small intestine
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Stomach
 Nerve supply – sympathetic and parasympathetic
fibers of the autonomic nervous system
 Blood supply – celiac trunk, and corresponding
veins (part of the hepatic portal system)
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Stomach
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Figure 23.14a
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
 The mucous surface layer traps a bicarbonaterich fluid beneath it
 Gastric pits contain gastric glands that secrete
gastric juice, mucus, and gastrin
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Microscopic Anatomy of the Stomach
Figure 23.15
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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 cells – secrete HCl and intrinsic factor
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Glands of the Stomach Fundus and Body
 Chief cells – produce pepsinogen
 Pepsinogen is activated to pepsin by:
 HCl in the stomach
 Pepsin itself via a positive feedback
mechanism
 Enteroendocrine cells – secrete gastrin, histamine,
endorphins, serotonin, cholecystokinin (CCK), and
somatostatin into the lamina propria
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Stomach Lining
 The stomach is exposed to the harshest conditions in
the digestive tract
 To keep from digesting itself, the stomach has a
mucosal barrier with:
 A thick coat of bicarbonate-rich mucus on the
stomach wall
 Epithelial cells that are joined by tight junctions
 Gastric glands that have cells impermeable to HCl
 Damaged epithelial cells are quickly replaced
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Digestion in the Stomach
 The stomach:
 Holds ingested food
 Degrades this food both physically and chemically
 Delivers chyme to the small intestine
 Enzymatically digests proteins with pepsin
 Secretes intrinsic factor required for absorption of
vitamin B12
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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
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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
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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
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Gastric Phase
 Inhibitory events include:
 A pH lower than 2
 Emotional upset that overrides the parasympathetic
division
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Intestinal Phase
 Excitatory phase – low pH; partially digested food
enters the duodenum and encourages gastric gland
activity
 Inhibitory phase – distension of duodenum, presence
of fatty, acidic, or hypertonic chyme, and/or irritants
in the duodenum
 Initiates inhibition of local reflexes and vagal nuclei
 Closes the pyloric sphincter
 Releases enterogastrones that inhibit gastric
secretion
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Release of Gastric Juice
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Figure 23.16
Regulation and Mechanism of HCl Secretion
 HCl secretion is stimulated by ACh, histamine, and
gastrin through second-messenger systems
 Release of hydrochloric acid:
 Is low if only one ligand binds to parietal cells
 Is high if all three ligands bind to parietal cells
 Antihistamines block H2 receptors and decrease HCl
release
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Regulation and Mechanism of HCl Secretion
Figure 23.17
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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
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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
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Gastric Contractile Activity
Figure 23.18
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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 quickly moves through the
duodenum
 Fat-laden chyme is digested more slowly causing
food to remain in the stomach longer
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Regulation of Gastric Emptying
Figure 23.19
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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
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Small Intestine: Microscopic Anatomy
 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
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Small Intestine: Microscopic Anatomy
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Figure 23.21
Small Intestine: Histology of the Wall
 The epithelium of the mucosa is made up of:
 Absorptive cells and goblet cells
 Enteroendocrine cells
 Interspersed T cells called intraepithelial
lymphocytes (IELs)
 IELs immediately release cytokines upon
encountering Ag
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Small Intestine: Histology of the Wall
 Cells of intestinal crypts secrete intestinal juice
 Peyer’s patches are found in the submucosa
 Brunner’s glands in the duodenum secrete alkaline
mucus
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Intestinal Juice
 Secreted by intestinal glands in response to
distension or irritation of the mucosa
 Slightly alkaline and isotonic with blood plasma
 Largely water, enzyme-poor, but contains mucus
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Liver
 The largest gland in the body
 Superficially has four lobes – right, left, caudate,
and quadrate
 The falciform ligament:
 Separates the right and left lobes anteriorly
 Suspends the liver from the diaphragm and anterior
abdominal wall
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Liver
 The ligamentum teres:
 Is a remnant of the fetal umbilical vein
 Runs along the free edge of the falciform ligament
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Liver: Associated Structures
 The lesser omentum anchors the liver to the stomach
 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
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Liver: Associated Structures
 Bile leaves the liver via:
 Bile ducts, which fuse into the common hepatic
duct
 The common hepatic duct, which fuses with the
cystic duct
 These two ducts form the bile duct
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Gallbladder and Associated Ducts
Figure 23.20
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Liver: Microscopic Anatomy
 Hexagonal-shaped liver lobules are the structural and
functional units of the liver
 Composed of hepatocyte (liver cell) plates radiating
outward from a central vein
 Portal triads are found at each of the six corners of each
liver lobule
 Portal triads consist of a bile duct and
 Hepatic artery – supplies oxygen-rich blood to the liver
 Hepatic portal vein – carries venous blood with nutrients
from digestive viscera
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Liver: Microscopic Anatomy
 Liver sinusoids – enlarged, leaky capillaries located
between hepatic plates
 Kupffer cells – hepatic macrophages found in liver
sinusoids
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Liver: Microscopic Anatomy
 Hepatocytes’ functions include:
 Production of bile
 Processing bloodborne nutrients
 Storage of fat-soluble vitamins
 Detoxification
 Secreted bile flows between hepatocytes toward the
bile ducts in the portal triads
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Microscopic Anatomy of the Liver
Figure 23.24c, d
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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
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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
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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
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Regulation of Bile Release
 Cholecystokinin causes:
 The gallbladder to contract
 The hepatopancreatic sphincter to relax
 As a result, bile enters the duodenum
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Regulation of Bile Release
Figure 23.25
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Digestive System
Part C
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Pancreas
 Location
 Lies deep to the greater curvature of the stomach
 The head is encircled by the duodenum and the tail
abuts the spleen
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Pancreas
 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
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Acinus of the Pancreas
Figure 23.26a
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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
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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
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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
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Regulation of Pancreatic Secretion
Figure 23.28
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Digestion in the Small Intestine
 As chyme enters the duodenum:
 Carbohydrates and proteins are only partially
digested
 No fat digestion has taken place
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Digestion in the Small Intestine
 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
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Motility in 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
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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
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Control of Motility
 Other impulses relax the circular muscle
 The gastroileal reflex and gastrin:
 Relax the ileocecal sphincter
 Allow chyme to pass into the large intestine
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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
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Large Intestine
 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
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Large Intestine
Figure 23.29a
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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
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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
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Mesenteries of Digestive Organs
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Figure 23.30b
Mesenteries of Digestive Organs
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Figure 23.30c
Mesenteries of Digestive Organs
Figure 23.30d
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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
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Structure of the Anal Canal
Figure 23.29b
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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
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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
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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
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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 defecation occurs
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Defecation
Figure 23.32
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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
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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
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Chemical Digestion: Proteins
Figure 23.34
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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
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Chemical Digestion: Fats
Figure 23.35
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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
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Fatty Acid Absorption
Figure 23.36
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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
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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+
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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)
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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
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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)
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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
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Embryonic Development of the Digestive
System
Figure 23.37
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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 (aids 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
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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
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Cancer
 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
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