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prepared by
Janice Meeking,
Mount Royal College
CHAPTER
23
The Digestive
System: Part A
Copyright © 2010 Pearson Education, Inc.
Digestive System
•
Two groups of organs
1. Alimentary canal (gastrointestinal or GI tract)
•
Digests and absorbs food
•
Mouth, pharynx, esophagus, stomach,
small intestine, and large intestine
•
continuous muscular digestive tube
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Digestive System
2. Accessory digestive organs
•
aid digestion physically and produce
secretions that break down foodstuff in
the GI tract
•
Teeth, tongue, gallbladder
•
Digestive glands
•
Salivary glands
•
Liver
•
pancreas
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Mouth (oral cavity)
Tongue
Esophagus
Liver
Gallbladder
Duodenum
Jejunum
Small
intestine Ileum
Anus
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Parotid gland
Sublingual gland Salivary
Submandibular
glands
gland
Pharynx
Stomach
Pancreas
(Spleen)
Transverse colon
Descending colon
Ascending colon
Large
Cecum
intestine
Sigmoid colon
Rectum
Vermiform appendix
Anal canal
Figure 23.1
Digestive Processes- 6 Essential Activities
1. Ingestion- act of putting food into the mouth
2. Propulsion-moves food through the alimentary canal and
includes both swallowing and peristalsis
3. Mechanical digestion-physical process of preparing the food
for chemical digestion and involves chewing, mixing, churning,
and segmentation
4. Chemical digestion-series of catabolic steps in which complex
food molecules are broken down to their chemical building
blocks by enzymes
5. Absorption-passage of digested end products from the lumen
of the GI tract through the mucosal cells into the blood or
lymph
6. Defecation-eliminates indigestible substances from the body
via the anus as feces
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Ingestion
Mechanical
digestion
• Chewing (mouth)
• Churning (stomach)
• Segmentation
(small intestine)
Chemical
digestion
Food
Pharynx
Esophagus
Propulsion
• Swallowing
(oropharynx)
• Peristalsis
Stomach (esophagus,
stomach,
small intestine,
large intestine)
Absorption
Lymph
vessel
Small
intestine
Large
intestine
Defecation
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Blood
vessel
Mainly H2O
Feces
Anus
Figure 23.2
From
mouth
(a) Peristalsis: Adjacent segments of
alimentary tract organs alternately contract
and relax, which moves food along the tract
distally.
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(b) Segmentation: Nonadjacent segments
of alimentary tract organs alternately
contract and relax, moving the food
forward then backward. Food mixing and
slow food propulsion occurs.
Figure 23.3
GI tract regulatory mechanisms
1. Mechanoreceptors and chemoreceptors
•
Respond to stretch, changes in osmolarity
and pH, and presence of substrate and end
products of digestion
•
Initiate reflexes that
•
Activate or inhibit digestive glands
•
Stimulate smooth muscle to mix and
move lumen contents
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GI tract regulatory mechanisms
2. Intrinsic and extrinsic controls
•
Enteric nerve plexuses (gut brain) initiate
short reflexes in response to stimuli in the GI
tract
•
Long reflexes in response to stimuli inside or
outside the GI tract involve CNS centers and
autonomic nerves
•
Hormones from cells in the stomach and
small intestine stimulate target cells in the
same or different organs
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Peritoneum and Peritoneal Cavity
• Peritoneum: serous membrane of the
abdominal cavity
• Visceral peritoneum on external surface of
most digestive organs
• Parietal peritoneum lines the body wall of the
abdominopelvic cavity
• Peritoneal cavity
• Between the two peritoneums
• Serous fluid lubricates mobile organs
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Abdominopelvic
cavity
Vertebra
Dorsal
mesentery
Parietal
peritoneum
Ventral
mesentery
Visceral
peritoneum
Peritoneal
cavity
Alimentary
canal organ
Liver
(a) Schematic cross sections of abdominal cavity
illustrate the peritoneums and mesenteries.
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Figure 23.5a
Peritoneum and Peritoneal Cavity
• Mesentery is a double layer of peritoneum
• Routes for blood vessels, lymphatics, and
nerves
• Holds organs in place and stores fat
• Extends to the digestive organs from the body
wall
• Retroperitoneal organs lie posterior to the
peritoneum & mesentery lying against the dorsal
abdominal wall
• Intraperitoneal (peritoneal) organs are
surrounded by the peritoneum
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Abdominopelvic
cavity
Mesentery
resorbed
and lost
Alimentary
Alimentary canal organ in
canal organ
a retroperitoneal position
(b) Some organs lose their mesentery and
become retroperitoneal during development.
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Figure 23.5b
Blood Supply: Splanchnic Circulation
• Splanchnic circulation serves the digestive
system and includes those arteries that branch
off the abdominal aorta to serve the digestive
organs and the hepatic portal circulation
• Arteries
• Hepatic, splenic, left gastric, inferior and
superior mesenteric
• Hepatic portal circulation
• Drains nutrient-rich blood from digestive
organs & delivers it to the liver for processing
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Histology of the Alimentary Canal
• Four basic layers (tunics)
• Mucosa
• Submucosa
• Muscularis externa
• Serosa
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Nerve
Artery
Vein
Mesentery
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Intrinsic nerve plexuses
• Myenteric nerve plexus
• Submucosal nerve plexus
Glands in submucosa
Mucosa
• Epithelium
• Lamina propria
• Muscularis
mucosae
Submucosa
Muscularis
externa
• Longitudinal
muscle
• Circular muscle
Serosa
• Epithelium
• Connective
tissue
Lumen
Gland in mucosa
Lymphatic
Mucosa-associated
Duct of gland outside
vessel
lymphoid tissue
alimentary canal
Figure 23.6
Mucosa
• Innermost, moist, epithelial membrane
• Lines the (lumen) entire digestive tract
• Functions
• Secretes mucus, digestive enzymes and
hormones
• Absorbs end products of digestion into the
blood
• Protects against infectious disease
• Three sublayers: epithelium, lamina propria,
and muscularis mucosae
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Mucosa
• Epithelium
• Simple columnar epithelium and mucussecreting cells
• Mucus
• Protects digestive organs from enzymes
• Eases food passage
• May secrete enzymes and hormones (e.g., in
stomach and small intestine)
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Mucosa
• Lamina propria
• Loose areolar connective tissue
• Capillaries for nourishment and absorption
• Lymphoid follicles (part of MALT)
• Muscularis mucosae: smooth muscle that
produces local movements of mucosa
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Submucosa and Muscularis Externa
• Submucosa
• Dense connective tissue layer
• Blood and lymphatic vessels, lymphoid follicles,
and submucosal nerve plexus
• Muscularis externa
• Consists of smooth muscle
• Responsible for segmentation and peristalsis
• Inner circular and outer longitudinal layers
• Myenteric nerve plexus
• Sphincters in some regions
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Serosa
• Visceral peritoneum is the serosa
• Protective outer layer of the intraperitoneal
organs
• Replaced by the fibrous adventitia in the
esophagus
• Retroperitoneal organs have both an adventitia
and serosa
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Nerve
Artery
Vein
Mesentery
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Intrinsic nerve plexuses
• Myenteric nerve plexus
• Submucosal nerve plexus
Glands in submucosa
Mucosa
• Epithelium
• Lamina propria
• Muscularis
mucosae
Submucosa
Muscularis
externa
• Longitudinal
muscle
• Circular muscle
Serosa
• Epithelium
• Connective
tissue
Lumen
Gland in mucosa
Lymphatic
Mucosa-associated
Duct of gland outside
vessel
lymphoid tissue
alimentary canal
Figure 23.6
Enteric Nervous System
• The alimentary canal has its own nerve supply
made up of enteric neurons that communicate
widely with each other to regulate digestive
activity
• Intrinsic nerve supply of the alimentary canal
• Submucosal nerve plexus
• Regulates glands and smooth muscle in the
mucosa
• Myenteric nerve plexus
• Controls GI tract motility
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Enteric Nervous System
• Linked to the CNS via afferent visceral fibers
• Long ANS fibers synapse with enteric
plexuses
• Sympathetic impulses inhibit secretion and
motility
• Parasympathetic impulses stimulate
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Mouth
• Oral (buccal) cavity
• Bounded by lips, cheeks, palate, and tongue
• Oral orifice is the anterior opening
• Lined with stratified squamous epithelium
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Soft palate
Palatoglossal arch
Hard palate
Uvula
Oral cavity
Palatine tonsil
Tongue
Oropharynx
Lingual tonsil
Epiglottis
Hyoid bone
Laryngopharynx
Esophagus
Trachea
(a) Sagittal section of the oral cavity and pharynx
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Figure 23.7a
Lips and Cheeks
• Contain orbicularis oris and buccinator
muscles
• helps to keep food between the teeth when
we chew and plays a small role in speech
• Vestibule: recess internal to lips and cheeks,
external to teeth and gums
• Oral cavity proper lies within the teeth and
gums
• Labial frenulum: median attachment of each
lip to the gum
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Gingivae (gums)
Palatine raphe
Hard palate
Soft palate
Uvula
Palatine tonsil
Sublingual fold
with openings of
sublingual ducts
Vestibule
Lower lip
Upper lip
Superior labial
frenulum
Palatoglossal arch
Palatopharyngeal
arch
Posterior wall
of oropharynx
Tongue
Lingual frenulum
Opening of
submandibular duct
Gingivae (gums)
Inferior labial
frenulum
(b) Anterior view
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Figure 23.7b
Palate
• The palate forms the roof of the mouth
• Hard palate: palatine bones and palatine
processes of the maxillae (anterior portion)
• Slightly corrugated to help create friction
against the tongue
• Soft palate: fold formed mostly of skeletal
muscle (posterior portion)
• Closes off the nasopharynx during swallowing
• Uvula projects downward from its free edge
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Tongue
• Functions include
• Repositioning and mixing food during chewing
• Formation of the bolus
• Initiation of swallowing, speech, and taste
• Intrinsic muscles change the shape of the tongue
• Extrinsic muscles alter the tongue’s position
• Lingual frenulum: attachment to the floor of the
mouth
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Tongue
•
Surface bears papillae
1. Filiform—whitish, give the tongue roughness and
provide friction
2. Fungiform—reddish, scattered over the tongue
3. Circumvallate (vallate)—V-shaped row in back of
tongue
•
These three house taste buds
4. Foliate—on the lateral aspects of the posterior
tongue
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Tongue
• Terminal sulcus marks the division between
• Body: anterior 2/3 residing in the oral cavity
• Root: posterior third residing in the oropharynx
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Epiglottis
Palatopharyngeal
arch
Palatine tonsil
Lingual tonsil
Palatoglossal
arch
Terminal sulcus
Foliate papillae
Circumvallate
papilla
Midline groove
of tongue
Dorsum of tongue
Fungiform papilla
Filiform papilla
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Figure 23.8
Salivary Glands
• Extrinsic salivary glands (parotid,
submandibular, and sublingual)
• Intrinsic (buccal) salivary glands are scattered
in the oral mucosa
• Produce secretion (saliva)
• Cleanses the mouth
• Moistens and dissolves food chemicals
• Aids in bolus formation
• Contains enzymes that begin the breakdown
of starch
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Salivary Glands
• Parotid gland
• Anterior to the ear external to the masseter
muscle
• Parotid duct opens into the vestibule next to
second upper molar
• Submandibular gland
• Medial to the body of the mandible
• Duct opens at the base of the lingual frenulum
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Salivary Glands
• Sublingual gland
• Anterior to the submandibular gland under the
tongue
• Opens via 10–12 ducts into the floor of the
mouth
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Tongue
Teeth
Parotid
gland
Ducts of
sublingual
gland
Frenulum
of tongue
Sublingual
gland
Mylohyoid
muscle (cut)
Anterior belly of
digastric muscle
(a)
Submandibular
gland
Parotid duct
Masseter muscle
Body of
mandible (cut)
Posterior belly
of digastric
muscle
Submandibular
duct
Mucous
cells
(b)
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Serous cells
forming
demilunes
Figure 23.9
Composition of Saliva
• Secreted by serous and mucous cells
• 97–99.5% water, slightly acidic solution containing
• Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3–
• Salivary amylase and lingual lipase
• Mucin
• Metabolic wastes—urea and uric acid
• Lysozyme, IgA, defensins, and a cyanide compound
protect against microorganisms
PLAY
Animation: Rotatable head
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Control of Salivation
• Intrinsic glands continuously keep the mouth moist
• Extrinsic salivary glands produce secretions when
• Ingested food stimulates chemoreceptors and
mechanoreceptors in the mouth
• Salivatory nuclei in the brain stem send impulses
along parasympathetic fibers in cranial nerves VII
and IX
• Strong sympathetic stimulation inhibits salivation and
results in dry mouth (xerostomia)
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Teeth
• The teeth tear and grind food, breaking it into
smaller pieces
• Primary and permanent dentitions are formed
by age 21
• 20 deciduous teeth erupt (6–24 months of
age)
• Roots are resorbed, teeth fall out (6–12 years
of age) as permanent teeth develop
• 32 permanent teeth
• All except third molars erupt by the end of
adolescence
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(b)
Deciduous teeth
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Permanent teeth
Figure 23.10b
Classes of Teeth
• Incisors
• Chisel shaped for cutting
• Canines
• Fanglike teeth that tear or pierce
• Premolars (bicuspids) and molars
• Have broad crowns with rounded cusps for
grinding or crushing
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Incisors
Central (6–8 mo)
Lateral (8–10 mo)
Canine (eyetooth)
(16–20 mo)
Molars
First molar
(10–15 mo)
Second molar
(about 2 yr)
(a)
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Deciduous
(milk) teeth
Incisors
Central (7 yr)
Lateral (8 yr)
Canine (eyetooth)
(11 yr)
Premolars
(bicuspids)
First premolar
(11 yr)
Second premolar
(12–13 yr)
Molars
First molar (6–7 yr)
Second molar
(12–13 yr)
Third molar
(wisdom tooth)
(17–25 yr)
Permanent
teeth
Figure 23.10a
Dental Formulas
• A shorthand way of indicating the number and
relative position of teeth
• Ratio of upper to lower teeth for one-half of the
mouth
• Primary: 2I,1C, 2M
• Permanent: 2I,1C, 2PM, 3M
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Tooth Structure
• Crown: the exposed part above the gingiva
(gum)
• Covered by enamel—the hardest substance in
the body (calcium salts and hydroxyapatite
crystals)
• Root: portion embedded in the jawbone
• Connected to crown by neck
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Tooth Structure
• Cementum: calcified connective tissue
• Covers root and attaches it to the periodontal
ligament
• Periodontal ligament
• Forms fibrous joint called a gomphosis
• Gingival sulcus: groove where gingiva borders
the tooth
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Tooth Structure
• Dentin: bonelike material under enamel
• Maintained by odontoblasts of pulp cavity
• Pulp cavity: cavity surrounded by dentin
• Pulp: connective tissue, blood vessels, and
nerves
• Root canal: extends from pulp cavity to the
apical foramen of the root
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Crown
Neck
Enamel
Dentin
Dentinal tubules
Pulp cavity (contains
blood vessels and
nerves)
Gingiva (gum)
Cementum
Root
Root canal
Periodontal
ligament
Apical foramen
Bone
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Figure 23.11
Tooth and Gum Disease
• Dental caries (cavities): gradual
demineralization of enamel and dentin
• Dental plaque (sugar, bacteria, and debris)
adheres to teeth
• Acid from bacteria dissolves calcium salts
• Proteolytic enzymes digest organic matter
• Prevention: daily flossing and brushing
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Tooth and Gum Disease
• Gingivitis
• Plaque calcifies to form calculus (tartar)
• Calculus disrupts the seal between the
gingivae and the teeth
• Anaerobic bacteria infect gums
• Infection reversible if calculus removed
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Tooth and Gum Disease
• Periodontitis
• Immune cells attack intruders and body tissues
• Destroy periodontal ligament
• Activate osteoclasts
• Consequences
• Possible tooth loss, promotion of
atherosclerosis and clot formation in
coronary and cerebral arteries
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Pharynx
• Oropharynx and laryngopharynx
• Allow passage of food, fluids, and air
• Stratified squamous epithelium lining
• Skeletal muscle layers: inner longitudinal,
outer pharyngeal constrictors
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Esophagus
• Flat muscular tube from laryngopharynx to
stomach
• Pierces diaphragm at esophageal hiatus
• Joins stomach at the cardiac orifice
• Provides a passageway for food and fluids
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Esophagus
• Esophageal mucosa contains stratified
squamous epithelium
• Changes to simple columnar at the stomach
• Esophageal glands in submucosa secrete
mucus to aid in bolus movement
• Muscularis: skeletal superiorly; smooth
inferiorly
• Adventitia instead of serosa
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(a)
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Mucosa
(contains a stratified
squamous epithelium)
Submucosa (areolar
connective tissue)
Lumen
Muscularis externa
• Longitudinal layer
• Circular layer
Adventitia (fibrous
connective tissue)
Figure 23.12a
Digestive Processes: Mouth
• Ingestion
• Mechanical digestion
• Mastication (chewing) is partly voluntary, partly
reflexive; mixes food with saliva
• Chemical digestion (salivary amylase and
lingual lipase)
• Propulsion
• Deglutition (swallowing)
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Deglutition
• Involves the tongue, soft palate, pharynx,
esophagus, and 22 muscle groups
• Buccal phase
• Voluntary contraction of the tongue and occurs
in the mouth where the bolus is forced into the
oropharynx
• Pharyngeal-esophageal phase
• Involuntary and occurs when food is squeezed
through the pharynx and into the esophagus
• Control center in the medulla and lower pons
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Bolus of food
Tongue
Uvula
Pharynx
Bolus
Epiglottis
Epiglottis
Glottis
Trachea
Bolus
Esophagus
1 Upper esophageal sphincter is
contracted. During the buccal phase, the
tongue presses against the hard palate,
forcing the food bolus into the oropharynx
where the involuntary phase begins.
Relaxed muscles
2 The uvula and larynx rise to prevent food
from entering respiratory passageways. The
tongue blocks off the mouth. The upper
esophageal sphincter relaxes, allowing food
to enter the esophagus.
4 Food is moved
through the esophagus
to the stomach by
peristalsis.
Circular muscles
contract
Bolus of food
3 The constrictor muscles of the
pharynx contract, forcing food
into the esophagus inferiorly. The
upper esophageal sphincter
contracts (closes) after entry.
Relaxed
muscles
5 The gastroesophageal
sphincter opens, and food
enters the stomach.
Longitudinal muscles
contract
Gastroesophageal
sphincter closed
Gastroesophageal
sphincter opens
Stomach
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Figure 23.13
Stomach: Gross Anatomy
• Temporary storage tank where the chemical
breakdown of proteins is initiated and food is
converted to chyme
• The adult stomach varies from 15–25 cm
long; its diameter and volume vary depending
on the amount of food it contains
• Cardiac region (cardia)
• Surrounds the cardiac orifice
• Fundus
• Dome-shaped region beneath the diaphragm
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Stomach: Gross Anatomy
• Body
• Midportion
• Pyloric region: antrum, pyloric canal, and
pylorus
• Pylorus is continuous with the duodenum
through the pyloric valve (sphincter)
• Greater curvature
• Convex lateral surface
• Lesser curvature
• Concave medial surface
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Cardia
Esophagus
Muscularis
externa
• Longitudinal layer
• Circular layer
• Oblique layer
Lesser
curvature
Fundus
Serosa
Body
Lumen
Rugae of
mucosa
Greater
curvature
Duodenum
(a)
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Pyloric
Pyloric
canal
antrum
Pyloric sphincter
(valve) at pylorus
Figure 23.14a
Stomach: Gross Anatomy
• Two omentums help to tie the stomach to
other digestive organs and the body wall
• Lesser omentum
• From the liver to the lesser curvature
• Greater omentum
• Drapes from greater curvature
• Anterior to the small intestine
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Falciform ligament
Liver
Gallbladder
Spleen
Stomach
Ligamentum teres
Greater omentum
Small intestine
Cecum
(a)
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Figure 23.30a
Liver
Gallbladder
Lesser omentum
Stomach
Duodenum
Transverse colon
Small intestine
Cecum
Urinary bladder
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(b)
Figure 23.30b
Stomach: Gross Anatomy
• ANS nerve supply
• Sympathetic via splanchnic nerves and celiac
plexus
• Parasympathetic via vagus nerve
• Blood supply
• Celiac trunk
• Veins of the hepatic portal system
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Stomach: Microscopic Anatomy
• Four tunics
• Muscularis and mucosa are modified
• Muscularis externa
• Three layers of smooth muscle
• Inner oblique layer allows stomach to churn,
mix, move, and physically break down food
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Surface
epithelium
Mucosa
Lamina propria
Submucosa
(contains submucosal
plexus)
Muscularis externa
(contains myenteric
plexus)
Serosa
Muscularis
mucosae
Oblique layer
Circular layer
Longitudinal
layer
(a) Layers of the stomach wall (l.s.)
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Stomach wall
Figure 23.15a
Stomach: Microscopic Anatomy
• Mucosa
• Simple columnar epithelium composed of
goblet cells
• Goblet cells produce a protective two-layer
coat of alkaline mucus that traps
bicarbonate-rich fluid beneath it
• Gastric pits lead into gastric glands
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Gastric pits
Surface epithelium
(mucous cells)
Gastric
pit
Mucous neck cells
Parietal cell
Chief cell
Gastric
gland
Enteroendocrine cell
(b) Enlarged view of gastric pits and gastric glands
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Figure 23.15b
Gastric Glands
• Cell types
• Mucous neck cells (secrete thin, acidic mucus)
• Parietal cells
• Chief cells
• Enteroendocrine cells
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Pepsinogen
HCl
Pepsin
Mitochondria
Parietal cell
Chief cell
Enteroendocrine
cell
(c) Location of the HCl-producing parietal cells and
pepsin-secreting chief cells in a gastric gland
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Figure 23.15c
Gastric Gland Secretions
• Glands in the fundus and body produce most of the
gastric juice, which may be composed of a
combination of mucus, hydrochloric acid, intrinsic
factor, pepsinogen, and a variety of hormones
• Parietal cell secretions
• HCl
•  pH 1.5–3.5 denatures protein in food, activates
pepsin, and kills many bacteria
• Intrinsic factor
• Glycoprotein required for absorption of vitamin B12
in small intestine
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Gastric Gland Secretions
• Chief cell secretions
• Inactive enzyme pepsinogen
• Activated to pepsin by HCl and by pepsin itself
(a positive feedback mechanism)
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Gastric Gland Secretions
• Enteroendocrine cells
• Secrete chemical messengers into the lamina
propria
• Paracrines
• Serotonin and histamine
• Hormones
• Somatostatin and gastrin
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Mucosal Barrier
• Layer of bicarbonate-rich mucus
• Tight junctions between epithelial cells
• Damaged epithelial cells are quickly replaced
by division of stem cells
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Homeostatic Imbalance
• Gastritis: inflammation caused by anything
that breaches the mucosal barrier
• Peptic or gastric ulcers: erosion of the
stomach wall
• Most are caused by Helicobacter pylori
bacteria
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Bacteria
Mucosa
layer of
stomach
(a) A gastric ulcer lesion
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(b) H. pylori bacteria
Figure 23.16
Digestive Processes in the Stomach
• Physical digestion
• Denaturation of proteins
• Enzymatic digestion of proteins by pepsin
(and rennin in infants)
• Secretes intrinsic factor required for
absorption of vitamin B12
• Lack of intrinsic factor  pernicious anemia
• Delivers chyme to the small intestine
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Regulation of Gastric Secretion
• Neural and hormonal mechanisms
• Stimulatory and inhibitory events occur in
three phases:
1. Cephalic (reflex) phase: few minutes prior to
food entry
2. Gastric phase: 3–4 hours after food enters
the stomach
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Regulation of Gastric Secretion
3. Intestinal phase: brief stimulatory effect as
partially digested food enters the duodenum,
followed by inhibitory effects (enterogastric
reflex and enterogastrones)
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Stimulatory events
Cephalic
phase
Gastric
phase
1 Sight and thought
of food
Cerebral cortex
Conditioned reflex
2 Stimulation of
taste and smell
receptors
Hypothalamus
and medulla
oblongata
1 Stomach
distension
activates
stretch
receptors
Vagovagal
reflexes
1 Presence of low
pH, partially digested
foods, fats, or
hypertonic solution
in duodenum when
stomach begins to
empty
Stimulate
Inhibit
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Medulla
Vagus
nerve
Vagus
nerve
Local
reflexes
2 Food chemicals
G cells
(especially peptides and
caffeine) and rising pH
activate chemoreceptors
Intestinal
phase
Inhibitory events
Gastrin
release
to blood
Intestinal
(enteric)
gastrin
release
to blood
Lack of
stimulatory
impulses to
parasympathetic
center
Cerebral
cortex
Gastrin
secretion
declines
G cells
Overrides
parasympathetic
controls
Sympathetic
nervous
system
activation
1 Excessive
acidity
(pH <2)
in stomach
2 Emotional
upset
Stomach
secretory
activity
Enterogastric
reflex
Brief
effect
1 Loss of
appetite,
depression
Local
reflexes
Vagal
nuclei
in medulla
Pyloric
sphincter
1 Distension
of duodenum;
presence of
fatty, acidic,
hypertonic
chyme, and/or
irritants in
the duodenum
2 Distension;
Release of intestinal
presence of
hormones (secretin,
cholecystokinin, vasoactive fatty, acidic,
partially
intestinal peptide)
digested food
in the
duodenum
Figure 23.17
Regulation and Mechanism of HCl Secretion
• Three chemicals (ACh, histamine, and
gastrin) stimulate parietal cells through
second-messenger systems
• All three are necessary for maximum HCl
secretion
• Antihistamines block H2 receptors and
decrease HCl release
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Blood
capillary
Chief cell
CO2
CO2 + H2O
Carbonic
H2CO3 anhydrase
H+
K+
Stomach lumen
H+-K+
ATPase
H+
K+
HCO3–
Alkaline
tide
HCI
Parietal cell
HCO3–
Cl–
Cl–
HCO3–- Cl–
antiporter
Cll–
Interstitial
fluid
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Figure 23.18
Response of the Stomach to Filling
• Stretches to accommodate incoming food
• Reflex-mediated receptive relaxation
• Coordinated by the swallowing center of the
brain stem
• Gastric accommodation
• Plasticity (stress-relaxation response) of
smooth muscle
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Gastric Contractile Activity
• Peristaltic waves move toward the pylorus at
the rate of 3 per minute
• Basic electrical rhythm (BER) initiated by
pacemaker cells (interstitial cells of Cajal)
• Distension and gastrin increase force of
contraction
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Gastric Contractile Activity
• Most vigorous near the pylorus
• Chyme is either
• Delivered in ~ 3 ml spurts to the duodenum, or
• Forced backward into the stomach
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Pyloric
valve
closed
1 Propulsion: Peristaltic
waves move from the
fundus toward the
pylorus.
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Pyloric
valve
closed
2 Grinding: The most
vigorous peristalsis and
mixing action occur
close to the pylorus.
Pyloric
valve
slightly
opened
3 Retropulsion: The pyloric
end of the stomach acts as a
pump that delivers small
amounts of chyme into the
duodenum, simultaneously
forcing most of its contained
material backward into the
stomach.
Figure 23.19
Regulation of Gastric Emptying
• As chyme enters the duodenum
• Receptors respond to stretch and chemical
signals
• Enterogastric reflex and enterogastrones
inhibit gastric secretion and duodenal filling
• Carbohydrate-rich chyme moves quickly through
the duodenum
• Fatty chyme remains in the duodenum 6 hours or
more
• The rate at which the stomach empties is
determined by the contents of the stomach &
processing in the small intestine
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Presence of fatty, hypertonic,
acidic chyme in duodenum
Duodenal enteroendocrine cells
Chemoreceptors and
stretch receptors
Secrete
Enterogastrones
(secretin,
cholecystokinin,
vasoactive intestinal
peptide)
Duodenal
stimuli
decline
Initial stimulus
Physiological response
Result
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Target
Via short
reflexes
Enteric
neurons
Contractile force and
rate of stomach
emptying decline
Via long
reflexes
CNS centers
sympathetic
activity;
parasympathetic
activity
Stimulate
Inhibit
Figure 23.20
Small Intestine: Gross Anatomy
•
Major organ of digestion and absorption
•
2–4 m long; from pyloric sphincter (where it
joins stomach) to ileocecal valve (where it
joins large intestine)
•
Subdivisions
1. Duodenum (retroperitoneal)
2. Jejunum (attached posteriorly by mesentery)
3. Ileum (attached posteriorly by mesentery)
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Mouth (oral cavity)
Tongue
Esophagus
Liver
Gallbladder
Duodenum
Jejunum
Small
intestine Ileum
Anus
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Parotid gland
Sublingual gland Salivary
Submandibular
glands
gland
Pharynx
Stomach
Pancreas
(Spleen)
Transverse colon
Descending colon
Ascending colon
Large
Cecum
intestine
Sigmoid colon
Rectum
Vermiform appendix
Anal canal
Figure 23.1
Duodenum
• The bile duct and main pancreatic duct
• Join at the hepatopancreatic ampulla
• Enter the duodenum at the major duodenal
papilla
• Are controlled by the hepatopancreatic
sphincter
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Right and left
hepatic ducts
of liver
Cystic duct
Common hepatic duct
Bile duct and sphincter
Accessory pancreatic duct
Mucosa
with folds
Gallbladder
Major duodenal
papilla
Hepatopancreatic
ampulla and sphincter
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Tail of pancreas
Pancreas
Jejunum
Duodenum
Main pancreatic duct
and sphincter
Head of pancreas
Figure 23.21
Structural Modifications
• Increase surface area of proximal part for
nutrient absorption
• Circular folds (plicae circulares)
• Villi
• Microvilli
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Structural Modifications
• Circular folds
• Permanent (~1 cm deep)
• Force chyme to slowly spiral through lumen
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Vein carrying blood to
hepatic portal vessel
Muscle
layers
Circular
folds
Villi
Lumen
(a)
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Figure 23.22a
Structural Modifications
• Villi
• Motile fingerlike extensions (~1 mm high) of
the mucosa
• Villus epithelium
• Simple columnar absorptive cells
(enterocytes)
• Goblet cells
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Structural Modifications
• Microvilli
• Projections (brush border) of absorptive
cells
• Bear brush border enzymes
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Intestinal Crypts
• Intestinal crypt epithelium (crypts of Lieberkühn)
• Secretory cells that produce intestinal juice serves as
a carrier fluid for absorbing nutrients from chyme
• Enteroendocrine cells
• Intraepithelial lymphocytes (IELs)
• Release cytokines that kill infected cells
• Paneth cells
• Secrete antimicrobial agents (defensins and
lysozyme)
• Stem cells
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Microvilli
(brush border)
Absorptive cells
Lacteal
Goblet cell
Blood
capillaries
Mucosa
associated
lymphoid tissue
Intestinal crypt
Muscularis
mucosae
Duodenal gland
(b)
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Vilus
Enteroendocrine
cells
Venule
Lymphatic vessel
Submucosa
Figure 23.22b
Submucosa
• Peyer’s patches protect distal part against
bacteria
• Duodenal (Brunner’s) glands of the duodenum
secrete alkaline mucus
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Intestinal Juice
• Secreted in response to distension or irritation
of the mucosa
• Slightly alkaline and isotonic with blood
plasma
• Largely water, enzyme-poor, but contains
mucus
• Facilitates transport and absorption of
nutrients
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Liver
• Largest gland in the body
• Accessory organ associated with the small
intestine
• Digestive function is to produce bile, which is
a fat emulsifier
• Four lobes—right, left, caudate, and quadrate
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Liver
• Falciform ligament
• Separates the (larger) right and (smaller) left
lobes
• Suspends liver from the diaphragm and
anterior abdominal wall
• Round ligament (ligamentum teres)
• Remnant of fetal umbilical vein along free
edge of falciform ligament
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Sternum
Nipple
Liver
Bare area
Falciform
ligament
Left lobe of liver
Right lobe
of liver
Gallbladder
(a)
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Round ligament
(ligamentum
teres)
Figure 23.24a
Sternum
Nipple
Liver
Lesser omentum
(in fissure)
Left lobe of liver
Porta hepatis
containing hepatic
artery (left) and
hepatic portal vein
(right)
Quadrate lobe
of liver
Ligamentum teres
Bare area
Caudate lobe
of liver
Sulcus for
inferior
vena cava
Hepatic vein
(cut)
Bile duct (cut)
Right lobe of
liver
Gallbladder
(b)
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Figure 23.24b
Liver: Associated Structures
• Lesser omentum anchors liver to stomach
• Hepatic artery and vein at the porta hepatis
• Bile ducts
• Common hepatic duct leaves the liver
• Cystic duct connects to gallbladder
• Bile duct formed by the union of the above two
ducts
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Right and left
hepatic ducts
of liver
Cystic duct
Common hepatic duct
Bile duct and sphincter
Accessory pancreatic duct
Mucosa
with folds
Gallbladder
Major duodenal
papilla
Hepatopancreatic
ampulla and sphincter
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Tail of pancreas
Pancreas
Jejunum
Duodenum
Main pancreatic duct
and sphincter
Head of pancreas
Figure 23.21
Liver: Microscopic Anatomy
• Liver lobules
• Hexagonal structural and functional units
• Made of plates of liver cells (hepatocytes)
• Filter and process nutrient-rich blood
• Composed of plates of hepatocytes (liver
cells)
• Longitudinal central vein
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(a)
Lobule
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(b)
Central vein
Connective
tissue septum
Figure 23.25a, b
Liver: Microscopic Anatomy
• Portal triad at each corner of lobule
• Bile duct receives bile from bile canaliculi
• Portal arteriole is a branch of the hepatic artery
• Hepatic venule is a branch of the hepatic portal
vein
• Liver sinusoids are leaky capillaries between
hepatic plates
• Kupffer cells (hepatic macrophages) in liver
sinusoids
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Interlobular veins
(to hepatic vein)
Central vein
Sinusoids
Bile canaliculi
Plates of
hepatocytes
Bile duct (receives
bile from bile
canaliculi)
Fenestrated
lining (endothelial
cells) of sinusoids
Portal vein
Hepatic
macrophages
in sinusoid walls
Bile duct
Portal venule
Portal arteriole
Portal triad
(c)
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Figure 23.25c
Liver: Microscopic Anatomy
• Hepatocyte functions
• Process bloodborne nutrients
• Store fat-soluble vitamins
• Perform detoxification
• Produce ~900 ml bile per day
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Bile
• Yellow-green, alkaline solution containing
• Bile salts: cholesterol derivatives that function
in fat emulsification and absorption
• Bilirubin: bile pigment formed from heme
• Cholesterol, neutral fats, phospholipids, and
electrolytes
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Bile
• Enterohepatic circulation
• Recycles bile salts
• Bile salts  duodenum  reabsorbed from
ileum  hepatic portal blood  liver 
secreted into bile
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The Gallbladder
• Accessory organ associated with the small
intestine
• Thin-walled muscular sac on the ventral surface
of the liver
• Stores and concentrates bile (produced by liver)
by absorbing its water and ions
• Releases bile as needed via the cystic duct,
which flows into the bile duct
• Bile enters the small intestine when the
gallbladder contracts after stimulated by
cholecystokinin (CCK)
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Pancreas
• Accessory gland that is retroperitoneal
• Location
• Mostly retroperitoneal, deep to the greater
curvature of the stomach
• Head is encircled by the duodenum; tail abuts
the spleen
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Pancreas
• Endocrine function
• Pancreatic islets secrete insulin and glucagon
• Exocrine function
• Acini (clusters of secretory cells) secrete
pancreatic juice
• Zymogen granules of secretory cells contain
digestive enzymes
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Small
duct
Acinar cells
Basement
membrane
Zymogen
granules
Rough
endoplasmic
reticulum
(a)
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Figure 23.26a
Pancreatic Juice
• Break downs all categories of foodstuffs and
electrolytes
• Watery alkaline solution (pH 8) neutralizes
chyme
• Electrolytes (primarily HCO3–)
• Enzymes
• Amylase, lipases, nucleases are secreted in
active form but require ions or bile for optimal
activity
• Proteases secreted in inactive form
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Pancreatic Juice
• Secretion of pancreatic juice is regulated by
local hormones and the parasympathetic
nervous system
• Protease activation in duodenum
• Trypsinogen is activated to trypsin by brush
border enzyme enteropeptidase
• Procarboxypeptidase and chymotrypsinogen
are activated by trypsin
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Stomach
Pancreas
Epithelial
cells
Membrane-bound
enteropeptidase
Trypsinogen
Trypsin
(inactive)
Chymotrypsin
Chymotrypsinogen
(inactive)
Carboxypeptidase
Procarboxypeptidase
(inactive)
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Figure 23.27
Regulation of Bile Secretion
• Bile secretion is stimulated by
• Bile salts in enterohepatic circulation
• Secretin from intestinal cells exposed to HCl
and fatty chyme
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Regulation of Bile Secretion
• Gallbladder contraction is stimulated by
• Cholecystokinin (CCK) from intestinal cells
exposed to proteins and fat in chyme
• Vagal stimulation (minor stimulus)
• CKK also causes the hepatopancreatic
sphincter to relax
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Regulation of Pancreatic Secretion
• CCK induces the secretion of enzyme-rich
pancreatic juice by acini
• Secretin causes secretion of bicarbonate-rich
pancreatic juice by duct cells
• Vagal stimulation also causes release of
pancreatic juice (minor stimulus)
Copyright © 2010 Pearson Education, Inc.
Slide 1
1
Chyme entering duodenum
causes release of
cholecystokinin
(CCK) and
secretin from
duodenal
enteroendocrine
cells.
2
CCK (red
dots) and
secretin (yellow
dots) enter the
bloodstream.
3
CCK induces
secretion of
enzyme-rich
pancreatic juice.
Secretin causes
secretion of
HCO3–-rich
pancreatic juice.
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4
Bile salts and,
to a lesser extent,
secretin
transported via
bloodstream
stimulate liver to
produce bile
more rapidly.
5
CCK (via
bloodstream)
causes
gallbladder to
contract and
hepatopancreatic
sphincter to
relax; bile enters
duodenum.
6 During
cephalic and
gastric phases,
vagal nerve
stimulation
causes weak
contractions of
gallbladder.
Figure 23.28
Digestion in the Small Intestine
• Food takes 3 to 6 hours to complete its
digestive path through the small intestine, the
site of virtually all nutrient absorption
• Chyme from stomach contains
• Partially digested carbohydrates and proteins
• Undigested fats
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Requirements for Digestion and Absorption
in the Small Intestine
• Optimal digestive activity in the small intestine
depends on a slow, measured delivery of
hypertonic chyme from the stomach
• Most substances required for chemical
digestion (bile, enzymes, and bicarbonate)
within the small intestine are imported from
the pancreas and the liver
• Mixing
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Motility of the Small Intestine
• Segmentation
• Most common motion of the small intestine
• Initiated by intrinsic pacemaker cells
• Mixes and moves contents slowly and steadily
toward the ileocecal valve
• Intensity is altered by long and short reflexes
•
Wanes in the late intestinal (fasting) phase
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Microvilli
(b)
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Absorptive
cell
Figure 23.3b
Motility of the Small Intestine
• Peristalsis
• Initiated by motilin in the late intestinal phase
• Each wave starts distal to the previous (the
migrating motility complex)
• Meal remnants, bacteria, and debris are
moved to the large intestine
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From mouth
(a) Peristalsis: Adjacent segments of alimentary
tract organs alternately contract and relax,
which moves food along the tract distally.
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Figure 23.3a
Motility of the Small Intestine
• Local enteric neurons coordinate intestinal
motility
• Cholinergic sensory neurons may activate the
myenteric plexus
• Causes contraction of the circular muscle
proximally and of longitudinal muscle distally
• Forces chyme along the tract
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Motility of the Small Intestine
• Ileocecal sphincter relaxes and admits chyme
into the large intestine when
• Gastroileal reflex enhances the force of
segmentation in the ileum
• Gastrin increases the motility of the ileum
• Ileocecal valve flaps close when chyme exerts
backward pressure
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Large Intestine
• Absorbs water from indigestible food residues
and eliminates the latter as feces
• Unique features
• Teniae coli
• Three bands of longitudinal smooth muscle in the
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
• Regions/subdivisions
• Cecum (pouch with attached vermiform
appendix)
• Colon
• Rectum
• Anal canal
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Left colic
(splenic) flexure
Transverse
mesocolon
Epiploic
appendages
Right colic
(hepatic)
flexure
Transverse
colon
Superior
mesenteric
artery
Haustrum
Descending
colon
Ascending
colon
IIeum
Cut edge of
mesentery
Teniae coli
IIeocecal
valve
Cecum
Vermiform appendix
Sigmoid
colon
Rectum
Anal canal
(a)
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External anal sphincter
Figure 23.29a
Colon
• Ascending colon and descending colon are
retroperitoneal
• Transverse colon and sigmoid colon are
anchored via mesocolons (mesenteries)
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Greater omentum
Transverse colon
Transverse
mesocolon
Descending colon
Jejunum
Mesentery
Sigmoid
mesocolon
Sigmoid colon
Ileum
(c)
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Figure 23.30c
Liver
Lesser omentum
Pancreas
Stomach
Transverse
mesocolon
Duodenum
Transverse colon
Mesentery
Greater omentum
Jejunum
Ileum
Visceral peritoneum
Parietal peritoneum
(d)
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Urinary bladder
Rectum
Figure 23.30d
Rectum and Anus
• Rectum
• Three rectal valves stop feces from being
passed with gas
• Anal canal
• The last segment of the large intestine
• Sphincters
• Internal anal sphincter—smooth muscle
• External anal sphincter—skeletal muscle
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Rectal valve
Rectum
Hemorrhoidal
veins
Levator ani
muscle
Anal canal
External anal
sphincter
Internal anal
sphincter
Anal columns
Pectinate line
Anal sinuses
Anus
(b)
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Figure 23.29b
Large Intestine: Microscopic Anatomy
• Mucosa of simple columnar epithelium except
in the anal canal (stratified squamous)
• Mucosa is thick and has abundant deep
crypts with a large number of mucusproducing goblet cells
• Superficial venous plexuses of the anal canal
form hemorrhoids if inflamed
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Bacterial Flora
• Enter from the small intestine or anus
• Colonize the colon
• Ferment indigestible carbohydrates
• Release irritating acids and gases
• Synthesize B complex vitamins and vitamin K
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Functions of the Large Intestine
• Vitamins, water, and electrolytes are
reclaimed
• Major function is propulsion of feces toward
the anus
• Colon is not essential for life
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Motility of the Large Intestine
• Haustral contractions
• Slow segmenting movements
• Haustra sequentially contract in response to
distension
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Motility of the Large Intestine
• Gastrocolic reflex
• Initiated by presence of food in the stomach
• Activates three to four slow powerful peristaltic
waves per day in the colon (mass movements)
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Defecation
• Mass movements force feces into rectum
• Distension (stretching) initiates spinal
defecation reflex
• Parasympathetic signals
• Stimulate contraction of the sigmoid colon and
rectum
• Relax the internal anal sphincter
• Conscious control allows relaxation of
external anal sphincter
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Impulses from
cerebral cortex
(conscious
control)
1
Sensory
nerve fibers
Distension, or stretch, of the
rectal walls due to movement
of feces into the rectum
stimulates stretch receptors
there. The receptors transmit
signals along afferent fibers to
spinal cord neurons.
2
Voluntary motor
nerve to external
anal sphincter
Sigmoid
colon
A spinal reflex is initiated in
which parasympathetic motor
(efferent) fibers stimulate
contraction of the rectal walls
and relaxation of the internal
anal sphincter.
Stretch receptors in wall
Rectum
External anal
sphincter
(skeletal muscle)
Involuntary motor nerve
(parasympathetic division)
Internal anal sphincter
(smooth muscle)
3
If it is convenient to defecate, voluntary motor
neurons are inhibited, allowing the external anal
sphincter to relax so that feces may pass.
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Figure 23.31
Chemical Digestion
• Catabolic
• Accomplished by enzymes, secreted by
intrinsic and accessory glands of the
alimentary canal, used in hydrolysis reactions
• Large food molecules are broken down to
chemical building blocks (monomers), which
are small enough to be absorbed by the GI
tract lining
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Chemical Digestion and Absorption of
Carbohydrates
• Begins in the mouth, where salivary amylase
breaks large polysaccharides into smaller
fragments
• Digestive enzymes
• Salivary amylase, pancreatic amylase, and
brush border enzymes (dextrinase,
glucoamylase, lactase, maltase, and sucrase)
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Chemical Digestion and Absorption of
Carbohydrates
• Absorption
• Glucose and galactose are transported into the
epithelial cells by common protein carriers and are
then moved by facilitated diffusion into the
capillary blood
• Secondary active transport (cotransport) with Na+
• Facilitated diffusion of some monosaccharides
• Enter the capillary beds in the villi
• Transported to the liver via the hepatic portal
vein
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Chemical Digestion and Absorption of
Carbohydrates
• Monosaccharides-simple sugars; absorbed
immediately (glucose, galactose, and
fructose)
• Disaccharides-two monosaccharides bonded
together (maltose, lactose, and sucrose)
• Starch-digestible polysaccharide found in the
diet
• Other polysaccharides-not able to be broken
down by humans (cellulose)
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Carbohydrate digestion
Foodstuff
Enzyme(s)
and source
Site of
action
Starch and disaccharides
Oligosaccharides
and disaccharides
Lactose Maltose Sucrose
Galactose Glucose Fructose
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Salivary
amylase
Pancreatic
amylase
Brush border
enzymes in
small intestine
(dextrinase, glucoamylase, lactase,
maltase, and sucrase)
Mouth
Small
intestine
Small
intestine
Path of absorption
• Glucose and galactose
are absorbed via
cotransport with
sodium ions.
• Fructose passes via
facilitated diffusion.
• All monosaccharides
leave the epithelial
cells via facilitated
diffusion, enter the
capillary blood in the
villi, and are
transported to the liver
via the hepatic portal
vein.
Figure 23.32 (1 of 4)
Chemical Digestion and Absorption of
Proteins
• Proteins digested into amino acids in the GI
tract include not only dietary proteins but also
enzyme proteins secreted into the GI tract
lumen.
• Pepsin, secreted by the chief cells, begins the
chemical digestion of proteins in the stomach
• Rennin is produced in infants and breaks
down milk proteins
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Chemical Digestion and Absorption of
Proteins
• Pancreatic proteases
• Trypsin, chymotrypsin, and carboxypeptidase
• Further break down proteins in the small intestine
• Brush border enzymes
• Aminopeptidases, carboxypeptidases, and
dipeptidases
• Work on freeing single amino acids in the small
intestine
• Absorption of amino acids is coupled to active transport
of Na+
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Amino acids of protein fragments
Brush border enzymes
Apical membrane (microvilli)
Lumen of
intestine
Pancreatic
proteases
1 Proteins and protein fragments
are digested to amino acids by
pancreatic proteases (trypsin,
chymotrypsin, and carboxypeptidase), and by brush border
enzymes (carboxypeptidase,
aminopeptidase, and dipeptidase)
of mucosal cells.
Na+
Na+
Absorptive
epithelial
cell
2 The amino acids are then
absorbed by active transport into
the absorptive cells, and move to
their opposite side (transcytosis).
Amino
acid
carrier
3 The amino acids leave the
Active transport
Passive transport
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Capillary
villus epithelial cell by facilitated
diffusion and enter the capillary
via intercellular clefts.
Figure 23.33
Protein digestion
Foodstuff
Protein
Large polypeptides
Small polypeptides,
small peptides
Amino acids
(some dipeptides
and tripeptides)
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Enzyme(s)
and source
Pepsin
(stomach glands)
in presence
of HCl
Pancreatic
enzymes
(trypsin, chymotrypsin,
carboxypeptidase)
Brush border
enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Site of
action
Path of absorption
• Amino acids are absorbed
by cotransport with
Stomach
sodium ions.
• Some dipeptides and
tripeptides are absorbed
via cotransport with H++
Small
and hydrolyzed to amino
intestine
acids within the cells.
• Amino acids leave the
epithelial cells by
Small
facilitated diffusion, enter
intestine
the capillary blood in the
villi, and are transported
to the liver via the hepatic
portal vein.
Figure 23.32 (2 of 4)
Chemical Digestion and Absorption of
Lipids
• Small intestine= sole site for lipid digestion
• Pre-treatment—emulsification by bile salts
• Enzymes—pancreatic lipases are the
enzymes that digest fats after they have been
pretreated with bile
• Absorption of glycerol and short chain fatty
acids
• Absorbed into the capillary blood in villi
• Transported via the hepatic portal vein
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Chemical Digestion and Absorption of
Lipids
• Absorption of monoglycerides and fatty acids
• Cluster with bile salts and lecithin to form
micelles
• Released by micelles to diffuse into epithelial
cells
• Combine with proteins to form chylomicrons
• Enter lacteals and are transported to systemic
circulation
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Fat globule
1 Large fat globules are emulsified
(physically broken up into smaller fat
droplets) by bile salts in the duodenum.
Bile salts
Fat droplets
coated with
bile salts
2 Digestion of fat by the pancreatic
enzyme lipase yields free fatty acids and
monoglycerides. These then associate
with bile salts to form micelles which
“ferry” them to the intestinal mucosa.
Micelles made up of fatty
acids, monoglycerides,
and bile salts
3 Fatty acids and monoglycerides leave
micelles and diffuse into epithelial cells.
There they are recombined and packaged
with other lipoid substances and proteins
to form chylomicrons.
4 Chylomicrons are extruded from the
Epithelial
cells of
small
intestine
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Lacteal
epithelial cells by exocytosis. The
chylomicrons enter lacteals. They are
carried away from the intestine by lymph.
Figure 23.34
Fat digestion
Foodstuff
Enzyme(s)
and source
Unemulsified
fats
Emulsification by
the detergent
action of bile
salts ducted
in from the liver
Pancreatic
lipases
Monoglycerides Glycerol
and fatty acids
and
fatty acids
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Site of
action
Path of absorption
• Fatty acids and monoglycerides
enter the intestinal cells via
diffusion.
Small
intestine • Fatty acids and monoglycerides
are recombined to form
triglycerides and then
combined with other lipids and
proteins within the cells, and
the resulting chylomicrons are
Small
extruded by exocytosis.
intestine
• The chylomicrons enter the
lacteals of the villi and are
transported to the systemic
circulation via the lymph in the
thoracic duct.
• Some short-chain fatty acids
are absorbed, move into the
capillary blood in the villi by
diffusion, and are transported
to the liver via the hepatic
portal vein.
Figure 23.32 (3 of 4)
Chemical Digestion and Absorption of
Nucleic Acids
• Enzymes
• Pancreatic ribonuclease & deoxyribonuclease
present in pancreatic juice
• Hydrolyze DNA & RNA to their nucleotide
monomers
• Absorption
• Active transport
• Transported to liver via hepatic portal vein
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Nucleic acid digestion
Foodstuff
Enzyme(s)
and source
Nucleic acids
Pentose sugars,
N-containing bases,
phosphate ions
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Pancreatic ribonuclease and
deoxyribonuclease
Brush border
enzymes
(nucleosidases
and phosphatases)
Site of
action
Path of absorption
• Units enter intestinal cells
by active transport via
Small
intestine membrane carriers.
• Units are absorbed into
capillary blood in the villi
Small
and transported to the
intestine
liver via the hepatic portal
vein.
Figure 23.32 (4 of 4)
Vitamin Absorption
• In small intestine dietary vitamins are absorbed
• Fat-soluble vitamins (A, D, E, and K) are
carried by micelles and then diffuse into
absorptive cells
• Water-soluble vitamins (vitamin C and B
vitamins) are absorbed by diffusion or by
passive or active transporters.
• Vitamin B12 binds with intrinsic factor, and is
absorbed by endocytosis
• In large intestine, Vitamin K and B vitamins from
bacterial metabolism are absorbed
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Nutrient Absorption
• Absorption occurs along the entire length of the small
intestine, and most of it is completed before the chyme
reaches the ileum
• Several types of carriers transport the different amino
acids before entering the capillary blood by diffusion
• Monoglycerides and free fatty acids of lipid digestion
become associated with bile salts and lecithin to form
micelles, which are necessary for lipid absorption
• Pentose sugars, nitrogenous bases, and phosphate ions
are transported actively across the epithelium by special
transport carriers in the villus epithelium
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Electrolyte Absorption
• Mostly along the length of small intestine
• Iron and calcium are absorbed in duodenum
• Na+ is coupled with absorption of glucose and
amino acids
• Ionic iron is stored in mucosal cells with ferritin
• K+ diffuses in response to osmotic gradients
• Ca2+ absorption is regulated by vitamin D and
parathyroid hormone (PTH)
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Water Absorption
• Most abundant substance in chyme
• 95% is absorbed in the small intestine by
osmosis
• Net osmosis occurs whenever a concentration
gradient is established by active transport of
solutes
• Water uptake is coupled with solute uptake
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Malabsorption of Nutrients
• Causes
• Anything that interferes with delivery of bile or
pancreatic juice
• Damaged intestinal mucosa (e.g., bacterial
infection)
• Gluten-sensitive enteropathy (celiac disease)
• Gluten damages the intestinal villi and brush
border
• Treated by eliminating gluten from the diet (all
grains but rice and corn)
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Developmental Aspects
• In the third week
• Endoderm has folded and foregut and hindgut
have formed
• Midgut is open and continuous with the yolk
sac
• Mouth and anal openings are nearly formed
• In the eighth week
• Accessory organs are budding from endoderm
• Alimentary canal is a continuous tube
stretching from the mouth to the anus
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Developmental Aspects
• The epithelial lining of the developing
alimentary canal forms from the endoderm with
the rest of the wall arising from the mesoderm
• The anteriormost endoderm touches the
depressed area of the surface ectoderm where
the membranes fuse to form the oral
membrane and ultimately the mouth
• The end of the hindgut fuses with an
ectodermal depression, called the proctodeum,
to form the cloacal membrane and ultimately
the anus
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Lung bud
Brain
Oral
membrane
Heart
Yolk sac
Cloacal
membrane
Body
stalk
Stomodeum
Foregut
Stomach
Liver
Site of
liver
development
Midgut
Spinal cord
Bile
duct
Gallbladder
Hindgut
Cystic duct
Ventral pancreatic bud
Dorsal
pancreatic
bud
Duodenum
Proctodeum
Endoderm
(a)
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(b)
Figure 23.35
Developmental Aspects
• During old age
• GI tract activity (motility) declines, digestive
juice production decreases, absorption is less
efficient, and peristalsis is slowed, resulting in
less frequent bowel movements and often
constipation
• Diverticulosis, fecal incontinence, and cancer
of the GI tract are fairly common
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Cancer
• Stomach and colon cancers rarely have early
signs or symptoms
• Metastasized colon cancers frequently cause
secondary liver cancer
• Prevention
• Regular dental and medical examination
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