Transcript Digestive System

PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
23
The Digestive
System
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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
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Digestive System**
2. Accessory digestive organs
•
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**
•
Six essential activities
1. Ingestion
2. Propulsion
3. Mechanical digestion
4. Chemical digestion
5. Absorption
6. Defecation
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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
**
Peristalsis – the contraction and relaxation of
muscles that move food along the alimentary
canal.
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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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(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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External stimuli
(sight, smell, taste,
thought of food)
Central nervous system
and extrinsic autonomic nerves
Long reflexes
Afferent impulses
Internal
(GI tract)
stimuli
Efferent impulses
Chemoreceptors,
osmoreceptors, or
mechanoreceptors
Local (intrinsic)
nerve plexus
(“gut brain”)
Effectors:
Smooth muscle
or glands
Short reflexes
Gastrointestinal
wall (site of short
reflexes)
Lumen of the
alimentary canal
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Response:
Change in
contractile or
secretory activity
Figure 23.4
Peritoneum and Peritoneal Cavity
• Peritoneum: serous membrane of the
abdominal cavity – has two parts
• 1. Visceral peritoneum on external surface of
most digestive organs
• 2. Parietal peritoneum lines the body wall
• Peritoneal cavity
• Between the two peritoneums
• 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
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Blood Supply: Splanchnic Circulation
• Arteries
• Hepatic (_____), splenic (______), and 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
• Lines the lumen
• Functions
• Secretes mucus, digestive enzymes and
hormones
• Absorbs end products of digestion
• 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
• Blood and lymphatic vessels, lymphoid
follicles, and submucosal nerve plexus
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Submucosa and Muscularis Externa
• Muscularis externa
• 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
• 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
• 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
• 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
• Hard palate: palatine bones and palatine
processes of the maxillae
• Slightly corrugated to help create friction
against the tongue
• Soft palate: fold formed mostly of skeletal
muscle
• 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)
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Salivary Glands
• Intrinsic (buccal) salivary glands are scattered
in the oral mucosa
• 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
• 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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Falciform ligament
Liver
Gallbladder
Spleen
Stomach
Ligamentum teres
Greater omentum
Small intestine
Cecum
(a)
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Figure 23.30a
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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Large Intestine: Microscopic Anatomy
• Mucosa of simple columnar epithelium except
in the anal canal (stratified squamous)
• Abundant deep crypts with 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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Chemical Digestion and Absorption of
Carbohydrates
• 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
• 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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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
• Enzymes: pepsin in the stomach
• Pancreatic proteases
• Trypsin, chymotrypsin, and carboxypeptidase
• Brush border enzymes
• Aminopeptidases, carboxypeptidases, and
dipeptidases
• 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
• Pre-treatment—emulsification by bile salts
• Enzymes—pancreatic lipase
• 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 and
deoxyribonuclease
• 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
• 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
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Vitamin Absorption
• In large intestine
• Vitamin K and B vitamins from bacterial
metabolism are absorbed
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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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Malabsorption of Nutrients
• Causes
• Anything that interferes with delivery of bile or
pancreatic juice
• Damaged intestinal mucosa (e.g., bacterial
infection)
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Malabsorption of Nutrients
• 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
• During old age
• GI tract activity declines, absorption is less
efficient, and peristalsis is slowed
• Diverticulosis, fecal incontinence, and cancer
of the GI tract
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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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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