Chapter 24
Digestive System
24-1
I. Introduction:
A. Anatomy of the Digestive System
• 1. Digestive tract: also called alimentary tract
• 2. GI tract: technically refers to stomach and
intestines
• 3. Accessory organs
• 4. Regions
– Mouth or oral cavity with salivary glands
and tonsils
– Pharynx (throat
– Esophagus
– Stomach
– Small intestine (duodenum, ileum,
jejunum)
– Large intestine including cecum, colon,
rectum
– anal canal with mucous glands
– Anus
24-2
B. Functions of the Digestive System
1. Ingestion:
2. Mastication:.
3. Propulsion
– Deglutition: swallowing
– Peristalsis:
– Mass movements
– chyme
24-3
4. Mixing: Segmental contractions
• http://www.youtube.com/watch?v=GdNtRomPvs&NR=1
24-4
Functions, cont.
5. Secretion: lubricate, liquefy, digest
–
–
–
–
Mucus:
Water:
Bile:
Enzymes:
6. Digestion: Mechanical and chemical
7. Absorption:
8. Elimination:
24-5
C. Histology of the Digestive Tract
•1. Mucosa
•2. Submucosa
24-6
• 3. Muscularis:
•
1. circular and longitudinal
•
2. smooth except for upper esophagus
• 4. Serosa or adventitia:
•
1. visceral peritoneum in abdominal cavity
•
2. tunica adventitia outside of the abdominal cavity
24-7
D. Nervous regulation of the Digestive System
– 1.
•
•
•
Local: enteric nervous system
Types of neurons: sensory, motor, interneurons
Coordinates peristalsis and regulates local reflexes
As stomach empties into small intestine, local reflex
regulates rate of emptying
– 2. General: coordination with the CNS. May initiate
reflexes because of sight, smell, or taste of food.
Parasympathetic primarily. Sympathetic input inhibits
muscle contraction, secretion, and decrease of blood
flow to the digestive tract.
24-8
E. Peritoneum
– 1. Visceral:
– 2. Parietal:
– 3. Retroperitoneal:
e.g., kidneys, pancreas,
duodenum
– 4. Mesenteries:
– 5. Greater omentum:
connects greater
curvature of the stomach
to the transverse colon.
– 6. Lesser omentum:
connects lesser curvature
of the stomach and the
proximal part of the
duodenum to the liver
and diaphragm
– G. Hand in balloon
explanation
24-9
Greater omentum
24-10
II. Terms of Oral Cavity
–
–
–
–
–
–
–
–
–
–
–
–
–
–
A. Vestibule:
B. Oral cavity proper:
C. Frenulum:
1. labial
2. lingual
D. Teeth
1. incisors
2. canines
3. premolars
4. molars
E. Gingiva
F. Hard palate
G. Soft palate
H. Uvula
24-11
I. Tongue
• 1. Muscular
– Intrinsic muscles: change shape
– Extrinsic muscles: protrude or
retract tongue, move side to side
• 2. Lingual frenulum
• 3. Terminal sulcus: groove divides
tongue into anterior 2/3; posterior 1/3
•
a. Anterior part: papillae, some of
which have taste buds
•
b. Posterior part: no papillae and a
few scattered taste buds.
•
c. Lymphoid tissue embedded in
posterior surface: lingual tonsil
• 4. Moves food in mouth, participates
in speech and swallowing
24-12
J. Muscles of Mastication
• 1. masseter
• 2. temporalis
• 3. medial and
lateral
pterygoidsproduce lateral
excursion when
acting together.
• 4.
http://www.you
tube.com/watch
?v=pV2TRZE7
pJM
24-13
K. Salivary Glands-three pairs
– 1. Parotid: largest.
– a.Serous.
– b. Parotid duct enters the
oral cavity adjacent to the
2nd upper molar
– 2. Sublingual: smallest.
– a.Mixed, but primarily
mucous.
– b.Each has 10-12 ducts
that enter the floor of the
oral cavity.
– 3. Submandibular: mixed,
– a.Posterior half of inferior
border of mandible.
– b.Duct enters oral cavity
on either side of lingual
frenulum
24-14
I. Functions of Saliva
– 1. Prevents bacterial
infection
– 2. Lubrication
– 3. Contains salivary
amylase that breaks
down starch into
disaccharides
– 4. Helps to form
bolus for deglutition
– 5. Parasympathetic
input causes salivary
production
– 6. Sympathetic
stimulation leads to
thick mucoid saliva
24-15
III. Pharynx
A. Common chamber of respiratory
and digestive system
B. Regions
C. Structures ensuring that food gets
to the right place
1. hard palate
2. soft palate and uvula
3. epiglottis
24-16
IV. Esophagus
A. 10” long
B. Tunical mucosa- Stratified
squamous epithelium
C. Passes through diaphragm
at esophageal hiatusHiatal hernia
D. sphinters
E. GERD
F. Peristalsis of bolus
G.
http://www.youtube.com/watc
h?v=Q-n_Q0qKXzg
H. Tunica muscularis-upper
third voluntary with lower two
thirds visceral
I. Swallowing (Deglutition)-three phases
– 1. Voluntary: bolus of food moved by tongue from
oral cavity to pharynx.
– 2. Pharyngeal: reflex.
– a. Controlled by swallowing center in medulla
oblongata.
– b. Soft palate elevates, upper esophageal sphincter
relaxes,
– c. elevated pharynx opens the esophagus,
– d. food pushed into esophagus by pharyngeal
constrictors’ successive contraction from superior to
inferior.
– e. Epiglottis is tipped posteriorly due to pressure of
the bolus,
– f. larynx elevated to prevent food from passing into
larynx.
– 3. Esophageal: reflex. Stretching of esophagus causes
enteric NS to initiate peristalsis of muscles in the
esophagus.
24-18
V. Stomach A. Parts
1. Openings
– Gastroesophageal
(cardiac): to esophagus
– Pyloric: to duodenum
2. Parts
–
–
–
–
Cardiac
Fundus
Body
Pyloric: antrum and
canal
– Greater and lesser
curvatures:
attachment sites for
omenta
– Sphincters
– Pyloric
24-19
B. Histology of the Stomach
1. Layers
– a. Serosa or visceral
peritoneum
– b. Muscularis: three
layers
• Outer
longitudinal
• Middle circular
• Inner oblique
– c. Submucosa
– d. Mucosa
– e. Rugae: folds in
mucosa and
submucosa when
empty.
24-20
2. Modifications of the tunica mucosa
•
•
a. Gastric pits: openings
for gastric glands. Lined
with simple columnar
epithelium
b. Cells of gastric pits
– Mucous neck:
mucus
– Parietal:
hydrochloric acid
and intrinsic factor
(aids in vitamin B12
absorption
– Chief: pepsinogen
– Endocrine:
regulatory hormones
• histamine that
stimulates acid
secretion
• Somatostatin
that inhibits
gastrin and
insulin secretion
24-21
3. Secretions of the Stomach
• a. Chyme: ingested food plus stomach secretions
• b. Mucus: surface and neck mucous cells
– Viscous and alkaline
– Protects from acidic chyme and enzyme pepsin
– Irritation of stomach mucosa causes greater mucus
• c. Intrinsic factor: parietal cells. Binds with vitamin B12 and helps
it to be absorbed. B12 necessary for DNA synthesis
• d. HCl: parietal cells
– Kills bacteria
– Stops carbohydrate digestion by inactivating salivary amylase
– Denatures proteins
– Helps convert pepsinogen to pepsin
• e. Pepsinogen: packaged in zymogen granules released by
exocytosis. Pepsin catalyzes breaking of covalent bonds in proteins
24-22
4. Hydrochloric Acid Production produces alkaline tide
in veins draining stomach
24-23
5. Regulation of stomach activity
a. Cephalic Phase
•
• The taste or smell or even
thoughts of food stimulate
the medulla oblongata.
• Parasympathetic action
potentials are carried by the
vagus nerves to the stomach
• Postganglionic 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
24-24
b. Gastric Phase
24-25
c. Intestinal Phase
-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-enterogastric reflex
-Secretin, and cholecystokinin produced by the duodenum decrease gastric secretions in
the stomach.
24-26
6. Movements of the Stomach-reflux mixing
• a. Initially both esophageal
and pyloric sphincters are
closed.
• b. A lot like kneading dough
• c. Mixture is turned back at
the pyloric sphincter and
folds over itself
• d. Liquefied mixture is
called chyme
• e. At some point pyloric
sphincter opens a little and a
bit of acidic chyme squirts
through
• f. The acidic additions to the
small intestine lead to the
enterogastric reflex
24-27
7. Rate of stomach emptying
• a. Goldilocks and the three bears
• b. Regulated by CCK (lipid rich meals) and
secretin
• c. Carbohydrate meals fastest through-1
hour
• d. Fatty meals slowest (5-6 hours)
• e. Greater the stretching the more reflux
24-28
VI. Small Intestine
• Site of greatest
amount of digestion
and absorption of
nutrients and water
• Divisions
– Duodenum- first 25
cm beyond the pyloric
sphincter.
– Jejunum- 2.5 m
– Ileum- 3.5 m. Peyer’s
patches or lymph
nodules
24-29
A. Modifications to Increase Surface Area
– 1. Plicae
circulares
(circular folds)
– 2. Villi that
contain
capillaries and
lacteals. Folds of
the mucosa
– 3. Microvilli:
folds of cell
membranes of
absorptive cells
– 4. total surface
area about that
of a tennis court
24-30
B. Mucosa and Submucosa
• 1. 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
– Granular cells (paneth
cells): may help protect from
bacteria
• 2. Intestinal glands (crypts of
Lieberkühn): tubular glands in
mucosa at bases of villi
• 3. Duodenal glands
(Brunner’s glands): tubular
mucous glands of the
submucosa.
24-31
C. Jejunum and Ileum
• 1. Gradual decrease in
diameter, thickness of
intestinal wall, number of
circular fold, and number of
villi the farther away from
the stomach
• 2. Peyer’s patches:
lymphatic nodules
numerous in mucosa and
submucosa
• 3. Ileocecal junction:
where ilium meets large
intestine. Ileocecal
sphincter and ileocecal
valve
24-32
D. Secretions of the Small Intestine
• 1. Fluid primarily composed of
water, electrolytes and mucus.
• 2. Mucus
– Protects against digestive
enzymes and stomach acids
• 3. Digestive enzymes: bound to
the membranes of the absorptive
cells
– Disaccharidases: Break down
disaccharides to
monosaccharides
– Peptidases: Hydrolyze
peptide bonds
– Nucleases: Break down
nucleic acids
• 4. Brunner’s glands
– Stimulated by vagus nerve,
secretin, chemical or tactile
irritation of duodenal mucosa
24-33
E. Movement in the Small Intestine
• 1. Segmental contractions mix
• 2. Peristalsis propels
• 3. Ileocecal sphincter remains slightly contracted
until peristaltic waves reach it; it relaxes, allowing
chyme to move into cecum
• 4. Cecal distention causes local reflex and
ileocecal valve constricts
– Prevents more chyme from entering cecum
– Increases digestion and absorption in small intestine by
slowing progress of chyme
– Prevents backflow
24-34
VII. Liver, Gallbladder, Pancreas and Ducts
24-35
VIII. Histology of the Liver
• A. Connective tissue
septa branch from the
visceral peritoneum into
the interior
– Divides liver into
lobules
– Nerves, vessels and
ducts follow the septa
• B. Lobules: portal triad
at each corner
– Three vessels: hepatic
portal vein, hepatic
artery, hepatic duct
– Central vein in
center of lobule
• C. Central veins unite to
form hepatic veins that
exit liver and empty into
inferior vena cava
24-36
• D. Hepatic cords: radiate
out from central vein.
Composed of hepatocytes
• E. Hepatic sinusoids:
between cords, lined with
endothelial cells and hepatic
phagocytic (Kupffer) cells
• F. Bile canaliculus:
between cells within cords
• G. Hepatocyte functions
– Bile production
– Storage
– Interconversion of
nutrients
– Detoxification
– Phagocytosis
– Synthesis of blood
components
24-37
H. Functions of the Liver
• 1. Bile production: 6001000 mL/day. Bile salts
(bilirubin), cholesterol,
fats, fat-soluble
hormones, lecithin
– Neutralizes and
dilutes stomach acid
– Bile salts emulsify
fats. Most are
reabsorbed in the
ileum.
– Secretin (from the
duodenum) stimulates
bile secretions,
increasing water and
bicarbonate ion
content of the bile
• 2. Storage
– Glycogen, fat,
vitamins, copper and
iron. Hepatic portal
blood comes to liver
from small intestine.
24-38
• 3. Nutrient interconversion
– Amino acids to energy producing compounds
– Hydroxylation of vitamin D. Vitamin D then travels to
kidney where it is hydroxylated again into its active
form-promotes bone growth and absorption of calcium
• 4. Detoxification
– Hepatocytes remove ammonia and convert to urea
• 5. Phagocytosis
– Kupffer cells phagocytize worn-out and dying red and
white blood cells, some bacteria
• 6. Synthesis
– Albumins, fibrinogen, globulins, heparin, clotting
factors
24-39
IX. Gallbladder
• A. Sac lined with mucosa
folded into rugae, inner
muscularis, outer serosa
• B. Bile arrives constantly
from liver is stored and
concentrated
• C. Stimulated by
cholecystokinin (from the
intestine) and vagal
stimulation
• D. Bile exits through cystic
duct then into common bile
duct
• E. Gallstones: precipitated
cholesterol
– Can block cystic duct
– Can occur because of
drastic dieting
24-40
Control of Bile Secretion and Release
24-41
X. Pancreas
• A. Pancreas both endocrine
and exocrine
• B. Endocrine: pancreatic
islets or Islets of
Langerhans.
• C. Exocrine: groups acini
(grape-like cluster) form
lobules separated by septa.
• D. Aqueous. Bicarbonate
lowers pH inhibiting pepsin
and providing proper pH
for enzymes
• E. Enzymatic portion:
– Trypsinogen
– Chymotrypsinogen
– Procarboxypeptidase
– Pancreatic amylase
– Pancreatic lipases
– Deoxyribonucleases
and ribonucleases
24-42
Histology of pancreas
24-43
F. Pancreatic Secretions
• 1. Interaction of duodenal and pancreatic enzymes
– Enterokinase from the duodenal mucosa and attached to the
brush border activates trypsinogen to trypsin.
– Trypsin activates chymotrypsinogen to chymotrypsin.
– Trypsin activates procarboxypeptidase to carboxypeptidase.
• 2. Trypsin, chymotrypsin, and carboxypeptidase digest proteins:
proteolytic.
• 3. Pancreatic amylase continues digestion of starch.
• 4. Pancreatic lipase digests lipids.
• 5. Deoxyribonucleases and ribonucleases digest DNA and
ribonucleic acid, respectively.
24-44
G. Control of Pancreatic Secretion
24-45
XI. Large Intestine
• A. Extends from
ileocecal junction to
anus
• B. Consists of cecum,
colon, rectum, anal
canal
• C. Movements sluggish
(18-24 hours); chyme
converted to feces.
• D. Absorption of water
and salts, secretion of
mucus, extensive action
of microorganisms.
• E. 1500 mL chyme
enter the cecum, 90% of
volume reabsorbed
yielding 80-150 mL of
feces
24-46
E. Anatomy of the Large Intestine
•
•
1. Cecum
– Blind sac, vermiform
appendix attached.
2. Colon
– Ascending, transverse,
descending, sigmoid
– Circular muscle layer
complete; longitudinal
incomplete (three teniae
coli). Contractions of
teniae form pouches
called haustra.
– Mucosa has numerous
straight tubular glands
called crypts. Goblet
cells predominate, but
there are also absorptive
and granular cells as in
the small intestine
24-47
• 3. Rectum
– Straight
muscular tube,
thick muscular
tunic
• 4. Anal canalsuperior epithelium
is simple columnar;
inferior epithelium is
stratified squamous
– Internal anal
sphincter
(smooth muscle)
– External anal
sphincter
(skeletal muscle)
– Hemorrhoids:
Vein
enlargement or
inflammation
24-48
F. Secretions of the Large Intestine
• 1. Mucus provides
protection
– Parasympathetic
stimulation increases
rate of goblet cell
secretion
• 2. Bacterial actions
produce gases (flatus)
from particular kinds of
carbohydrates found in
legumes and in artificial
sugars like sorbitol
• 3. Bacteria produce
vitamin K which is then
absorbed
• 4. Feces consists of water,
undigested food
(cellulose),
microorganisms,
sloughed-off epithelial
cells
• 5. Lactose intolerance
24-49
G. Movement in the Large Intestine
• 1. Mass movements
– Common after meals
– Integrated by the enteric plexus
• 2. Local reflexes instigated by the presence of food in the stomach and
duodenum
– Gastrocolic: initiated by stomach
– Duodenocolic: initiated by duodenum
• 3. 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.
24-50
Movement in the Large Intestine
24-51
XII. Digestion and absorption
A. Carbohydrates
1.
2.
3.
4.
5.
Monosaccharide vs.
disaccharide vs.
polysaccharide
Carbohydrate digestion
begins in the mouthsalivary amylase
Nothing occurs in the
stomach as salivary
amylase is denatured
Pancreatic amylase
takes the carbo to the
disaccharide level
Epithelium of the small
intestine produces
disaccharidases
24-52
B. Protein digestion
• 1. amino acid vs. protein
• 2. initial breakdown
begins in stomach with
pepsin
• 3. continues in intestine
with trypsin,
chymotrypsin, and
carboxypeptidase
• 4. dipeptides are broken
down by peptidases from
epithelial cells of the
mucosa
24-53
C. Lipid digestion
• 1. categories of lipids
•
a. phospholipids
•
b. Triglycerides
•
c. Cholesterol
• 2. diverse group but all
are hydrophobic
• 3. digestion
commences in the small
intestine with
pancreatic lipase and
bile salts
24-54
4. Transport of Lipids Across Intestinal Epithelium
24-55
5. Transport of Lipids in blood stream
• All lipids carried in the blood are done so in combination
with protein to make them soluble in plasma.
• Cholesterol: 15% ingested; 85% manufactured in liver and
intestinal mucosa
• Lipids are lower density than water; proteins are higher
density than water
• Chylomicrons: 99% lipid and 1% protein (extremely low
density); enter lymph
• VLDL: 92% lipid, 8% protein
– Form in which lipids leave the liver
– Triglycerides removed from VLDL and stored in
adipose cells. VLDL has been converted to LDL.
• LDL: 75% lipid, 25% protein
– Transports cholesterol to cells
– Cells have LDL receptors
– # of LDL receptors become less once cell’s
lipid/cholesterol needs are met.
• HDL: 55% lipid, 45% protein
– Transports excess cholesterol from cells to liver
24-56
Water and Ions
• Water: can move in
either direction across
wall of small intestine
depending on osmotic
gradients
• Ions: sodium,
potassium, calcium,
magnesium, phosphate
are actively
transported
24-57