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The Components of the Digestive System
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 24.1
Functions of the digestive system
• Ingestion
• Mechanical processing
• Digestion
• Secretion
• Absorption
• Excretion
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Movement of digestive materials
• Visceral smooth muscle shows rhythmic cycles of
activity
• Pacemaker cells
• Peristalsis
• Waves that move a bolus
• Segmentation
• Churn and fragment a bolus
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Peristalsis
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Figure 24.4
Control of the digestive system
• Movement of materials along the digestive tract is
controlled by:
• Neural mechanisms
• Hormonal mechanisms
• Enhance or inhibit smooth muscle
contraction
• Local mechanisms
• Coordinate response to changes in pH or
chemical stimuli
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]The Regulation of Digestive Activities
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Figure 24.5
The mouth opens into the oral or buccal cavity
• Its functions include:
• Analysis of material before swallowing
• Mechanical processing by the teeth, tongue,
and palatal surfaces
• Lubrication
• Limited digestion
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Anatomy of the Mouth and Throat
8
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Human Deciduous and Permanent Teeth
9
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Dorsal Surface of the Tongue
10
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The tongue
• primary functions include:
• Mechanical processing
• Assistance in chewing and swallowing
• Sensory analysis by touch, temperature, and
taste receptors
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The pharynx
• Common passageway
for food, liquids, and
air
• Lined with stratified
squamous epithelium
• Pharyngeal muscles
assist in swallowing
• Pharyngeal
constrictor muscles
• Palatal muscles
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The Esophagus
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Figure 24.10a-c
Deglutition (swallowing)
• Sequence
• Voluntary stage
• Push food to back of mouth
• Pharyngeal stage
• Raise
• Larynx + hyoid
• Tongue to soft palate
• Esophageal stage
• Contract pharyngeal muscles
• Open esophagus
• Start peristalsis
14
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Deglutition (swallowing)
• Control
• Nerves
• Glossopharyngeal
• Vagus
• Accessory
• Brain stem
• Deglutition center
• Medulla oblongata
• Pons
• Disorders
• Dysphagia
• Aphagia
15
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Esophagus
• Sphincters
• Upper
• Lower
16
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• Abnormalities
• Achalasia – lower esophagus muscles don’t relax- doesn’t
allow food to pass through
• Atresia- esophogus openings are closed shut (Birth
Defect)
• Hernia- Esophogus pushes into stomach
• Barret’s esophagus- tissue is more like intestinal lining
tissue- can develop a rare cancer from this
• Esophageal varices- enlarged veins that can leak blood
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Functions of the stomach
• Bulk storage of undigested food
• Mechanical breakdown of food
• Disruption of chemical bonds via acids and
enzymes
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Digestion and absorption in the stomach
• Preliminary digestion of proteins
• Pepsin
• Permits digestion of carbohydrates
• Very little absorption of nutrients
• Some drugs, however, are absorbed
• Mucous secretion containing several hormones
• Enteroendocrine cells (Endocrine= Hormones!)
• G cells secrete gastrin – stimulates pareital
cells to secrete HCL
• D cells secrete somatostatin- inhibits gastric
secretions
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The Stomach
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Figure 24.12b
The Stomach Lining
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Figure 24.13a, b
The Stomach Lining
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Figure 24.13c, d
Stomach Histology
•
•
Gastric pits: openings for gastric glands. Lined with
simple columnar epithelium
Cells of gastric pits
• Surface mucus: mucus that protects stomach
lining from acid and digestive enzymes
• Mucous neck: mucus
• Parietal: hydrochloric acid and intrinsic factor
• Chief: pepsinogen
• Endocrine: regulatory hormones
• Gastrin-containing cells: secrete gastrin (a
hormone that stimulates acid secretion)
• Somatostatin-containing cells: secrete
somatostatin that inhibits gastrin and insulin
secretion
24-23
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Secretions of the Stomach
• Chyme: ingested food plus stomach secretions
• Mucus: surface and neck mucous cells
• Viscous and alkaline
• Protects from acidic chyme and enzyme pepsin
• Irritation of stomach mucosa causes greater mucus
• Intrinsic factor: parietal cells. Binds with vitamin B12 and helps
it to be absorbed in the ileum. B12 necessary for DNA synthesis
and RBC production (lack of B12 absorption leads to pernicious anemia)
24-24
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Secretions of the Stomach
• HCl: parietal cells
• Kills bacteria (found in ingested food)
• Stops carbohydrate digestion by inactivating salivary amylase
• Denatures proteins
• Helps convert pepsinogen to pepsin (optimal activity at pH 3 or
less)
• Pepsinogen: packaged and released by exocytosis. Pepsin
catalyzes breaking of covalent bonds in proteins (breaks them into
smaller peptide chains)
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Cephalic Phase
• The taste or smell of
food, tactile sensations of
food in the mouth, or
even thoughts of food
stimulate the medulla
oblongata.
• Parasympathetic action
potentials are carried by
the vagus nerves to the
stomach, where enteric
plexus
neurons are activated.
• Neurons stimulate secretion by parietal and chief cells (HCl and
pepsin) and stimulate the secretion of the hormone gastrin and
histamine.
• Gastrin is carried through the circulation back to the stomach where
it and histamine stimulate further secretion of HCl and pepsin.
24-26
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Gastric Phase
• Distention of the stomach activates a parasympathetic reflex.
Action potentials are carried by the vagus nerves to
the medulla oblongata.
• Medulla oblongata stimulates further secretions of the stomach.
• Distention also stimulates local reflexes that amplify stomach
secretions.
24-27
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Intestinal Phase
1.
2.
Chyme in the duodenum with a pH less than 2 or containing lipids
inhibits gastric secretions by three mechanisms
Sensory input to the medulla from the duodenum inhibits the motor
input from the medulla to the stomach. Stops secretion of pepsin and
HCl.
Local reflexes inhibit gastric secretion
Secretin, and cholecystokinin produced by the duodenum decrease
gastric secretions in the stomach.
3.
4.
24-28
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Movements in Stomach
• Combination of
mixing waves
(80%) and
peristaltic waves
(20%)
• Both esophageal
and pyloric
sphincters are
closed.
24-29
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Small intestine
• Important digestive and absorptive functions
• Secretions and buffers provided by pancreas,
liver, gall bladder
• Three subdivisions:
• Duodenum
• Jejunum
• Ileum
• Ileocecal sphincter
• Transition between small and large intestine
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Figure 24.16 Regions of the Small Intestine
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Figure 24.16a
Histology of the small intestine
• Plicae
• Transverse folds of the intestinal lining
• Villi
• Fingerlike projections of the mucosa
• Lacteals
• Terminal lymphatic in villus
• Intestinal glands
• Lined by enteroendocrine, goblet and stem
cells
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Modifications to Increase Surface Area
• Increase surface area
600 fold
• Plicae circulares
(circular folds)
• Villi that contain
capillaries and lacteals.
Folds of the mucosa
• Microvilli: folds of cell
membranes of
absorptive cells
24-33
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The Intestinal Wall
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Figure 24.17a
The Intestinal Wall
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Figure 24.17b, c
The Intestinal Wall
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Figure 24.17d, e
Intestinal juices
• Moisten chyme
• Help buffer acids
• Maintain digestive material in solution
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Mucosa and Submucosa of the Duodenum
• Cells and glands of the mucosa
• Absorptive cells: cells with microvilli, produce digestive
enzymes and absorb digested food
• Goblet cells: produce protective mucus
• Endocrine cells: produce regulatory hormones (Secretin,
and cholecystokinin)
• Granular cells (paneth cells): may help protect from
bacteria (contain lysozymes)
• Intestinal glands (crypts of Lieberkühn): tubular
glands in mucosa at bases of villi [secrete sucrase ,maltase,
trypsin, chymotrypsin, and pepsin (endopeptidases and exopeptidases) ]
• Duodenal glands (Brunner’s glands): tubular
mucous glands of the submucosa. Open into intestinal
glands [produce a mucus-rich alkaline secretion (containing bicarbonate)
24-38
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Jejunum and Ileum
• Gradual decrease in diameter, thickness of intestinal
wall, number of circular fold, and number of villi the
farther away from the stomach
• Major site of nutrient absorption
• Peyer’s patches: lymphatic nodules numerous in
mucosa and submucosa
• Ileocecal junction: where ileum meets large intestine.
Ileocecal sphincter (ring of smooth muscle) and ileocecal
valve (one-way valve)
24-39
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Liver, Gallbladder, Pancreas and Ducts
24-40
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The Pancreas
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Figure 24.18a-c
Pancreatic Secretions: Pancreatic Juice
• Enzymatic portion: (without the enzymes produced by pancreas, lipids, proteins, & carbs not
adequately digested)
•
•
•
•
•
•
Trypsinogen- active form is trypsin--------proteolytic enzyme
Chymotrypsinogen- active form is chymotrypsin--------proteolytic enzyme
Procarboxypeptidase- active form is carboxypeptidase-------proteolytic enzyme
Pancreatic amylase- continues digestion of starch.
Pancreatic lipases- lipid digesting enzyme
Deoxyribonucleases and ribonucleases- reduce DNA & RNA to their nucleotide
• Interaction of duodenal and pancreatic enzymes
• Enterokinase is a proteolytic enzyme from the duodenal mucosa and it
activates trypsinogen to trypsin.
• Trypsin activates chymotrypsinogen to chymotrypsin.
• Trypsin activates procarboxypeptidase to carboxypeptidase.
24-42
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Liver Histology
• Hepatic cords: radiate out
from central vein.
Composed of hepatocytes
• Hepatic sinusoids: between
cords, lined with endothelial
cells and hepatic
phagocytic (Kupffer) cells
• Bile canaliculus: between
cells within cords
• Hepatocyte functions
• Bile production
• Storage
• Interconversion of nutrients
• Detoxification
• Synthesis of blood components
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24-43
Functions of the Liver
• Bile production: 600-1000 mL/day. Bile salts, bilirubin (bile pigment
that results from breakdown of hemoglobin), cholesterol, fats, fat-soluble
hormones, lecithin
• Neutralizes and dilutes stomach acid (neutralizes chyme so that
pancreatic enzymes can function)
• Bile salts emulsify fats. Most are reabsorbed in the ileum. (90% bile
salts reabsorbed in the ileum & carried back to liver)
• Secretin (from the duodenum) stimulates bile secretions, increasing
water and bicarbonate ion content of the bile
• Storage
• Glycogen, fat, vitamins (A, B12, D, E, and K), copper and iron.
Hepatic portal blood comes to liver from small intestine (nutrients
are stored and secreted back into circulation when needed)
• Synthesis
• Blood proteins: Albumins, fibrinogen, globulins, heparin, clotting
factors (liver produces its own new compounds)
24-44
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Liver
• Lobes
• Major: Left and right
• Minor: Caudate and quadrate
• Porta: on inferior surface. Vessels,
ducts, nerves, exit/enter liver
• Hepatic portal vein, hepatic artery,
hepatic nerve plexus enter
• Lymphatic vessels, two hepatic ducts
exit
•Ducts
–Right and left hepatics (which transport
bile out of liver) unite to form
–Common hepatic
–Cystic: from gallbladder
–Common bile: union of cystic duct and
common hepatic duct
24-45
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The Anatomy of the Liver
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Figure 24.19b, c
Liver Histology
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Figure 24.20a, b
The gallbladder
• Hollow, pear-shaped organ
• Stores, modifies and concentrates bile
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The Gallbladder
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Figure 24.21a, b
Large Intestine
• Extends from ileocecal
junction to anus
• Consists of cecum, colon,
rectum, anal canal
• Movements sluggish (18-24
hours); chyme converted to
feces.
• Absorption of water and salts,
secretion of mucus, extensive
action of microorganisms are
involved in the formation of
feces.
• 1500 mL chyme enter the
cecum, 90% of volume
reabsorbed yielding 80-150
mL of feces
24-50
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The Large Intestine
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Figure 24.23a
Anatomy of Large Intestine
• Cecum
• Blind sac, appendix attached.
Appendix walls contain numerous
lymph nodules
• Colon
• Ascending, transverse,
descending, sigmoid
• Mucosa has numerous straight
tubular glands called crypts.
Goblet cells predominate, but
there are also absorptive and
granular cells as in the small
intestine
24-52
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The Large Intestine
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Figure 24.23b, c
Anatomy of Large Intestine
• Rectum
• Straight muscular tube,
thick muscular tunic
• Anal canal- superior epithelium
is simple columnar; inferior
epithelium is stratified
squamous
• Internal anal sphincter
(smooth muscle)
• External anal sphincter
(skeletal muscle)
• Hemorrhoids: Vein
enlargement or
inflammation
24-54
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Secretions of Large Intestine
• Mucus provides protection
• Pumps: bacteria remove acid from the epithelial
cells that line the large intestine
• Bacteria produce gases (flatus) from particular
kinds of carbohydrates found in legumes and in
artificial sugars like sorbitol
• Bacteria produce vitamin K which is then absorbed
• Feces consists of water, undigested food (cellulose),
microorganisms, sloughed-off epithelial cells
24-55
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Movement in Large Intestine
• Mass movements (strong contractions)
• Common after meals
• Local reflexes instigated by the presence
of food in the stomach and duodenum
• Gastrocolic: initiated by stomach
• Duodenocolic: initiated by duodenum
• Defecation
• Defecation reflex: distension of the
rectal wall by feces
• Parasympathetic stimulation
• Usually accompanied by voluntary
movements to expel feces. Abdominal
cavity pressure caused by inspiration
and by contraction of muscles of
abdominal wall.
24-56
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Histology of the large intestine
• Absence of villi
• Presence of goblet cells
• Deep intestinal glands
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The Defecation Reflex
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Figure 24.25
Carbohydrate digestion and absorption
• Begins in the mouth
• Salivary and pancreatic enzymes
• Disaccharides and trisaccharides
• Brush border enzymes
• Monosaccharides
• Absorption of monosaccharides occurs across the
intestinal epithelia
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Lipid digestion and absorption
• Lipid digestion utilizes lingual and pancreatic
lipases
• Bile salts improve chemical digestion by
emulsifying lipid drops
• Lipid-bile salt complexes called micelles are
formed
• Micelles diffuse into intestinal epithelia which
release lipids into the blood as chylomicrons
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Protein digestion and absorption
• Low pH destroys tertiary and quaternary
structure
• Enzymes used include pepsin, trypsin,
chymotrypsin, and elastase
• Liberated amino acids are absorbed
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Absorption
• Water
• Nearly all that is ingested is reabsorbed via
osmosis
• Ions
• Absorbed via diffusion, cotransport, and active
transport
• Vitamins
• Water soluble vitamins are absorbed by
diffusion
• Fat soluble vitamins are absorbed as part of
micelles
• Vitamin B12 requires intrinsic factor
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Digestive Secretion and Absorption of Water
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Figure 24.27