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Essentials of Human Anatomy & Physiology
Chapter 38-2
The Digestive System
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
The
Digestive
System
Vocabulary
Headings
Important
Info
Overview of GI tract
Functions
• Mouth: bite, chew, swallow
• Pharynx and Esophagus:
transport
• Stomach: mechanical
disruption; absorption of
water & alcohol
• Small Intestine: chemical &
mechanical digestion &
absorption
• Large Intestine: absorb
electrolytes & vitamins
(B and K)
• Rectum & Anus: defecation
Layers of the GI Tract
1. Mucosal Layer
– secretes enzymes &
absorbs nutrients
2. Submucosal Layer
– containing BV, glands &
lymphatic tissue
3. Muscularis Layer
– Skeletal:control over swallowing and defecation
– Smooth: mixes, crushes & propels food along by
peristalsis
4. Serosa Layer
– Covers all organs and walls of cavities not open to
outside of body
– Secretes slippery fluid
Peritoneum
• Peritoneum
– visceral layer covers
organs
– parietal layer lines the
walls of body cavity
• Peritoneal Cavity
– potential space
containing a bit of
serous fluid
Salivary Glands
•
•
•
•
Parotid below your ear and over the masseter
Submandibular is under lower edge of mandible
Sublingual is deep to the tongue in floor of mouth
All have ducts that empty into the oral cavity
Composition and Functions of Saliva
• Wet food for easier
swallowing
• Dissolves food for tasting
• Protects mouth from infection
with its rinsing action:
– 1 to 1 ½ qt/day
• Bicarbonate ions buffer acidic
foods
– Bulimia---vomiting hurts the
enamel on your teeth
• Salivary Amylase (enzyme):
– Starts chemical digestion
of starch
• Lysozyme (enzyme):
– helps destroy bacteria
Structure and Function of the
Tongue
• Muscle of
tongue is
attached to
hyoid, mandible,
hard palate and
styloid process
• Papillae are the
bumps:
– taste buds are
protected by
being on the
sides of papillae
Tooth Structure & Composition
•
•
•
•
Crown
Neck
Roots
Pulp cavity
• Enamel
– hardest substance in
body
– calcium phosphate or
carbonate
• Dentin
– calcified connective
tissue
• Cementum
– bone-like
– periodontal ligament
penetrates it
Dentition
• Primary/baby teeth
– 20 teeth that start
erupting @ 6months
– 1 new pair of teeth
per month
• Permanent teeth
– 32 teeth erupt b/t
6 & 12 years of age
– differing structures indicate function
• incisors for biting
• Canines/cuspids for tearing
• premolars & molars for crushing/grinding food
Primary and Secondary
Dentition
Digestion in the Mouth
• Mechanical Digestion
(Mastication/Chewing)
• breaks into pieces
• mixes with saliva forming: Bolus
• Chemical Digestion
– Amylase
• begins starch digestion at:
– pH of 6.5 or 7.0 found in
mouth
• when bolus & enzyme hit pH 2.5
gastric juices hydrolysis ceases
– Lingual Lipase
• secreted by glands in tongue
• begins breakdown of
triglycerides
into fatty acids and glycerol
Pharynx
•
•
•
Funnel-shaped tube extending
from internal nares to the
esophagus (posteriorly) and
larynx (anteriorly)
Skeletal muscle lined by
mucous membrane
Deglutition/swallowing is
facilitated by saliva and mucus
1. starts when bolus is pushed
into the oropharynx
2. sensory nerves send signals
to deglutition center in
brainstem
3. soft palate is lifted to close
nasopharynx
4. larynx is lifted as epiglottis
is bent to cover glottis
Esophagus
• Collapsed muscular
tube
• In front of
vertebrae
• Posterior to trachea
• Posterior to the
heart
• Pierces the
diaphragm at hiatus
– hiatal hernia or
diaphragmatic hernia
Physiology of the Esophagus - Swallowing
• Voluntary Phase: tongue pushes food to back of oral cavity
• Involuntary Phase: pharyngeal stage
– breathing stops & airways are closed
– soft palate & uvula are lifted to close off nasopharynx
– vocal cords close
– epiglottis is bent over airway as larynx is lifted
Swallowing
• Upper sphincter relaxes
when larynx is lifted
• Peristalsis pushes food down
– circular fibers behind bolus
– longitudinal fibers in front of
bolus shorten the distance of
travel
• Travel time:
– 4-8 seconds for solids
– 1 sec for liquids
• Lower sphincter relaxes
as food approaches
Gastroesophageal Reflex Disease
• If lower sphincter fails to open
– distension of esophagus feels like
chest pain or heart attack
• If lower sphincter fails to close
– stomach acids enter esophagus &
cause heartburn (GERD)
– for a weak sphincter---don't eat
a large meal & lay down in front
of TV
– smoking & alcohol make sphincter
relax worsening situation
• Control symptoms by avoiding
– coffee, chocolate, tomatoes,
fatty foods, onions & mint
– take Tagamet HB or Pepcid AC
60 minutes before eating
– neutralize existing stomach acids
with Tums
Anatomy of Stomach
• Parts of stomach
–
–
–
–
Cardia
Fundus
Body
Pylorus:
• starts to narrow
as approaches
Pyloric Sphincter
• Empties as small
squirts of chyme
leave the stomach
through the pyloric
valve
Pylorospasm and Pyloric Stenosis
• Abnormalities of the pyloric
sphincter in infants
• Pylorospasm
– muscle fibers of sphincter
fail to relax trapping food in
the stomach
– vomiting occurs to relieve
pressure
• Pyloric Stenosis
– narrowing of sphincter
indicated by projectile
vomiting
– must be corrected surgically
Histology of the Stomach
Mucosa & Gastric Glands
• Hydrochloric acid converts
pepsinogen from chief cell
to pepsin
• Intrinsic factor
– absorption of vitamin B12
for RBC production
• Gastrin hormone (g cell)
– “get it out of here”
– release more gastric
juice
– increase gastric motility
– relax pyloric sphincter
– constrict esophageal
sphincter preventing
entry
Muscularis
• Three layers of
smooth muscle:
– Outer
Longitudinal
– Circular
– Inner Oblique
• Permits greater
churning & mixing
of food with
gastric juice
Physiology of Digestion—
Mechanical
vs.
Chemical
• Gentle mixing waves
– every 15 to 25
seconds
– mixes bolus with 2
quarts/day of gastric
juice to turn it into
chyme (a thin liquid)
• More vigorous waves
– travel from body of
stomach to pyloric
region
• Intense waves near
pylorus
– opens & squirts out
1-2 teaspoons full
w/each wave
•Protein digestion begins
–HCl denatures protein molecules
–HCl transforms Pepsinogen into
Pepsin
–breaks peptide bonds b/t certain
a.a.
•Fat digestion continues
–gastric lipase splits the
triglycerides in milk fat
–most effective at pH 5 to 6
(infant stomach)
•HCl kills microbes in food
•Mucous cells protect stomach
walls from being digested w/13 mm thick layer of mucous
Cephalic Phase =
“Stomach Getting Ready”
• Cerebral Cortex:
– sight, smell, taste
& thought
– stimulate PNS
• Vagus Nerve:
– increases stomach
muscle and
glandular activity
Gastric Phase =
“Stomach Working”
• Nervous control keeps stomach
active
– stretch receptors &
chemoreceptors provide info
– vigorous peristalsis and glandular
secretions continue
– chyme is released into duodenum
• Endocrine influences over
stomach activity
– distention & presence of caffeine or
protein cause G cells secretion of
gastrin into bloodstream
– gastrin hormone increases stomach
glandular secretion
– gastrin hormone increases stomach
churning and sphincter relaxation
Intestinal Phase = “Stomach Emptying”
• Stretch receptors in
duodenum slow stomach
activity & increase intestinal
activity
• Distension, fatty acids or
sugar signals medulla
•
– SNS slow stomach activity
Hormonal influences
– secretin hormone decreases
stomach secretions
– Cholecystokinin (CCK)
decreases stomach emptying
– Gastric Inhibitory
Peptide (GIP) decreases
stomach secretions, motility
& emptying
Absorption of Nutrients by the Stomach
•
•
•
•
Water especially if it is cold
Electrolytes
Some drugs (especially aspirin) & alcohol
Fat content in the stomach slows the passage of
alcohol to the intestine where absorption is more
rapid
• Gastric mucosal cells contain alcohol
dehydrogenase that converts some alcohol to
acetaldehyde
– more of this enzyme found in males than females
• Females have less total body fluid that same size
male so end up w/higher blood alcohol levels with
same intake of alcohol
Digestive Hormones
• Gastrin
– stomach, gastric & ileocecal
sphincters
• Gastric inhibitory peptide--GIP
– stomach & pancreas
• Secretin
– pancreas, liver
& stomach
• Cholecystokinin--CCK
– pancreas,
gallbladder,
sphincter of
Oddi, & stomach
Digestive Enzymes
• When chyme enters the duodenum, proteins &
carbohydrates are only partly digested, & fat digestion
needs to be carried out
• Enzymes aid in digestive breakdown & absorption of chyme
_____________________________________________________________
Bile Salts (Liver & Gallbladder)
Pancreatic & Salivary Amylase (Pancreas & Mouth
Fat  Fat Droplets
Starch + H2O  Maltose
Bile is a thick
digestive fluid
secreted by the liver
and stored in
gallbladder.
Facilitates digestion
by emulsifying fats
into fatty acids,
which can be
absorbed by the
digestive tract
Trypsin & Pepsin (Pancreas)
Protein + H2O  Peptides
Lipase (Pancreas)
Fat Droplets + H2O  Glycerol + Fatty Acids
Peptidases (Intestinal Juice)
Peptides + H2O  Amino Acids
Maltase (Intestinal Juice)
Maltose + H2O  Glucose
Vomiting (Emesis): Reverse Peristalsis
1.
A deep breath is taken, the glottis is closed, and the larynx is raised
to open the upper esophageal sphincter. Soft Palate is elevated to
close of external nares
2.
Diaphragm contracts down to create a negative pressure in the
thorax, which facilitates opening of the esophagus and esophageal
sphincter
3.
Simultaneously with the downward movement of the diaphragm, the
abdominal muscles contract elevating inner gastric pressure. With
the pylorus closed and the esophagus open the exit route is clear
Anatomy of the Pancreas
• 5" long by 1" thick
• Head close to curve in
C-shaped duodenum
• Main duct joins
common bile duct from
liver
• Sphincter of Oddi on
major duodenal papilla
– Bile and pancreatic secretions enter the
digestive system through this point
• Opens 4" below pyloric sphincter
Composition and Functions of
Pancreatic Juice
• 1.5 Quarts/day
• pH 7.1 to 8.2
• Contains water,
enzymes & sodium
bicarbonate
• Digestive enzymes
– pancreatic amylase,
pancreatic lipase,
proteases
– Ribonuclease
• to digest nucleic acids
– deoxyribonuclease
Pancreatitis
Pancreatitis:
• inflammation of the
pancreas occurring with
the mumps
Acute Pancreatitis:
• associated with heavy
alcohol intake or biliary
tract obstruction
– result is patient
secretes trypsin in
pancreas & starts to
digest themselves
Regulation of Pancreatic Secretions
• Secretin
– acidity in intestine
causes increased
sodium bicarbonate
release
• GIP
– fatty acids & sugar
causes increased insulin
release
• CCK
– fats and proteins cause
increased digestive
enzyme release
Anatomy of the
Liver and Gallbladder
• Liver
–
–
–
–
–
weighs 3 lbs.
below diaphragm
right lobe larger
gallbladder on right lobe
size causes right kidney
to be lower than left
• Gallbladder
– fundus, body & neck
– The gallbladder stores about 50 ml of bile
Liver Functions—Carbohydrate Metabolism
• Turn proteins 
glucose
• Turn triglycerides 
glucose
• Turn excess glucose 
glycogen & store in the
liver
• Turn glycogen back 
glucose as needed
Liver Functions: Lipid Metabolism
• Synthesize cholesterol
• Synthesize lipoproteins
– HDL
(high-density lipoprotein)
• helps move cholesterol
back to liver for removal
from bloodstream (GOOD)
– LDL
(low-density lipoprotein)
• helps cholesterol stick to
artery walls (BAD)
• Breaks down some fatty
acids
Liver Functions:
Protein Metabolism
• Deamination = removes NH2 (amine group) from
amino acids so can use what is left as energy source
• Converts resulting toxic ammonia (NH3) into urea
for excretion by the kidney
• Synthesizes plasma proteins utilized in clotting
mechanism and immune system
• Convert one amino acid into another
Other Liver Functions
• Detoxifies blood by removing or
altering drugs & hormones
(thyroid & estrogen)
• Removes the waste product:
– Bilirubin
• Releases bile salts help
digestion by emulsification
• Stores fat soluble vitamins:
– A, B12, D, E, K
• Stores iron and copper
• Phagocytizes worn out blood
cells & bacteria
• Activates Vitamin D:
– the skin can also do this with
1 hr of sunlight a week
Bile Production
• 1 quart of bile/day is
secreted by the liver
– yellow-green in color &
pH 7.6 to 8.6
• Components
– water & cholesterol
– Bile Salts = Na & K salts
of bile acids
– Bile Pigments (bilirubin)
from hemoglobin
molecule
• Globin = a reuseable
protein
• Heme = broken down
into iron and bilirubin
Pathway of Bile Secretion
• Bile capillaries
• Hepatic ducts connect to form common hepatic
duct
• Cystic duct from gallbladder & common hepatic
duct join to form common bile duct
• Common bile duct & pancreatic duct empty into
duodenum
Regulation of Bile Secretion
Blood Supply to the Liver
• Hepatic Portal Vein
– nutrient rich blood
from stomach,
spleen & intestines
• Hepatic artery
branch off the
aorta
Anatomy of the Small Intestine
• 20 feet long: 1 inch in diameter
• Large surface area for majority of
absorption
• 3 parts
– Duodenum: 10 inches
– Jejunum: 8 feet
– Ileum: 12 feet
• ends at
Ileocecal Valve
• Plica Circularis
– permanent ½ inch tall folds
that contain part of
submucosal layer
– not found in lower ileum
– cannot stretch out like
stomach
• Villi
– 1 Millimeter tall
– Contains vascular capillaries
and lacteals (lymphatic
capillaries)
• Microvilli
– Absorption and digestion
– Digestive enzymes found at
cell surface on microvilli
– Digestion occurs at cell
surfaces
Small
Intestine
Small Intestine: Regions and Structures
Mechanical Digestion in Small Intestine
• Weak peristalsis in
comparison to the
stomach:
– chyme remains for
3 to 5 hours
• Segmentation:
– local mixing of chyme
with intestinal juices
• sloshing back &
forth
Digestion of
Carbohydrates
• Mouth: salivary amylase
• Esophagus & stomach:
nothing happens
• Duodenum:
pancreatic amylase
• Brush Border Enzymes
(maltase, sucrase &
lactose) act on
disaccharides
– produces monosaccharides-fructose, glucose &
galactose
– lactose intolerance (no
enzyme; bacteria ferment
sugar)--gas & diarrhea
Digestion of
Nucleic Acids
•Pancreatic juice contains
2 nucleases
–ribonuclease which digests
RNA
–deoxyribonuclease which
digests DNA
•Nucleotides produced
are further digested by
brush border enzymes
•Absorbed by active
transport
Digestion of
Proteins
• Stomach
– HCl denatures or unfolds
proteins
– pepsin turns proteins into
peptides
• Pancreas
– digestive enzymes:
split peptide bonds b/t
different amino acids
– brush border enzymes:
split off amino acid at
amino end of molecule or
split dipeptide
Digestion of
Lipids
•Mouth: lingual lipase
•Small intestine
–emulsification by bile
–Pancreatic Lipase:
– splits into fatty
acids &
monoglyceride
–No enzymes in
brush border
Absorption in Small
Intestine
Absorption of
Monosaccharides
• Absorption into epithelial
cell
– glucose & galactose:
sodium symporter(active
transport)
– Fructose: facilitated
diffusion
• Movement out of epithelial
cell into bloodstream
– by facilitated diffusion
Absorption of
Amino Acids &
Dipeptides
•Absorption into
epithelial cell
–active transport
w/Na+ or H+ ions
•Movement out of
epithelial cell into
blood
–diffusion
Absorption of Lipids
• Small fatty acids enter cells & then blood by simple
diffusion
• Larger lipids exist only within micelles (bile salts coating)
• Lipids enter cells by simple diffusion leaving bile salts
behind in gut
• Bile salts reabsorbed into blood & reformed into bile in
the liver
Absorption of Electrolytes
• Sources of electrolytes
– GI secretions & ingested foods and liquids
• Enter epithelial cells by diffusion & secondary active
transport
– sodium & potassium move = Na+/K+ pumps (active
transport)
– chloride, iodide and nitrate = passively follow
– iron, magnesium & phosphate ions = active transport
• Intestinal Ca+
absorption requires
vitamin D &
parathyroid hormone
Absorption of Vitamins
• Fat-Soluble Vitamins
– A, D, E, K
– travel in micelles & are
absorbed by simple diffusion
• Water-Soluble Vitamins
– Vitamin B complex, Ca
– absorbed by diffusion
• B12 combines with intrinsic
factor before it is
transported into the cells
– receptor mediated
endocytosis
Absorption of Water
• 9 liters of fluid dumped
into GI tract each day
• Small intestine reabsorbs
8 liters
• Large intestine
reabsorbs 90% of that
last liter
• Absorption is by osmosis
through cell walls into
vascular capillaries inside
villi
•
•
•
•
Anatomy of Large Intestine
5 feet long by 2½ inches in diameter
Ascending & descending colon are retroperitoneal
Cecum & appendix
Rectum = last 8 inches of GI tract anterior to the sacrum &
coccyx
• Anal canal = last 1 inch of GI tract
– internal sphincter----smooth muscle & involuntary
– external sphincter----skeletal muscle & voluntary control
Appendicitis
• Inflammation of the appendix due to blockage of
the lumen by chyme, foreign body, carcinoma,
stenosis, or kinking
• Symptoms
– high fever, elevated WBC
count, neutrophil count above
75%
– referred pain, anorexia, nausea
and vomiting
– pain localizes in right lower
quadrant
• Infection may progress to gangrene and
perforation within 24 to 36 hours
Mechanical Digestion in Large Intestine
• Smooth muscle: mechanical digestion
• Peristaltic Waves
– 3 to 12 contractions/minute
• Haustral Churning:
– relaxed pouches filled from
below by muscular contractions
(elevator)
• Gastroileal Reflex
– when stomach is full, gastrin
hormone relaxes ileocecal
sphincter so small intestine will
empty and make room
• Gastrocolic Reflex
– when stomach fills, a strong
peristaltic wave moves contents
of transverse colon into rectum
Chemical Digestion in Large Intestine
• No enzymes are secreted
only mucous
• Bacterial Fermentation
– undigested carbohydrates
into carbon dioxide &
methane gas
– undigested proteins into
simpler substances
(indoles)----odor
– turn bilirubin into simpler
substances that produce
color
• Bacteria produce vitamin
K and B in colon
Absorption & Feces Formation in Large Intestine
• Some electrolytes:
– Na+ and Cl-
• After 3 to 10 hours, 90%
of H2O has been removed
from chyme
• Feces are semisolid by
time reaches transverse
colon
• Feces = dead epithelial
cells, undigested food
such as cellulose,
bacteria (live & dead)
Defecation
• Gastrocolic reflex
moves feces into rectum
• Stretch receptors signal
sacral spinal cord
• Parasympathetic nerves
contract muscles of
rectum & relax internal
anal sphincter
• External sphincter is
voluntarily controlled
Defecation Problems
• Diarrhea
– chyme passes too quickly through
intestine
– H20 not reabsorbed
• Constipation
– decreased intestinal motility
– too much
water is
reabsorbed
– Remedy =
fiber,
exercise
and water
Dietary Fiber
• Insoluble Fiber
– Woody parts of
plants (wheat bran,
veggie skins)
– speeds up transit
time & reduces
colon cancer
• Soluble Fiber
– gel-like consistency
– beans, oats, citrus
white parts, apples
– lowers blood cholesterol
by preventing reabsorption of bile salts so liver
has to use cholesterol to make more
Aging and the Digestive System
• Changes that occur:
– decreased secretory
mechanisms & motility
– loss of strength & tone of
muscular tissue
– changes in neurosensory
feedback
– diminished response to pain &
internal stimuli
• Symptoms:
– sores, loss of taste, peridontal
disease, difficulty swallowing,
hernia, gastritis, ulcers,
malabsorption, jaundice,
cirrhosis, pancreatitis,
hemorrhoids and constipation
– Colon or rectum is common
Categories of Nutrients
• Carbohydrates
– Most are derived from plants
– Exceptions: lactose from milk and small amounts of glycogens from
meats
• Lipids
– Saturated fats from animal products
– Unsaturated fats from nuts, seeds, and vegetable oils
– Cholesterol from egg yolk, meats, and milk products
• Proteins
– Complete proteins – contain all essential amino acids
• Most are from animal products
– Legumes and beans also have proteins, but are incomplete
• Vitamins
– Most vitamins are used as cofactors and act with enzymes
– Found in all major food groups
• Minerals
– Play many roles in the body
– Most mineral-rich foods are vegetables, legumes, milk, & some meats
Metabolism
The sum of all chemical reactions
necessary to maintain life
•Catabolism
•Substance is
broken down to
simpler products
•Energy is
released
•Anabolism
•Larger molecules
are built from
smaller ones
•Energy Required