Chapter 25 Lecture Outline - Palm Beach State College...25-7 Digestive Function •Chemical...
Transcript of Chapter 25 Lecture Outline - Palm Beach State College...25-7 Digestive Function •Chemical...
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Chapter 25
Lecture Outline
Copyright © McGraw-Hill Education. Permission required for reproduction or display.
See separate PowerPoint slides for all figures and tables pre-
inserted into PowerPoint without notes.
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Introduction
• Most nutrients we eat cannot be used in existing
form
– Must be broken down into smaller components before
body can make use of them
• Digestive system—acts as a disassembly line
– To break down nutrients into forms that can be used by
the body
– To absorb them so they can be distributed to the tissues
• Gastroenterology—the study of the digestive tract
and the diagnosis and treatment of its disorders
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General Anatomy and
Digestive Processes
• Expected Learning Outcomes
– List the functions and major physiological processes of the
digestive system.
– Distinguish between mechanical and chemical digestion.
– Describe the basic chemical process underlying all
chemical digestion, and name the major substrates and
products of this process.
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General Anatomy and
Digestive Processes
(Continued)
– List the regions of the digestive tract and the accessory
organs of the digestive system.
– Identify the layers of the digestive tract and describe its
relationship to the peritoneum.
– Describe the general neural and chemical controls over
digestive function.
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Digestive Function
• Digestive system—organ system that processes
food, extracts nutrients, and eliminates residue
• Five stages of digestion
– Ingestion: selective intake of food
– Digestion: mechanical and chemical breakdown of food
into a form usable by the body
– Absorption: uptake of nutrient molecules into the
epithelial cells of the digestive tract and then into the blood
and lymph
– Compaction: absorbing water and consolidating the
indigestible residue into feces
– Defecation: elimination of feces
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Digestive Function
• Mechanical digestion—the physical breakdown of
food into smaller particles
– Cutting and grinding action of the teeth
– Churning action of stomach and small intestines
– Exposes more food surface to digestive enzymes
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Digestive Function
• Chemical digestion—a series of hydrolysis reactions
that breaks dietary macromolecules into their
monomers (residues)
– Carried out by digestive enzymes produced by salivary
glands, stomach, pancreas, and small intestine
– Results
• Polysaccharides into monosaccharides
• Proteins into amino acids
• Fats into monoglycerides and fatty acids
• Nucleic acids into nucleotides
• Some nutrients are present in a usable form in
ingested food and can be directly absorbed
– Vitamins, amino acids, minerals, cholesterol, and water
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General Anatomy
• Digestive system has two subdivisions: digestive tract and accessory organs
• Digestive tract (alimentary canal)
– 30 ft long muscular tube extending from mouth to anus
– Mouth, pharynx, esophagus, stomach, small intestine, and large intestine
– Gastrointestinal (GI) tract is the stomach and intestines
Figure 25.1
Small intestine
Cecum
Appendix
Liver
Gallbladder
Diaphragm
Rectum
Anal canal
Pancreas
Bile duct
Anus
Stomach
Esophagus
Pharynx
Parotid
gland
Sublingual gland
Tongue
Teeth
Oral cavity
Submandibular
gland
Transverse
colon
Descending
colon
Ascending
colon
Sigmoid
colon
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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(Continued)
• Accessory organs
– Teeth, tongue, salivary glands, liver, gallbladder, and pancreas
Figure 25.1
Small intestine
Cecum
Liver
Gallbladder
Diaphragm
Rectum
Anal canal
Pancreas
Bile duct
Anus
Stomach
Esophagus
Pharynx
Parotid
gland
Sublingual gland
Tongue
Teeth
Oral cavity
Submandibular
gland
Transverse
colon
Descending
colon
Ascending
colon
Sigmoid
colon
Appendix
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
General Anatomy
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General Anatomy
• Digestive tract is open to environment at both
ends
• Most material in it has not entered the body
tissues
– Considered to be external to the body until it is absorbed
by the epithelial cells of the alimentary canal
• On a strict sense, defecated food residue was
never in the body
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General Anatomy
• Most of digestive tract follows a basic structural plan
with the digestive tract wall consisting of layers:
– Mucosa
• Epithelium
• Lamina propria
• Muscularis mucosae
– Submucosa
– Muscularis externa
• Inner circular layer
• Outer longitudinal layer
– Serosa
• Areolar tissue
• Mesothelium
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General Anatomy
• Mucosa (mucous membrane)—lines the lumen and
consists of:
– Inner epithelium
• Simple columnar in most of digestive tract
• Stratified squamous from mouth through esophagus, and in
lower anal canal
– Lamina propria: loose connective tissue layer
– Muscularis mucosa: thin layer of smooth muscle
• Tenses mucosa creating grooves and ridges that enhance
surface area and contact with food
• Improves efficiency of digestion and nutrient absorption
– Mucosa-associated lymphatic tissue (MALT): the mucosa
exhibits an abundance of lymphocytes and lymphatic nodules
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General Anatomy
• Submucosa—thicker layer of loose connective
tissue
– Contains blood vessels, lymphatic vessels, a nerve
plexus, and in some places mucus-secreting glands
that dump lubricating mucus into the lumen
– MALT extends into the submucosa in some parts of the
GI tract
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General Anatomy
• Muscularis externa—consists of usually two
layers of muscle near the outer surface
– Inner circular layer
• In some places, this layer thickens to form valves
(sphincters) that regulate the passage of material through
the tract
– Outer longitudinal layer
– Responsible for the motility that propels food and
residue through the tract
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General Anatomy
• Serosa—composed of a thin layer of areolar tissue
topped by simple squamous mesothelium
– Begins in the lower 3 to 4 cm of the esophagus
– Ends just before the rectum
– Adventitia: fibrous connective tissue layer that binds and
blends the pharynx, most of the esophagus, and the
rectum into adjacent connective tissue of other organs
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Tissue Layers of the Digestive Tract
Figure 25.2
Mucosa:
Lamina propria
Muscularis mucosae
Submucosa:
Esophageal gland
Lumen
Blood vessels
Diaphragm
Esophageal hiatus
Myenteric plexus
Submucosal plexus
Muscularis externa:
Inner circular layer
Outer longitudinal layer
Serosa
Enteric nervous system:
Stratified squamous
epithelium
Parasympathetic ganglion of
myenteric plexus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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General Anatomy
• Enteric nervous system—nervous network in esophagus, stomach, and intestines that regulates digestive tract motility, secretion, and blood flow– Thought to have over 100 million neurons
– Can function independently of central nervous system
• But CNS usually exerts influence on its action
– Often considered part of autonomic nervous system
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General Anatomy
• Enteric nervous system is composed of two
networks of neurons
– Submucosal (Meissner) plexus: in submucosa
• Controls glandular secretions of mucosa
• Controls movements of muscularis mucosae
– Myenteric (Auerbach) plexus: parasympathetic ganglia
and nerve fibers between the two layers of the
muscularis externa
• Controls peristalsis and other contractions of muscularis
externa
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25-19
Relationship to the Peritoneum
• Mesenteries—connective tissue sheets that
suspend stomach and intestines from abdominal wall
– Looseness allows stomach and intestines to undergo
strenuous contractions with freedom of movement in the
abdominal cavity
– Hold abdominal viscera in proper relationship to each other
– Prevent intestines from becoming twisted and tangled by
changes in body position and by its own contractions
– Provide passage of blood vessels and nerves that supply
digestive tract
– Contain many lymph nodes and lymphatic vessels
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Relationship to the Peritoneum
• Parietal peritoneum—a serous membrane that
lines the wall of the abdominal cavity
– Turns inward along posterior midline
– Forms dorsal mesentery: a translucent two-layered
membrane extending to the digestive tract
– The two layers of the mesentery separate and pass
around opposite sides of the organ forming the serosa
– Come together on the far side of the organ and continue
as another sheet of tissue, called the anterior (ventral)
mesentery
• May hang freely in the abdominal cavity
• May attach to the anterior abdominal wall or other organs
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Relationship to the Peritoneum
• Lesser omentum—a ventral mesentery that extends from the lesser curvature of the stomach to the liver
• Greater omentum—hangs from the greater curvature of the stomach (its left inferior margin)– Covers small intestine like an apron
– The inferior margin turns back on itself and passes upward
– Forming a deep pouch between its deep and superficial layers
– Inner superior margin forms serous membranes around the spleen and transverse colon—mesocolon
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Relationship to the Peritoneum
• Mesocolon—extension of the mesentery that anchors
the colon to the abdominal wall
• Intraperitoneal—when an organ is enclosed by
mesentery on both sides
– Considered within the peritoneal cavity
– Stomach, liver, and parts of small and large intestine
• Retroperitoneal—when an organ lies against the
posterior body wall and is covered by peritoneum on its
anterior side only
– Considered to be outside the peritoneal cavity
– Duodenum, pancreas, and parts of the large intestine
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Serous Membranes
• Lesser omentum—attaches stomach to liver
• Greater omentum—covers small intestines like an apron
Figure 25.3a
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Serous Membranes
• Mesentery of small intestines holds many blood vessels
• Mesocolon anchors colon to body wall
Figure 25.3b
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25-25
Regulation of the Digestive Tract
• Motility and secretion of the digestive tract are
controlled by neural, hormonal, and paracrine
mechanisms
• Neural control
– Short (myenteric) reflexes: stretch or chemical
stimulation acts through myenteric plexus
• Stimulates paristaltic contractions of swallowing
– Long (vagovagal) reflexes: parasympathetic
stimulation of digestive motility and secretion
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25-26
Regulation of the Digestive Tract
• Hormones
– Chemical messengers secreted into bloodstream that
stimulate distant parts of the digestive tract
– Gastrin and secretin
• Paracrine secretions
– Chemical messengers that diffuse through the tissue
fluids to stimulate nearby target cells
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The Mouth Through Esophagus
• Expected Learning Outcomes
– Describe the gross anatomy of the digestive tract from
the mouth through the esophagus.
– Describe the composition and functions of saliva.
– Describe the neural control of salivation and swallowing.
25-27
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25-28
The Mouth
• The mouth is known as the oral, or buccal
cavity
• Functions
– Ingestion (food intake)
– Taste and other sensory responses to food
– Chewing and chemical digestion
– Swallowing, speech, and respiration
• Mouth enclosed by cheeks, lips, palate, and
tongue
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The Mouth
• Oral fissure—anterior opening between lips
• Fauces—posterior opening to the throat
• Stratified squamous epithelium lines mouth
– Keratinized in areas subject to food abrasion: gums
and hard palate
– Nonkeratinized in other areas: floor of mouth, soft
palate, and insides of cheeks and lips
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The Oral Cavity
Figure 25.4
Upper lip
Vestibule
Tongue
Lingual frenulum
Lower lip
Submandibular
Sublingual
Palatine tonsil
Palatine raphe
Soft palate
Uvula
Palatoglossal
arch
Palatopharyngeal
arch
Inferior labial
frenulum
Hard palate
and palatine
rugae
Superior
labial
frenulum
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Salivary duct
orifices:
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The Cheeks and Lips
• Cheeks and lips
– Retain food and push it between the teeth
– Essential for speech
– Essential for sucking and blowing actions, including
suckling by infants
– Fleshiness due to subcutaneous fat, buccinator muscle
of the cheek, and orbicularis oris of the lips
– Labial frenulum: median fold that attaches each lip to
the gum between the anterior incisors
– Vestibule: space between cheek or lips and the teeth
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25-32
The Cheeks and Lips
• Lips divided into three areas
– Cutaneous area: colored like the rest of the face
• Has hair follicles and sebaceous glands
– Red (vermillion) area: hairless region where lips meet
• Tall dermal papilla that allows blood vessels and nerves
to come closer to epidermal surface
• Redder and more sensitive than cutaneous area
– Labial mucosa: the inner surface of the lips facing the
gums and teeth
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The Tongue
• Tongue—muscular, bulky, but remarkably agile and
sensitive organ
– Manipulates food between teeth
– Senses taste and texture of food
– Can extract food particles from the teeth after a meal
– Nonkeratinized stratified squamous epithelium covers
its surface
– Lingual papillae: bumps and projections that are the
sites of most taste buds
– Body: anterior two-thirds of tongue; it occupies oral cavity
– Root: posterior one-third of the tongue; it occupies the
oropharynx
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25-34
The Tongue
– Vallate papillae: a V-shaped row of papillae that mark the
boundary between the body and root of the tongue
– Terminal sulcus: groove behind the vallate papillae
– Lingual frenulum: median fold that attaches the body of
the tongue to the floor of the mouth
– Intrinsic muscles—contained entirely within the tongue
• Produce subtle tongue movements of speech
– Extrinsic muscles: with origins elsewhere and insertions
in the tongue
• Produce stronger movements of food manipulation
• Genioglossus, hyoglossus, palatoglossus, and styloglossus
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25-35
The Tongue
– Lingual glands: serous and mucous glands amid the
extrinsic muscles
• Secrete a portion of the saliva
– Lingual tonsils: contained in the root
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The Tongue
Figure 25.5a,b
Epiglottis
Root
Body
Hyoid bone
Mylohyoid m.
Sublingual gland
Submandibular gland
Mandible
Genioglossus m.
Hyoglossus m.
Styloglossus m.
1st molar
Buccinator m.
Intrinsic muscles ofthe tongue
(a) Superior view (b) Frontal section, anterior view
Lingual
tonsils
Palatine
tonsil
Terminal
sulcusVallate
papillae
Foliate
papillae
Fungiform
papillae
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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The Palate
• Palate—separates oral cavity from nasal cavity
– Makes it possible to breathe while chewing food
• Hard (bony) palate—anterior portion that is
supported by the palatine processes of the
maxillae and the palatine bones
– Palatine rugae: transverse ridges that help the tongue
hold and manipulate food
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The Palate
• Soft palate—posterior with a more spongy texture– Composed of skeletal muscle and glandular tissue
– No bone
– Uvula: conical medial projection visible at the rear of the mouth
• Helps retain food in the mouth until one is ready to swallow
• Pair of muscular arches on each side of the oral
cavity– Palatoglossal arch: anterior arch
– Palatopharyngeal arch: posterior arch
– Palatine tonsils are located on the wall between the arches
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25-39
The Teeth
• Dentition—the teeth
• Masticate (chew) food into smaller pieces
– Makes food easier to swallow
– Exposes more surface area for action of digestive
enzymes, speeding chemical digestion
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25-40
The Teeth
• 32 adult teeth– 16 in mandible
– 16 in maxilla
– From midline to the rear of each jaw
• 2 incisors—chisel-like cutting teeth used to bite off a piece of food
• 1 canine—pointed and act to puncture and shred food
• 2 premolars—broad surface for crushing, shredding, and grinding
• 3 molars—even broader surface for crushing, shredding, and grinding
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25-41
The Teeth
• Alveolus—tooth socket in bone
– Gomphosis joint formed between
tooth and bone
• Periodontal ligament—modified
periosteum whose collagen fibers
penetrate into the bone on one side
and into the tooth on the other
– Anchors tooth firmly in alveolus
– Allows slight movement under
pressure of chewing
• Gingiva (gum)—covers the
alveolar boneFigure 25.6
Names of teeth
2nd molar
1st molar
Canine
Lateral incisor
Central incisor 6–9
16–20
12–16
20–26
Names of teeth
(a) Deciduous (baby) teeth
(b) Permanent teeth
Central incisor
Lateral incisor
1st premolar
2nd premolar
1st molar
Canine
2nd molar
6–8
7–9
9–12
10–12
10–12
6–7
11–13
17–25
Age at eruption
(months)
7– 11
Age at eruption
(years)
3rd molar
(wisdom tooth)
Copyright© The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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25-42
The Teeth
• Regions of a tooth
– Crown: portion above the gum
– Root: the portion below the gum,
embedded in alveolar bone
– Neck: the point where crown,
root, and gum meet
– Gingival sulcus: space between
the tooth and the gum
• Hygiene in the sulcus is
important to dental health
Figure 25.6
Names of teeth
2nd molar
1st molar
Canine
Lateral incisor
Central incisor 6–9
16–20
12–16
20–26
Names of teeth
(a) Deciduous (baby) teeth
(b) Permanent teeth
Central incisor
Lateral incisor
1st premolar
2nd premolar
1st molar
Canine
2nd molar
6–8
7–9
9–12
10–12
10–12
6–7
11–13
17–25
Age at eruption
(months)
7– 11
Age at eruption
(years)
3rd molar
(wisdom tooth)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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• Dentin—hard yellowish
tissue that makes up most
of the tooth
• Enamel—covers crown
and neck
– A noncellular secretion that
cannot regenerate
• Cementum—covers root
• Cementum and dentin
are living tissue and can
regenerate
The Teeth
Figure 25.7
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• Root canal—space in a
root leading to pulp cavity
in the crown
– Nerves and blood vessels
– Apical foramen: pore at the
basal end of each root canal
• Occlusion—meeting of
the teeth with the mouth
closed
The Teeth
Figure 25.7
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Permanent and Deciduous Teeth
Figure 25.8
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The Teeth
• 20 deciduous teeth (milk teeth or baby teeth)
• Teeth develop beneath gums and erupt in a
predictable order
– Erupt from 6 to 30 months
– Beginning with incisors
– Between 6 and 25 years of age, are replaced by 32
permanent teeth
• Third molars (wisdom teeth) erupt from age 17 to
25 years
– May be impacted: crowded against neighboring teeth
and bone so they cannot erupt
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Tooth and Gum Disease
• The human mouth is home to more than 700
species of microorganisms, especially bacteria
• Plaque—sticky residue on the teeth made up of
bacteria and sugars
– Calculus: calcified plaque
– Bacteria metabolize sugars and release acids that dissolve the
minerals of enamel and dentin to form dental caries
(cavities)
• Root canal therapy is necessary if cavity reaches
pulp
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Tooth and Gum Disease
• Calculus in the gingival sulcus wedges the tooth
and gum apart
– Allows bacterial invasion of the sulcus
– Gingivitis: inflammation of the gums
– Periodontal disease: destruction of the supporting
bone around the teeth which may result in tooth loss
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Mastication
• Mastication (chewing)—breaks food into smaller
pieces to be swallowed and exposes more surface
to digestive enzymes
– First step in mechanical digestion
– Food stimulates oral receptors that trigger an
involuntary chewing reflex
– Tongue, buccinator, and orbicularis oris manipulate
food
– Masseter and temporalis elevate the lower teeth to
crush food
– Medial and lateral pterygoids, and masseters swing
teeth in side-to-side grinding action
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Saliva and the Salivary Glands
• Saliva
– Moistens mouth
– Begins starch and fat digestion
– Cleanses teeth
– Inhibits bacterial growth
– Dissolves molecules so they can stimulate the taste
buds
– Moistens food and binds it together into bolus to aid in
swallowing
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Saliva and the Salivary Glands
• Saliva is a hypotonic solution of 97.0% to 99.5%
water and the following solutes:– Salivary amylase: enzyme that begins starch digestion in
the mouth
– Lingual lipase: enzyme that is activated by stomach acid and digests fat after food is swallowed
– Mucus: binds and lubricates a mass of food and aids in swallowing
– Lysozyme: enzyme that kills bacteria
– Immunoglobulin A (IgA): an antibody that inhibits bacterial growth
– Electrolytes: Na+, K+, Cl−, phosphate, and bicarbonate
• pH: 6.8 to 7.0
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Saliva and the Salivary Glands
• Intrinsic salivary glands—small glands
dispersed amid other oral tissues
– Lingual glands: in the tongue; produce lingual lipase
– Labial glands: inside of the lips
– Palatine glands: roof of mouth
– Buccal glands: inside of the cheek
– All secrete saliva at a fairly constant rate
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Saliva and the Salivary Glands
• Extrinsic salivary glands—three pairs connected to
oral cavity by ducts
– Parotid: located beneath the skin anterior to the earlobe
• Mumps is an inflammation and swelling of the parotid
gland caused by a virus
– Submandibular gland: located halfway along the body
of the mandible
• Its duct empties at the side of the lingual frenulum, near
the lower central incisors
– Sublingual gland: located in the floor of the mouth
• Has multiple ducts that empty posterior to the papilla of
the submandibular duct
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The Extrinsic Salivary Glands
Figure 25.9
Parotid duct
Mandible
Parotid
gland
Sublingual
ducts
Masseter
muscle
Submandibular
duct
Submandibular
gland
Sublingual
glandOpening of
submandibular
duct
Lingual
frenulum
Tongue
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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25-55
Histology of Salivary Glands
• Compound tubuloacinar
glands
– Branched ducts ending in
acini
• Mucous cells secrete mucus
• Serous cells secrete thin
fluid rich in enzymes and
electrolytes
• Mixed acinus has both
mucous and serous cells
Figure 25.10a,b
Mucous acinus
Mixed acinus
Mucous cells
Serous cells
Salivary duct
(a)
Serous
acinus
Serous
demilune
on mixed
acinus
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Salivation
• Extrinsic salivary glands secrete about 1 to
1.5 L of saliva per day
• Cells of acini filter water and electrolytes from blood and add amylase, mucin, and lysozyme
• Salivatory nuclei in medulla oblongata and pons respond to signals generated by presence of food
– Excited by tactile, pressure, and taste receptors
– Salivatory nuclei receive input from higher brain centers as well
• Odor, sight, thought of food stimulates salivation
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Salivation
Salivatory nuclei (Continued)
– Send signals by way of autonomic fibers in the
facial and glossopharyngeal nerves to the glands
• Parasympathetic fibers stimulate the glands to
produce an abundance of thin, enzyme-rich saliva
• Sympathetic activity stimulates the glands to
produce less, and thicker, saliva with more mucus
• Bolus—mass swallowed as a result of saliva
binding food particles into a soft, slippery, easily
swallowed mass
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25-58
The Pharynx
• Pharynx—muscular funnel connecting oral cavity to
esophagus and nasal cavity to larynx
– Digestive and respiratory tracts intersect
– Has deep layer of longitudinal skeletal muscle
– Has superficial layer of circular skeletal muscles that form
pharyngeal constrictors (superior, middle, and inferior)
that force food downward during swallowing
• When not swallowing, the inferior constrictor (upper
esophageal shincter) remains contracted to exclude air from the
esophagus
• Disappears at the time of death when the muscles relax, so it is a
physiological sphincter, not an anatomical structure
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The Esophagus
• Esophagus—straight muscular tube 25 - 30 cm long
– Begins at level between C6 and the cricoid cartilage
– Extends from pharynx to cardiac orifice of stomach
passing through esophageal hiatus in diaphragm
– Lower esophageal sphincter: food pauses here because
of constriction
• Prevents stomach contents from regurgitating into the
esophagus
• Protects esophageal mucosa from erosive stomach acid
• Heartburn—burning sensation produced by acid reflux into
the esophagus
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The Esophagus
(Continued)
– Nonkeratinized stratified squamous epithelium
– Esophageal glands in submucosa secrete mucus
– Deeply folded into longitudinal ridges when empty
– Skeletal muscle in upper one-third, mix of muscle types
in middle one-third, and only smooth muscle in bottom
one-third
– Meets stomach at level of T7
– Covered with adventitia
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Swallowing
Figure 25.11
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Swallowing
• Swallowing (deglutition)—a complex action
involving over 22 muscles in the mouth, pharynx,
and esophagus
– Swallowing center: pair of nuclei in medulla oblongata
that coordinates swallowing
• Communicates with muscles of the pharynx and
esophagus by way of trigeminal, facial, glossopharyngeal,
and hypoglossal nerves
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Swallowing
• Swallowing occurs in three phases: oral, pharyngeal, and esophageal
– Oral phase: under voluntary control
• Tongue collects food, presses it against palate forming bolus, and pushes it posteriorly
• Food accumulates in oropharynx in front of epiglottis
• Epiglottis tips posteriorly and food bolus slides around it and into laryngopharynx
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Swallowing
(Continued)
– Pharyngeal phase: involuntary
• Prevents food and drink from reentering mouth or entering
the nasal cavity
• Breathing is suspended
• Infrahyoid muscles pull larynx up to meet epiglottis and
cover laryngeal opening
• Vocal cords adduct to close airway
• Upper esophagus widens
• Food bolus is driven downward by constriction of the
upper, then middle, and finally the lower pharyngeal
constrictors
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Swallowing
(Continued)– Esophageal phase—peristalsis: involuntary wave of
muscular contraction that pushes the bolus ahead of it
• When standing or sitting upright, food and liquid drops through esophagus by gravity faster than peristalsis can keep up with it
• Peristalsis ensures you can swallow regardless of body position
• Liquid reaches the stomach in 1 to 2 seconds; food bolus in 4 to 8 seconds
• When it reaches lower end of the esophagus, the lower esophageal sphincter relaxes to let food pass into the stomach
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The Stomach
• Expected Learning Outcomes
– Describe the gross and microscopic anatomy of the
stomach.
– State the function of each type of epithelial cell in the
gastric mucosa.
– Identify the secretions of the stomach and state their
functions.
– Explain how the stomach produces hydrochloric acid and
pepsin.
– Describe the contractile responses of the stomach to food.
– Describe the three phases of gastric function and how
gastric activity is activated and inhibited.
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The Stomach
• Stomach—a muscular sac in upper left abdominal
cavity immediately inferior to the diaphragm
– Primarily functions as a food storage organ
• Internal volume of about 50 mL when empty
• 1.0 to 1.5 L after a typical meal
• Up to 4 L when extremely full – can extend nearly as far
as the pelvis
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The Stomach
• Mechanically breaks up food, liquefies it, and
begins chemical digestion of protein and fat
– Chyme: soupy or pasty mixture of semi-digested
food in the stomach
• Most digestion occurs after the chyme passes
on to the small intestine
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Gross Anatomy
• Stomach—J-shaped
– Relatively vertical in tall people, horizontal in short people
– Divided into four regions
• Cardiac region (cardia)—small area within about 3 cm of
the cardiac orifice
• Fundic region (fundus)—dome-shaped portion superior to
esophageal attachment
• Body (corpus)—makes up the greatest part of stomach
• Pyloric region—narrower pouch at the inferior end
– Subdivided into the funnel-like antrum
– Narrower pyloric canal that terminates at pylorus
– Pylorus: narrow passage to duodenum
– Pyloric (gastroduodenal) sphincter—regulates the
passage of chyme into the duodenum
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Gross Anatomy
• Stomach has greater and lesser curvatures
– Curvatures are the margins of the stomach
– Greater curvature is about 40 cm long, on inferolateral
surface
• Greater omentum hangs from greater curvature
– Lesser curvature is about 10 cm long, on superomedial
margin
• Lesser omentum connects lesser curvature of stomach to liver
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Gross Anatomy
• Note the bulge of fundus, narrowing of pyloric region, thickness of pyloric sphincter, and greater and lesser curvatures
Figure 25.12a
Lesser omentum
Duodenum
(a)
Lesser curvature
Fundic region
Cardiac region
Body
Circular muscle
Oblique muscle
Greater omentum
Greater curvature
Gastric rugae
Diaphragm
Pyloric region:
Antrum
Pyloric canal
Pylorus
Longitudinal
muscle
Pyloric
sphincter
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Gross Anatomy
Figure 25.12b
• Longitudinal wrinkles called rugae can be seen in empty stomach wall
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Innervation and Circulation
• Stomach receives:
– Parasympathetic fibers from vagus
– Sympathetic fibers from celiac ganglia
• Supplied with blood by branches of the celiac trunk
• Blood drained from stomach and intestines enters
hepatic portal circulation and is filtered through
liver before returning to heart
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Microscopic Anatomy
• Stomach has a simple columnar epithelium covers mucosa– Apical regions of its surface cells are filled with mucin
– Mucin swells with water and becomes mucus after it is secreted
• Mucosa and submucosa are flat when stomach is full, but form longitudinal wrinkles called gastric rugae when empty
• Muscularis externa has three layers (instead of the two seen elsewhere)– Outer longitudinal, middle circular, and inner oblique
layers
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Microscopic Anatomy
Figure 25.13a
(a) Stomach wall
Lumen of stomach
Epithelium
Gastric pit
Gastric gland
Muscularis mucosae
Artery
Oblique layer of muscle
Circular layer of muscle
Mucosa
Submucosa
Serosa
Lamina propria
Muscularis
externa
Longitudinal layer
of muscle
Vein
Lymphatic nodule
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Microscopic Anatomy
• Gastric pits—depressions in gastric mucosa
– Lined with simple columnar epithelium
– Two or three tubular glands open into the bottom of
each gastric pit
• Cardiac glands in cardiac region
• Pyloric glands in pyloric regions
• Gastric glands in the rest of the stomach
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Gastric Pits
Figure 25.13d
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25-78
Microscopic Anatomy
• Mucous cells—secrete mucus
– Predominate in cardiac and pyloric glands
– In gastric glands, called mucous neck
cells since they are concentrated at the
neck of the gland
• Regenerative (stem) cells—found in
base of pit and in neck of gland
– Divide rapidly and produce continual
supply of new cells to replace cells that die
• Parietal cells—found mostly in the upper
half of the gland
– Secrete hydrochloric acid (HCl), intrinsic
factor, and a hunger hormone called
ghrelinFigure 25.13c
(c) Gastric gland
G cell
Mucous neck cell
Parietal cell
Chief cell
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25-79
Microscopic Anatomy
• Chief cells—most numerous
– Secrete gastric lipase and pepsinogen
– Dominate lower half of gastric glands
– Absent from pyloric and cardiac glands
• Enteroendocrine cells—concentrated
in lower end of gland
– Secrete hormones and paracrine
messengers that regulate digestion
Figure 25.13c
(c) Gastric gland
G cell
Mucous neck cell
Parietal cell
Chief cell
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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(c) Gastric gland(b) Pyloric gland
G cell
Mucous neck cell
Parietal cell
Chief cell
Mucous cell
25-80
Pyloric and Gastric Glands
Figure 25.13b,c
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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25-81
Gastric Secretions
• Gastric juice—2 to 3 L per day produced by the gastric glands
• Mainly a mixture of water, hydrochloric acid, and pepsin
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25-82
Hydrochloric Acid
• Gastric juice has a high concentration of hydrochloric acid (HCl)
– pH as low as 0.8
Figure 25.14
Blood Parietal cell Lumen of gastric gland
HCO3–
Cl–
H2CO3
H+
K+
Cl–
CO2
HCO3–
H2OCO2+
H+–K+ ATPase
Stomach
acid
Alkaline
tide
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Hydrochloric Acid
• Parietal cells produce HCl and contain carbonic anhydrase (CAH)
CAH– CO2 + H2O H2CO3 HCO3
− + H+
– H+ is pumped into gastric gland lumen by H+–K+ ATPase pump
• Antiporter uses ATP to pump H+ out and K+ in
– HCO3− exchanged for Cl− (chloride shift) from blood
plasma
• Cl− (chloride ion) pumped into the lumen of gastric gland to join H+ forming HCl
• Elevated HCO3− (bicarbonate ion) in blood causes alkaline
tide (high pH of blood leaving stomach) when digestion is occurring
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25-84
Hydrochloric Acid
• HCl activates pepsin and lingual lipase
• Breaks up connective tissues and plant cell walls– Helps liquefy food to form chyme
• Converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+)– Fe2+ absorbed and used for hemoglobin synthesis
• Contributes to nonspecific disease resistance by destroying most ingested pathogens
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25-85
Pepsin
• Zymogens—digestive enzymes secreted as inactive proteins– Converted to active enzymes by removing some of their
amino acids
• Pepsinogen—zymogen secreted by chief cells– Hydrochloric acid removes some of its amino acids and
forms pepsin that digests proteins
– Autocatalytic effect—as some pepsin is formed, it converts more pepsinogen into more pepsin
• Pepsin digests dietary proteins into shorter peptides– Protein digestion is completed in the small intestine
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25-86
The Production and Action of Pepsin
Figure 25.15
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
HCl
Parietal cell
Chief cell
Gastric gland
Pepsin
(active enzyme)
Pepsinogen
(zymogen)
Removed
peptide
Dietary
proteins
Partially digested
protein
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Gastric Lipase
• Gastric lipase—produced by chief cells
• Gastric lipase and lingual lipase play a minor role in
digesting dietary fats
– Digests 10% to 15% of dietary fats in the stomach
– Rest digested in the small intestine
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Intrinsic Factor
• Intrinsic factor—a glycoprotein secreted by
parietal cells
• Essential to absorption of vitamin B12 by the small
intestine
– Binds vitamin B12 and then intestinal cells absorb this
complex by receptor-mediated endocytosis
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25-89
Intrinsic Factor
• Vitamin B12 is needed to synthesize
hemoglobin
– Deficiency causes anemia
• Secretion of intrinsic factor is the only
indispensable function of the stomach
– Digestion can continue if stomach is removed (gastrectomy),
but B12 supplements will be needed
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25-90
Chemical Messengers
• Gastric and pyloric glands have a variety of cells
that produce a variety of chemical messengers
– Most are hormones that enter blood and stimulate distant
cells
– Some are paracrine secretions that stimulate neighboring
cells
– Several are peptides produced in both the digestive tract
and the central nervous system: gut–brain peptides
• Substance P, vasoactive intestinal peptide (VIP), secretin,
gastric inhibitory peptide (GIP), cholecystokinin, and
neuropeptide Y (NPY)
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25-91
Gastric Motility
• Swallowing center of medulla oblongata
signals stomach to relax
• Vagus nerve relays message from medulla and
activates a receptive-relaxation response in
stomach
– Resists stretching briefly, but relaxes to hold more food
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Gastric Motility
• Soon stomach shows a rhythm of peristaltic
contractions controlled by pacemaker cells in
longitudinal layer of muscularis externa
– A ring of constriction every 20 seconds
– Becomes stronger contraction at pyloric region
– After 30 minutes or so these contractions become quite
strong
• They churn the food, mix it with gastric juice, and
promote its physical breakup and chemical digestion
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25-93
Gastric Motility
(Continued)
– Thick muscularis of antrum acts as a strong pump that
breaks up semidigested food and prepares it for intestine
– Antral contractions come in waves that churn and break
up the chyme into small particles
– Only about 3 mL of chyme is squirted into the duodenum
at a time; this small amount allows duodenum to:
• Neutralize the stomach acid
• Digest nutrients little by little
– If duodenum is overfilled it inhibits gastric motility
– Typical meal emptied from stomach in 4 hours
• Less time if the meal is more liquid
• As long as 6 hours for a high-fat meal
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Vomiting
• Vomiting—forceful ejection of stomach and
intestinal contents (chyme) from the mouth
• Emetic center in the medulla oblongata integrates
multiple muscle actions
• Vomiting induced by:
– Overstretching of the stomach or duodenum
– Chemical irritants such as alcohol and bacterial toxins
– Visceral trauma
– Intense pain or psychological and sensory stimuli
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25-95
Vomiting
• Vomiting is usually preceded by nausea and
retching
• Retching—thoracic expansion and abdominal
contraction creates a pressure difference that
dilates the esophagus
– Lower esophageal sphincter relaxes while the stomach
and duodenum contract spasmodically
– Chyme enters esophagus but then drops back to the
stomach as the stomach relaxes
– Does not get past the upper esophageal sphincter
– Usually accompanied by tachycardia, profuse salivation,
and sweating
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25-96
Vomiting
• Vomiting—occurs when abdominal contractions and
rising thoracic pressure force the upper esophageal
sphincter to open
– Esophagus and body of the stomach relax
– Chyme is driven out of the stomach and mouth by
strong abdominal contractions combined with
reverse peristalsis of gastric antrum and duodenum
• Projectile vomiting—sudden vomiting with no prior
nausea or retching
– Common in infants after feeding
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Vomiting
• Chronic vomiting
– Results in dangerous fluid, electrolyte, and acid–base
imbalances
– Bulimia: eating disorder; hydrochloric acid in vomit
causes tooth enamel to erode
– Aspiration (inhalation) of acid is very destructive to the
respiratory tract
– Surgical anesthesia may induce nausea and must be
preceded by fasting until the stomach and small intestine
are empty
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Digestion and Absorption
• Salivary and gastric enzymes partially digest
protein and lesser amounts of starch and fat
in the stomach
• Most digestion and nearly all absorption occur
after the chyme has passed into the small
intestine
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Digestion and Absorption
• Stomach does not absorb any significant
amount of nutrients
– But does absorb aspirin and some lipid-soluble drugs
• Alcohol is absorbed mainly by small intestine
– Intoxicating effects depend partly on how rapidly the
stomach is emptied
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25-100
Protection of the Stomach
• Stomach is protected in three ways from the harsh
acidic and enzymatic environment it creates
– Mucous coat: thick, highly alkaline mucus resists action
of acid and enzymes
– Tight junctions between epithelial cells prevent gastric
juice from seeping between them and digesting deeper
tissue
– Epithelial cell replacement: cells live only 3 to 6 days
• Sloughed off into the chyme and digested with food
• Replaced rapidly by cell division in gastric pits
• Breakdown of these protective measures can result
in inflammation and peptic ulcer
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Peptic Ulcer
• Gastritis, inflammation of the stomach, can lead to a peptic ulcer as
pepsin and hydrochloric acid erode the stomach wall.
• Most ulcers are caused by acid-resistant bacteria Helicobacter
pylori, that can be treated with antibiotics and Pepto-Bismol.25-101
Figure 25.16a,b
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Regulation of Gastric Function
• Nervous and endocrine systems collaborate
– Increase gastric secretion and motility when food is
eaten; suppress them when the stomach empties
• Gastric activity is divided into three phases
– Cephalic phase: stomach being controlled by brain
– Gastric phase: stomach controlling itself
– Intestinal phase: stomach being controlled by small
intestine
• Phases overlap and can occur simultaneously
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25-103
Regulation of Gastric Function
Figure 25.17
0+
+
+
0
Vagus nerveVagus nerveVagus nerve
Long (vagovagal) reflex:
Sensory fibers
Motor fibers
GastrinHistamine
Intestinal gastrin
Sympathetic nerve
1 2 3
Stimulation
Inhibition
Reduced or no effect
Key
+
Sensory and
mental input
Secretin
and CCK
Intestinal phase
Intestinal gastrin briefly stimulates the
stomach, but then secretin, CCK, and the
enterogastric reflex inhibit gastric secretion
and motility while the duodenum processes
the chyme already in it. Sympathetic nerve
fibers suppress gastric activity, while vagal
(parasympathetic) stimulation of the
stomach is now inhibited.
Gastric phase
Food stretches the stomach and
activates myenteric and
vagovagal reflexes. These
reflexes stimulate gastric
secretion. Histamine and gastrin
also stimulate acid and enzyme
secretion.
Cephalic phase
Vagus nerve stimulates
gastric secretion even
before food is swallowed.
–
++
– –
–
Enterogastric
reflex
Short
(myenteric)
reflex
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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25-104
Regulation of Gastric Function
• Cephalic phase– Stomach responds to sight, smell, taste, or thought of
food
– Sensory and mental inputs converge on hypothalamus
– Hypothalamus relays signals to medulla oblongata
– Vagus nerve fibers from medulla stimulate the enteric nervous system of stomach, stimulating gastric secretion
– 40% of stomach’s acid secretion occurs in cephalic phase
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25-105
Regulation of Gastric Function
• Gastric phase
– Period in which swallowed food and semi-digested
protein activate gastric activity
• Two-thirds of gastric secretion and one-half of acid
secretion occur in this phase
– Ingested food stimulates gastric activity in two ways
• By stretching the stomach
– Activates short reflex mediated through myenteric plexus
– Activates long reflex mediated through the vagus nerves
and the brainstem
• By increasing the pH of its contents
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25-106
Regulation of Gastric Function
(Continued)
– Gastric secretion is stimulated by three chemicals
• Acetylcholine (ACh)—secreted by parasympathetic
nerve fibers of both reflexes
• Histamine—a paracrine secretion from enteroendocrine
cells in the gastric glands
• Gastrin—a hormone produced by the enteroendocrine
G cells in pyloric glands
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25-107
Regulation of Gastric Function
• Intestinal phase
– Duodenum responds to arriving chyme and moderates
gastric activity through hormones and nervous reflexes
– Duodenum initially enhances gastric secretion, but soon
inhibits it
• Stretching of duodenum accentuates vagovagal reflex that
stimulates stomach
• Peptides and amino acids in chyme stimulate G cells of
duodenum to secrete more gastrin, further stimulating stomach
• Soon acids and fats trigger enterogastric reflex—duodenum sends
inhibitory signals to stomach by way of enteric nervous system
• Duodenum also signals medulla to: inhibit vagal nuclei (reducing
vagal stimulation of stomach) and stimulate sympathetic neurons
(sending inhibitory signals to the stomach)
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25-108
Regulation of Gastric Function
Intestinal phase (Continued)
– Chyme also stimulates duodenal enteroendocrine cells
to release secretin and cholecystokinin
• They stimulate the pancreas and gallbladder
• Also suppress gastric secretion
– Gastrin secretion declines and pyloric sphincter
contracts tightly to limit chyme entering duodenum
• Gives duodenum time to work on chyme
– Enteroendocrine cells also secrete glucose-dependent
insulinotropic peptide (GIP) originally called gastrin-
inhibiting peptide
• Stimulates insulin secretion in preparation for processing nutrients
about to be absorbed by small intestine
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Feedback Control of Gastric Secretion
Pyloric gland Gastric gland
G cells secretegastrin
G cell
Partially digested protein Pepsin (active enzyme)
Mucous cell
Parietal cell
Chief cell
HCI Pepsinogen
(zymogen)
HCI converts
pepsinogen
to pepsin
Pepsin digests
dietary protein
Oligopeptides
and amino
acids buffer
stomach acid
Oligopeptides
directly
stimulate
G cells
Elevated pH
stimulates
G cells
Ingested food
buffers
stomach acid
Chief cells secrete
pepsinogen
Parietal
cells
secrete
HCI
Gastrin
stimulates
chief cells
and
parietal
cells
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 25.18
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The Liver, Gallbladder, and Pancreas
• Expected Learning Outcomes
– Describe the gross and microscopic anatomy of the liver,
gallbladder, bile duct system, and pancreas.
– Describe the digestive secretions and functions of the
liver, gallbladder, and pancreas.
– Explain how hormones regulate secretion by the liver and
pancreas.
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25-111
The Liver, Gallbladder, and Pancreas
• Small intestine receives chyme from stomach
and secretions from liver and pancreas
– These secretions enter digestive tract near the junction
of stomach and small intestine
– These secretions are important to the digestive process
of the small intestine
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The Liver
• Liver—reddish brown gland located immediately
inferior to the diaphragm
• The body’s largest gland
– Weighs about 1.4 kg (3 lb)
• Variety of functions
– Secretes bile which contributes to digestion
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25-113
Gross Anatomy of Liver
• Four lobes—right, left, quadrate, and caudate
– Falciform ligament separates left and right lobes
• Sheet of mesentery that suspends the liver from the
diaphragm
– Round ligament (ligamentum teres)—fibrous remnant
of umbilical vein
• Carries blood from umbilical cord to liver of the fetus
• From inferior view, squarish quadrate lobe next to
the gallbladder and a tail-like caudate lobe
posterior to that
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Gross Anatomy of Liver
• Porta hepatis—irregular opening between quadrate and caudate lobes– Point of entry for hepatic portal vein and proper
hepatic artery
– Point of exit for the bile passages
– All travel in lesser omentum
• Gallbladder—adheres to a depression on the inferior surface of the liver, between right and quadrate lobes
• Bare area on superior surface where it attaches to diaphragm
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25-115
Gross Anatomy of the Liver
Figure 25.19b,c
Right lobe
Left lobe
Porta hepatis:Round ligament
(c) Inferior view
Anterior
Posterior
Gallbladder
Inferior vena cavaBare area
Right lobe
Caudate lobe
Proper hepatic artery
Hepatic portal vein
Quadrate lobe
Falciform
ligament
(b) Anterior view
Common hepatic
duct
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Microscopic Anatomy of the Liver
Figure 25.20a
Stroma
Bile ductule
(a)
Hepatocytes
Stroma
Hepatic triad:
Central vein
Branch of
hepatic
portal vein
Branch of
proper hepatic
artery
Bile
canaliculi
Hepatic
sinusoid
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Microscopic Anatomy of the Liver
• Hepatic lobules—tiny cylinders that fill the interior
of the liver
– About 2 mm long and 1 mm in diameter
– Central vein: passes down the core
– Hepatocytes: cuboidal cells surrounding central vein in
radiating sheets or plates
• Each plate of hepatocytes is an epithelium one or two cells
thick
– Hepatic sinusoids: blood-filled channels that fill spaces
between the plates
• Lined by a fenestrated endothelium that separates
hepatocytes from blood cells
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Microscopic Anatomy of the Liver
(Continued)
• Allows plasma into the space between the hepatocytes
and endothelium
• Hepatocytes have brush border of microvilli that project
into this space
• Blood filtered through the sinusoids comes directly from
the stomach and intestines
– Hepatic macrophages (Kupffer cells): phagocytic
cells in the sinusoids that remove bacteria and debris
from the blood
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Microscopic Anatomy of the Liver
• Hepatocytes
– After a meal, hepatocytes absorb from the blood:
glucose, amino acids, iron, vitamins, and other nutrients
for metabolism or storage
– Between meals, hepatocytes break down stored
glycogen and release glucose into the blood
– Remove and degrade: hormones, toxins, bile pigments,
and drugs
– Secrete into the blood: albumin, lipoproteins, clotting
factors, angiotensinogen, and other products
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Microscopic Anatomy of the Liver
• Hepatic lobules are separated by a sparse
connective tissue called stroma
• Between lobules is a hepatic triad of two vessels
and a bile ductule
– Other vessel: branch of hepatic portal vein
– One vessel: branch of hepatic artery proper
– Both vessels supply blood to sinusoids which receive a
mixture of nutrient-laden venous blood from the
intestines, and freshly oxygenated arterial blood from
the celiac trunk
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Microscopic Anatomy of the Liver
• After filtering through the sinusoids, blood is
collected in the central vein
• Ultimately flows into the right and left hepatic
veins
• Blood leaves the liver at its superior surface and
immediately drains into the inferior vena cava
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Microscopic Anatomy of the Liver
• Bile canaliculi—narrow channels into which the
liver secretes bile
– Bile passes into bile ductules of the triads
– Ultimately into the right and left hepatic ducts
– Common hepatic duct: formed from convergence of
right and left hepatic ducts on inferior side of the liver
– Cystic duct coming from gallbladder joins common
hepatic duct
– Bile duct: formed from union of cystic and common
hepatic ducts
• Descends through lesser omentum toward the duodenum
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Microscopic Anatomy of the Liver
(Continued)
– Near duodenum, bile duct joins duct of pancreas
– Forms expanded chamber: hepatopancreatic ampulla
• Terminates in a fold of tissue—major duodenal papilla on
duodenal wall
– Major duodenal papilla contains muscular
hepatopancreatic sphincter (sphincter of Oddi)
• Regulates passage of bile and pancreatic juice into
duodenum
• Between meals, sphincter closes and prevents release of
bile into the intestines
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Microscopic Anatomy of the Liver
Figure 25.20b
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Gross Anatomy of the Gallbladder,
Pancreas, and Bile Passages
Hepatic ducts
Common hepatic duct
Cystic duct
Bile duct
Gallbladder:NeckBodyHead
Pancreatic duct
Jejunum
Duodenum
Circular folds
Pancreas:TailBodyHead
Minor duodenal
papilla
Hepatopancreatic
sphincter
Major duodenal
papilla
Hepatopancreatic
ampulla
Accessory
pancreatic duct
Duodenojejunal
flexure
Figure 25.21
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The Gallbladder and Bile
• Gallbladder—a pear-shaped sac on underside of
liver
– Serves to store and concentrate bile by absorbing
water and electrolytes
– About 10 cm long
– Internally lined by highly folded mucosa with simple
columnar epithelium
– Head (fundus) usually projects slightly beyond inferior
margin of liver
– Neck (cervix) leads into the cystic duct
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The Gallbladder and Bile
• Bile—yellow-green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments, and bile acids– Bilirubin: principal pigment derived from the
decomposition of hemoglobin
– Bacteria in large intestine metabolize bilirubin to urobilinogen
• Responsible for the brown color of feces
– Bile acids (bile salts): steroids synthesized from cholesterol
• Bile acids and lecithin, a phospholipid, aid in fat digestion and absorption
– Gallstones may form if bile becomes excessively concentrated with wastes
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The Gallbladder and Bile
(Continued)
– Bile gets to the gallbladder by first filling the bile duct
then overflowing into the gallbladder
– Liver secretes about 500 to 1,000 mL of bile daily
– 80% of bile acids are reabsorbed in the ileum and
returned to the liver
• Hepatocytes absorb and resecrete them
• Enterohepatic circulation—route of secretion, reabsorption,
and resecretion of bile acids two or more times during digestion
of an average meal
– 20% of the bile acids are excreted in the feces
• Body’s only way of eliminating excess cholesterol
• Liver synthesizes new bile acids from cholesterol to replace
those lost in feces
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The Gallbladder and Bile
• Gallstones (biliary calculi)—hard masses in either
the gallbladder or bile ducts
– Composed of cholesterol, calcium carbonate, and
bilirubin
• Cholelithiasis—formation of gallstones
– Most common in obese women over 40 - excess
cholesterol
• Painful obstruction of ducts– Result in jaundice (yellowing of skin), poor fat digestion,
and impaired absorption of fat-soluble vitamins
• Lithotripsy—use of ultrasonic vibration to pulverize stones without surgery
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The Pancreas
• Pancreas—spongy retroperitoneal gland posterior
to greater curvature of stomach
– Measures 12 to 15 cm long, and 2.5 cm thick
– Has a head encircled by duodenum, a body (midportion),
and a tail on the left
– Both an endocrine and exocrine gland
• Endocrine portion—pancreatic islets that secrete insulin
and glucagon
– Concentrated in the tail of the gland
• Exocrine portion—99% of pancreas that secretes 1,200 to
1,500 mL of pancreatic juice per day
– Secretory acini release their secretion into small ducts that
converge on the main pancreatic duct
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The Pancreas
Pancreas (Continued)
– Pancreatic duct runs lengthwise through middle of the
gland
• Joins the bile duct at the hepatopancreatic ampulla
• Hepatopancreatic sphincter controls release of both bile
and pancreatic juice into the duodenum
– Accessory pancreatic duct: smaller duct that
branches from the main pancreatic duct
• Opens independently into the duodenum
• Bypasses the sphincter and allows pancreatic juice to
be released into duodenum even when bile is not
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The Pancreas
(Continued)
– Pancreatic juice: alkaline mixture of water, enzymes,
zymogens, sodium bicarbonate, and other electrolytes
• Acini secrete the enzymes and zymogens
• Ducts secrete bicarbonate
– Bicarbonate buffers HCl arriving from the stomach
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The Pancreas
• Pancreatic zymogens are:
– Trypsinogen
• Secreted into intestinal lumen
• Converted to trypsin by enterokinase that is secreted by
mucosa of small intestine
• Trypsin is autocatalytic—converts trypsinogen into still
more trypsin
– Chymotrypsinogen: converted to trypsinogen by trypsin
– Procarboxypeptidase: converted to carboxypeptidase
by trypsin
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The Pancreas
• Other pancreatic enzymes
– Pancreatic amylase: digests starch
– Pancreatic lipase: digests fat
– Ribonuclease and deoxyribonuclease: digest RNA
and DNA respectively
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Microscopic Anatomy of the Pancreas
Figure 25.22a,b
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The Activation of Pancreatic Enzymes
in the Small Intestine
Figure 25.23
Enterokinase
Trypsinogen
Chymotrypsinogen
Procarboxypeptidase
Chymotrypsin
Carboxypeptidase
Trypsin
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Regulation of Secretion
• Three stimuli are chiefly responsible for the
release of pancreatic juice and bile:
acetylcholine, cholecystokinin, and secretin
– Acetylcholine (ACh): from vagus and enteric nerves
• Stimulates acini to secrete enzymes during cephalic phase
of gastric control even before food is swallowed
– Enzymes remain in acini and ducts until chyme enters the
duodenum
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Regulation of Secretion
(Continued)
– Cholecystokinin (CCK): secreted by mucosa of
duodenum in response to arrival of fats in small intestine
• Stimulates pancreatic acini to secrete enzymes
• Strongly stimulates gallbladder
• Induces contractions of gallbladder and relaxation of
hepatopancreatic sphincter to discharge bile into duodenum
– Secretin: released from duodenum in response to acidic
chyme arriving from the stomach
• Stimulates ducts of both liver and pancreas to secrete more
sodium bicarbonate
• Raises pH to the level required for activity of the pancreatic
and intestinal digestive enzymes
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The Small Intestine
• Expected Learning Outcomes
– Describe the gross and microscopic anatomy of the small
intestine.
– State how the mucosa of the small intestine differs from
that of the stomach, and explain the functional significance
of the differences.
– Define contact digestion and describe where it occurs.
– Describe the types of movement that occur in the small
intestine.
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The Small Intestine
• Nearly all chemical digestion and nutrient
absorption occurs in the small intestine
• The longest part of the digestive tract
– About 5 m long in a living person
– Up to 8 m long in a cadaver - no muscle tone
• “Small” intestine refers to the diameter—not
length
– Diameter is about 2.5 cm (1 in.)
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The Small Intestine
Figure 25.24
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Gross Anatomy
• Small intestine—coiled mass filling most of the
abdominal cavity inferior to stomach and liver
• Small intestine divided into three regions:
duodenum, jejunum, and ileum
– Duodenum: first 25 cm (10 in.)
• Begins at pyloric valve
– Major and minor duodenal papilla distal to pyloric valve
– Receives major and minor pancreatic ducts respectively
• Arches around head of the pancreas
• Ends at a sharp bend called the duodenojejunal flexure
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Gross Anatomy
Small intestine regions
Duodenum (Continued)
• Most is retroperitoneal
• Receives stomach contents, pancreatic juice, and bile
• Stomach acid is neutralized here
• Fats are physically broken up (emulsified) by bile acids
• Pepsin is inactivated by increased pH
• Pancreatic enzymes perform chemical digestion
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Gross Anatomy
• Small intestine regions (Continued)
– Jejunum: first 40% of small intestine beyond duodenum
• Roughly 1.0 to 1.7 m in a living person
• Has large, tall, closely spaced circular folds
• Its wall is relatively thick and muscular
• Especially rich blood supply which gives it a red color
• Most digestion and nutrient absorption occurs here
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Gross Anatomy
Small intestine regions (Continued)
– Ileum: forms last 60% of the postduodenal small intestine
• About 1.6 to 2.7 m
• Thinner, less muscular, less vascular, and paler pink color
• Peyer patches—prominent lymphatic nodules in clusters on
the side opposite the mesenteric attachment
– Visible to naked eye; become larger near large intestine
• Ileocecal junction—end of the small intestine
– Where the ileum joins the cecum of the large intestine
• Ileocecal valve—a sphincter formed by the thickened muscularis of the ileum
– Protrudes into the cecum
– Regulates passage of food residue into the large intestine
– Both jejunum and ileum are intraperitoneal and covered with serosa
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Microscopic Anatomy
• Small intestine tissues designed for nutrient
digestion and absorption
– Lumen lined with simple columnar epithelium
– Muscularis externa is noted for a thick inner circular layer
and a thinner outer longitudinal layer
– Large internal surface area - great length and three
types of internal folds or projections
• Circular folds (plicae circulares)—increase surface area
by a factor of 2 to 3
• Villi—increase surface area by a factor of 10
• Microvilli—increase the surface area by a factor of 20
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Microscopic Anatomy
• Circular folds (plicae circulares)—largest folds of
intestinal wall
– Up to 10 mm high
– Involve only mucosa and submucosa
– Occur from duodenum to middle of ileum
• Relatively small and sparse in ileum; not found in distal half,
as most nutrient absorption is completed by this point
– Cause chyme flow in spiral path causing more contact
with mucosa
– Promote more thorough mixing and nutrient absorption
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Microscopic Anatomy
• Villi—finger-like projections 0.5 to 1 mm tall
– Make mucosa look fuzzy
– Villus covered with two types of epithelial cells
• Absorptive cells (enterocytes)
• Goblet cells—secrete mucus
– Epithelia joined by tight junctions that prevent digestive
enzymes from seeping between them
– Core of villus filled with areolar tissue of lamina propria
• Contains arteriole, capillaries, venule, and lymphatic
capillary called a lacteal
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Microscopic Anatomy
Figure 25.25c
(c)
Absorptive cell
Lacteal
Capillary network
Goblet cell
Intestinal crypts
Arteriole
Paneth cell
Villi
Brush border
of microvilli
Lymphatic vessel
Venule
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Microscopic Anatomy
• Microvilli—form a fuzzy brush border on apical
surface of each absorptive cell
– About 1 μm high
– Increases absorptive surface area
• Brush border enzymes—contained in plasma
membrane of microvilli
– Carry out some of the final stages of enzymatic digestion
– Not released into the lumen
– Contact digestion: chyme must contact the brush border
for digestion to occur
– Intestinal churning of chyme ensures contact with the
mucosa
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• Intestinal crypts (crypts of Lieberkühn)—numerous
pores that open into tubular glands on the floor of the
small intestine between bases of the villi
– Similar to gastric glands
– In upper half, have enterocytes and goblet cells like the villi
– In lower half, dominated by dividing stem cells
• Life span of 3 to 6 days
• New epithelial cells migrate up the crypt to the tip of the villus
where they are sloughed off and digested
– A few Paneth cells are clustered at the base of each crypt
• Secrete lysozyme, phospholipase, and defensins—defensive
proteins that resist bacterial invasion of the mucosa
Microscopic Anatomy
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Intestinal Villi
Figure 25.25a–c
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• Duodenal glands—in submucosa of duodenum
– Secrete an abundance of bicarbonate-rich mucus
– Neutralize stomach acid and shield the mucosa from its
erosive effects
• Large population of lymphocytes throughout
lamina propria and submucosa of small intestine
– Intercept pathogens before they can invade bloodstream
– Aggregated into lymphatic nodules in ileum: Peyer
patches
Microscopic Anatomy
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Intestinal Secretion
• Intestinal crypts secrete 1 to 2 L of intestinal
juice per day
– In response to acid, hypertonic chyme, and distension of
intestines
– pH of 7.4 to 7.8
– Contains water, mucus, and little enzyme
• Most enzymes that function in small intestine are found in
brush border and pancreatic juice
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Intestinal Motility
• Contractions of small intestine serve three
functions
– To mix chyme with intestinal juice, bile, and
pancreatic juice
• To neutralize acid
• Digest nutrients more effectively
– To churn chyme and bring it in contact with the
mucosa for contact digestion and nutrient absorption
– To move residue toward large intestine
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Intestinal Motility
• Segmentation—movement in which stationary ring-
like constrictions appear in several places along the
intestine
– They relax and new constrictions form elsewhere
– Most common kind of intestinal contraction
– Pacemaker cells in muscularis externa set rhythm of
segmentation
• Contractions about 12 times per minute in the duodenum
• 8 to 9 times per minute in the ileum
• When most nutrients have been absorbed and little remains
but undigested residue, segmentation declines and
peristalsis begins
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Contractions of the Small Intestine
• Purpose of segmentation is to mix and churn, not to move material along as in peristalsis
Figure 25.26a(a) Segmentation
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Intestinal Motility
• Peristalsis moves contents of small intestine toward
colon
• Peristaltic wave begins in duodenum, travels 10 to 70 cm
and dies out
• Followed by another wave starting further down the tract
• Migrating motor complex—successive, overlapping waves
of contraction
• Milk chyme toward colon over a period of 2 hours
Figure 25.26b
(b) Peristalsis
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Intestinal Motility
(Continued)
• Ileocecal valve usually closed
– Food in stomach triggers gastroileal reflex that enhances
segmentation in the ileum and relaxes the valve
– As cecum fills with residue, pressure pinches the valve shut
• Prevents reflux of cecal contents into the ileum
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Chemical Digestion and Absorption
• Expected Learning Outcomes
– Describe how each major class of nutrients is
chemically digested, name the enzymes involved, and
discuss the functional differences among these
enzymes.
– Describe how each type of nutrient is absorbed by the
small intestine.
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Carbohydrates
• Starch—most digestible dietary carbohydrate
– Cellulose is indigestible
– Starch is first digested to oligosaccharides (up to eight
glucose residues long)
– Oligosaccharides then digested to the disaccharide
maltose
– Maltose finally digested to glucose which is absorbed
by the small intestine
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Carbohydrates
• Process begins in the mouth
– Salivary amylase hydrolyzes starch into
oligosaccharides
– Amylase works best at pH of 6.8 to 7.0 of oral cavity
– Amylase quickly denatured on contact with stomach
acid and digested by pepsin
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Carbohydrates
– About 50% of dietary starch is digested before it reaches small intestine
– Pancreatic amylase resumes starch digestion in intestine
Figure 25.27
Starch
Contact digestion
Absorption
Glucose
Pancreatic
amylase
Maltose and
oligosaccharides
Maltase, dextrinase,
and glucoamylase
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Carbohydrates
• When reaching small intestine, pancreatic amylase
quickly converts starch to oligosaccharides and
maltose
• Brush border enzymes continue carbohydrate
digestion:
– Dextrinase and glucoamylase hydrolyze oligosaccharides
– Maltase hydrolyzes maltose (a disaccharide)
– Sucrase and lactase hydrolyze the disaccharides sucrose
and lactose
• In most people, lactase production stops in childhood
– Monosaccharides produced by disaccharide hydrolysis
(such as glucose) are immediately absorbed
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Carbohydrates
• Plasma membrane of absorptive cells has transport proteins that absorb monosaccharides as soon as brush border enzymes release them
• 80% of absorbed sugar is glucose– Taken up by sodium–glucose transport (SGLT)
proteins
– Glucose is transported out the base of absorptive cell into ECF by facilitated diffusion
– Sugar entering ECF increases its osmolarity
– Draws water osmotically from lumen of intestine, through leaky tight junctions between epithelial cells
– Water carries more glucose and other nutrients with it by solvent drag
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Carbohydrates
• SGLT also absorbs galactose
• Fructose is absorbed by facilitated diffusion (by a different carrier protein) and converted to glucose
• Glucose, galactose, and any remaining fructose are transported out of the base of the cell by facilitated diffusion
• Absorbed by blood capillaries in the villus
• Hepatic portal system delivers them to the liver
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Lactose Intolerance
• In people without lactase, lactose passes undigested into large intestine– Increases osmolarity of intestinal contents
– Causes water retention in the colon and diarrhea
– Gas production by bacterial fermentation of the lactose
• Occurs in many people– 15% of American whites, 90% of American blacks, 70%
of Mediterraneans; and nearly all of Asian descent
• Can consume yogurt and cheese since bacteria have broken down the lactose
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Proteins
• Amino acids absorbed by the small intestine come
from three sources
– Dietary proteins
– Digestive enzymes digested by each other
– Sloughed epithelial cells digested by enzymes
• Endogenous amino acids from last two sources
total about 30 g/day
• Exogenous amino acids from our diet total about
44 to 60 g/day
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Proteins
• Proteases (peptidases)—enzymes that digest
proteins
– Begin their work in stomach in optimum pH of 1.5 to 3.5
– Pepsin hydrolyzes any peptide bond between tyrosine
and phenylalanine
• Pepsin digests 10% to 15% of dietary protein into shorter
peptides and some free amino acids
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Proteins
• Protein digestion continues in small intestine
– Pepsin inactivated when it passes into the duodenum
and mixes with alkaline pancreatic juice (pH 8)
– Pancreatic enzymes trypsin and chymotrypsin take
over the process
– Hydrolyze polypeptides into even shorter oligopeptides
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Proteins
(Continued)
– Oligopeptides taken apart one amino acid at a time by
three more enzymes
• Carboxypeptidase—removes amino acids from – COOH
end of the chain
– Carboxypeptidase is a pancreatic secretion
• Aminopeptidase—removes amino acids from –NH2 end
• Dipeptidase—splits dipeptides in the middle and release
two free amino acids
– Aminopeptidase and dipeptidase are brush border enzymes
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Protein Digestion and Absorption
• Pancreatic enzymes take over protein digestion in small intestine by hydrolyzing polypeptides into shorter oligopeptides
Figure 25.29
OC
HO
HN
H
OC
HO
HN
H
OC
OH
H
HN
H
HN
OC
OH
OC
HO N
HN
H
CO OH
H
HN
OC
OH
H
HN
OC
OH
OC
OHH
HNH
NH
CO OH
OC
OH
H
HN
OC
OHH
HN
OC
OH
H
HN
OC
OH
H
HN
OC
OH
H
HN
OC
OH
H
HN
Polypeptides Oligopeptides
Trypsin ( ) and chymotrypsin ( )
hydrolyze other peptide bonds, breaking
polypeptides down into smaller
oligopeptides.
Carboxypeptidase ( ) removes one
amino acid at a time from the carboxyl
(–COOH) end of an oligopeptide.
Small intestine
Actions of pancreatic enzymes
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Protein Digestion and Absorption
• Brush border enzymes finish task, producing free amino acids
that are absorbed into intestinal epithelial cells
– Sodium-dependent amino acid cotransporters move amino acids
into epithelial cells
– Facilitated diffusion moves amino acids out into bloodstream
• Infants absorb proteins by pinocytosis (maternal IgA) and
release them into the blood by exocytosis
Figure 25.29
OC
OH
H
HN
OC
OH
H
HN O
COH
H
HN
Carboxypeptidase Aminopeptidase Dipeptidase
Carboxypeptidase ( ) of the brush
border continues to remove amino acids
from the carboxyl (–COOH) end.
Aminopeptidase ( ) of the brush border
removes one amino acid at a time from
the amino (–NH) end.
Dipeptidase ( ) splits dipeptides ( )
into separate amino acids ( ).
Small intestine
Actions of brush border enzymes (contact digestion)
Blood capillary
of intestinal villus
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Lipids
• Hydrophobicity of lipids makes their digestion
and absorption complicated
• Lipases—fat-digesting enzymes
– Lingual lipase secreted by intrinsic salivary glands of the
tongue
• Active in mouth, but more active in stomach along with
gastric lipase
• 10% to 15% of lipids digested before reaching duodenum
– Before digestion in duodenaum, vigorous pumping in
stomach’s antrum emulsifies the fat (breaks up globs)
• Emulsification droplets are passed to small intestine
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Lipids
(Continued)
– Emulsification droplets are broken down further by bile,
lecithin, and agitation produced by intestinal segmentation
• Exposes more fat surface to enzymatic action
– There is enough pancreatic lipase in the small intestine
after a meal to digest the average daily fat intake in as little
as 1 to 2 minutes
– Lipase acts on triglycerides
• Removes first and third fatty acids from glycerol backbone,
but leaves the middle one
• The product of lipase action are two free fatty acids (FFAs)
and a monoglyceride
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Fat Digestion and Absorption
Figure 25.30
Emulsification
Bile acid
Lecithin Hydrophilic region
Hydrophobic region
Fat globule
Emulsification
droplets
Fat globule is broken up and coated by
lecithin and bile acids.
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Lipids
• Absorption of free fatty acids, monoglycerides, and
other lipids depends on minute droplets in the bile
called micelles
– Made in the liver
– Consist of 20 to 40 bile acid molecules aggregated with
their hydrophilic side groups facing outward and their
hydrophobic steroid rings facing inward
– Bile phospholipids and cholesterol diffuse into the
center of the micelle to form its core
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Lipids
(Continued)
– Micelles pass down the bile duct into the duodenum
• There they absorb fat-soluble vitamins, cholesterol, and
the FFAs and monoglycerides produced by fat digestion
– They transport lipids to the surface of the intestinal
absorptive cells
– Lipids leave the micelles and diffuse through the plasma
membrane into the cells
– Micelles are reused, picking up another cargo of lipid,
transporting them to the absorptive cells
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Fat Digestion and Absorption
Figure 25.30
Fat hydrolysis
Lipid uptake by micelles
Pancreatic lipasePancreatic lipase
Monoglyceride
Free fatty acid
Free fatty acid
Lecithin
Bile acid
Dietary lipid
Lipid core
MicellesFatty acids Fat-soluble vitamins
Bile acid
Monoglycerides Cholesterol
Emulsification droplets are acted upon by
pancreatic lipase, which hydrolyzes the
first and third fatty acids from triglycerides,
usually leaving the middle fatty acid.
Micelles in the bile pass to the small
intestine and pick up several types of
dietary and semidigested lipids.
Triglyceride
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Lipids
• Within the intestinal cell, free fatty acids
and monoglycerides are transported to
the smooth ER
• Resynthesized into triglycerides
• Golgi complex coats these with
phospholipids and protein to form
chylomicrons
– Packaged into secretory vesicles that
migrate to basal surface of cell
– Release their contents into core of villus
– Taken up by lacteal into lymph
– White, fatty intestinal lymph (chyle)
flows into larger and larger
lymphatic vessels until it enters the
bloodstream
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Figure 25.30
Fatty acids
Monoglycerides
Triglycerides
Phospholipids
Cholesterol
Protein shell
Chylomicron
Chylomicron formation
Micelles
Brush border
Absorptive cell
Intestinal cells absorb lipids from micelles, resynthesize
triglycerides, and package triglycerides, cholesterol, and
phospholipids into 85 protein-coated chylomicrons.
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Chylomicrons and the Lymphatics
Figure 25.30
Chylomicrons are released into the lymphatic system in the lacteals of the
villi. They enter the bloodstream when lymphatic fluid enters the subclavian
vein via the thoracic duct.
Chylomicron exocytosis and lymphatic uptake
Lacteal
Golgi complex packages chylomicrons into secretory vesicles;
chylomicrons are released from basal cell membrane by exocytosis
and enter the lacteal (lymphatic capillary) of the villus.
Chylomicrons
in lymph
Chylomicrons
in secretory
vesicles
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Summary of Nutrient Digestion
25-182Figure 25.31
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Nucleic Acids
• Nucleic acid
– Nucleases (deoxyribonuclease and ribonuclease) of
pancreatic juice hydrolyze DNA and RNA to nucleotides
– Nucleosidases and phosphatases of brush border split
them into phosphate ions, ribose or deoxyribose sugar,
and nitrogenous bases
– Membrane carriers allow absorption
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Vitamins
• Vitamins
– Absorbed unchanged
– Fat-soluble vitamins: A, D, E, and K absorbed with
other lipids
• If ingested without fat-containing food, they are not
absorbed at all, but are passed in the feces and wasted
– Water-soluble vitamins, B complex and C, absorbed
by simple diffusion and B12 if bound to intrinsic factor
from the stomach
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Minerals
• Minerals (electrolytes)
– Absorbed all along small intestine
– Na+ cotransported with sugars and amino acids
– Cl− exchanged for bicarbonate reversing chloride–
bicarbonate exchange that occurs in the stomach
– K+ absorbed by simple diffusion
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Minerals
(Continued)
– Iron and calcium absorbed as needed
• Iron absorption is stimulated by liver hormone hepcidin
• Absorptive cells bind ferrous ions (Fe2+) and internalize
them by active transport
• Unable to absorb ferric ions (Fe3+) but stomach acid
reduces ferric ions to absorbable ferrous ions
• Transferrin (extracellular protein) transports iron in
blood to bone marrow, muscle, and liver
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Minerals
• Calcium is absorbed throughout the intestine by
different mechanisms
– Transcellular absorption in the duodenum
• Enters through calcium channels in apical cell membrane
• Binds to calbindin protein so concentration gradient will
continue to favor calcium influx
• Actively transported out of base of cell into bloodstream by
calcium–ATPase and Na+–Ca2+ antiport
– Diffusion between epithelial cells in jejunum and ileum
– Most absorbed calcium is from meat and dairy
• Dietary fat retards calcium absorption
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Minerals
• Parathyroid hormone—secreted in response to a
drop in blood calcium levels
– Stimulates kidney to synthesize vitamin D from
precursors made by epidermis and liver
– Vitamin D affects absorptive cells of the duodenum in
three ways
• Increases number of calcium channels in apical membrane
• Increases the amount of calbindin in cytoplasm
• Increases the number of calcium–ATPase pumps at basal
membrane
– Parathyroid hormone increases the level of calcium in
the blood
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Water
• Digestive system is one of several systems
involved in water balance
• Digestive tract receives about 9 L of water/day
– 0.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal
secretions
– 8 L is absorbed by small intestine and 0.8 L by large
intestine
– 0.2 L voided in daily fecal output
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Water
• Water is absorbed by osmosis following the
absorption of salts and organic nutrients
• Diarrhea—occurs when large intestine absorbs too
little water
– Feces pass through too quickly if intestine is irritated
– Feces contain high concentrations of a solute (such as
lactose)
• Constipation—occurs when fecal movement is
slow, too much water gets reabsorbed, and feces
become hardened
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The Large Intestine
• Expected Learning Outcomes
– Describe the gross anatomy of the large intestine.
– Contrast the mucosa of the colon with that of the small
intestine.
– State the physiological significance of intestinal bacteria.
– Discuss the types of contractions that occur in the colon.
– Explain the neurological control of defecation.
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Introduction to Large Intestine
• Large intestine receives about 500 mL of
indigestible residue per day
– Reduces it to about 150 mL of feces by absorbing
water and salts
– Eliminates feces by defecation
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Gross Anatomy
• Large intestine
– Measures 1.5 m (5 ft) long and 6.5 cm (2.5 in.) in
diameter in cadaver
– Begins as cecum inferior to ileocecal valve
– Appendix attached to lower end of cecum
• Densely populated with lymphocytes—a significant source
of immune cells
– Ascending colon, right colic (hepatic) flexure,
transverse colon, left colic (splenic) flexure, and
descending colon frame the small intestine
– Sigmoid colon is S-shaped portion leading down into
pelvic cavity
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Gross Anatomy
(Continued)
– Rectum: portion ending at anal canal
• Has three curves and three infoldings, called the transverse rectal folds (rectal valves)
– Anal canal: final 3 cm of the large intestine
• Passes through levator ani muscle and pelvic floor, terminates at the anus
• Anal columns and sinuses—exude mucus and lubricant into anal canal during defecation
• Large hemorrhoidal veins for superficial plexus in anal columns and around orifice
• Hemorrhoids—permanently distended veins that protrude into anal canal or bulge outside the anus
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Gross Anatomy
(Continued)
– Muscularis externa of colon is unusual
• Taenia coli—longitudinal fibers concentrated in three thickened, ribbon-like strips
• Haustra—pouches in the colon caused by the muscle tone of the taeniae coli
• Internal anal sphincter—smooth muscle of muscularis externa
• External anal sphincter—skeletal muscle of pelvic diaphragm
– Omental appendages—club-like, fatty pouches of peritoneum adhering to the colon; unknown function
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Large Intestine Gross Anatomy
Haustrum
Ileum
Cecum
Appendix
Rectum
Anal canal
(a) Gross anatomy
Mesocolon
Right colic
flexure
Transverse
colon
Superior
mesenteric
artery
Ascending
colon
Ileocecal
valve
Greater
omentum
(retracted)
Left colic
flexure
Descending
colon
Omental
appendages
Sigmoid
colon
External anal
sphincter
Taeniae coli
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 25.32a
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The Large Intestine – Anal Canal
Rectal valve
Rectum
Anal canal
Anal columns
Anal sinuses
Anus
Levator ani
muscle
Hemorrhoidal
veins
Internal anal
sphincter
External anal
sphincter
(b) Anal canal
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 25.32b
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Microscopic Anatomy
• Mucosa—simple columnar epithelium
– Anal canal has nonkeratinized stratified squamous
epithelium in its lower half for abrasion resistance
• No circular folds or villi in large intestine
• Intestinal crypts—glands sunken deep into lamina
propria with a high density of mucus-secreting
goblet cells
• Lamina propria and submucosal have a lot of
lymphatic tissue
– Provides protection from large population of bacteria in
large intestine
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Intestinal Microbes and Gas
• Gut microbiome—about 800 species of bacteria
that populate the large intestine
– Bacteria digest cellulose, pectin, and other carbohydrates
for which our cells lack enzymes
– Help in synthesis of vitamins B and K
• Flatus—intestinal gas
– Average person produces 500 mL of flatus per day
• Most gas in large intestine is reabsorbed instead
– Much of flatus is swallowed air, but bacteria add to it
– Hydrogen sulfide, indole, and skatole produce odor
• Hydrogen gas may explode during electrical cauterization
used in surgery
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Absorption and Motility
• Large intestine takes about 36 to 48 hours to
reduce residue of a meal to feces
– Most time in transverse colon
– Does not chemically change the residue
– Reabsorbs water and electrolytes
• Feces consist of about 75% water and 25% solids
– Solids: 30% bacteria, 30% undigested fiber, 10% to 20%
fat, small amount of mucus, proteins, salts, digestive
secretions, and sloughed epithelial cells
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Absorption and Motility
• Haustral contractions occur every 30 minutes
– Distension of a haustrum stimulates it to contract
– Churns and mixes residue promoting water and salt
absorption
• Mass movements—stronger contractions that occur
one to three times a day
– Triggered by gastrocolic and duodenocolic reflexes
• Filling of the stomach and duodenum stimulates motility of
the colon
• Move residue several centimeters
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Defecation
• Stretching of rectum stimulates two defecation
reflexes
– Accounts for urge to defecate often felt soon after a meal
– Intrinsic defecation reflex works entirely within
myenteric plexus to produce relatively weak response
• Stretch signals travel through plexus to the muscularis,
causing it to contract and the internal sphincter to relax
– Parasympathetic defecation reflex involves spinal cord
• Stretching of rectum sends sensory signals to spinal cord
• Pelvic nerves return signals, intensifying peristalsis and
relaxing the internal anal sphincter
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Defecation
• Defecation occurs only if external anal sphincter
and puborectalis muscles are voluntarily relaxed
• Abdominal contractions (Valsalva maneuver)
increase abdominal pressure and compresses
rectum
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Neural Control of Defecation
1. Filling of the rectum
2. Reflex contraction of
rectum and relaxation
of internal anal
sphincter
3. Voluntary relaxation of
external sphincter
Figure 25.33
Anal canal
Rectum
1
1
2
2
3
3
4
4
Impulses from
cerebral cortex
Sensory
fibers
Parasympathetic
motor fibers
Voluntary
motor
fibers
Stretch
receptors
Sigmoid
colon
Stretch
receptors
Internal anal
sphincter
External anal
sphincter
Feces stretch the rectum and stimulate stretch
receptors, which transmit signals to the spinal cord.
A spinal reflex stimulates contraction of the rectum.
The spinal reflex also relaxes the internal anal sphincter.
Impulses from the brain prevent untimely defecation
by keeping the external anal sphincter contracted.
Defecation occurs only if this sphincter also relaxes.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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The Man with a Hole in His Stomach
• Canadian shot accidentally by shotgun in 1822
– A hole “large enough to receive forefinger” covered by a
flap of tissue remained after wound healed
• Physician William Beaumont experimented on
him
– Removed samples and made observations
• Proved digestion required HCl
– Published book in 1833 that laid foundation for modern
gastric physiology