Organs of the
Digestive System
For student copy
The Mouth
• aka oral or buccal cavity
• cheeks form lateral walls
– internally covered by mucous membrane:
nonkeratinized, stratified sq epith
– wall of cheeks: buccinator muscle then subq &
skin
• lips or labia surround opening
– inner surface of each lip attached to its gum by
a midline fold of mucous membrane called a
labial frenulum “small bridle”
Mouth - 2
• vestibule: space between buccal mucosa &
teeth
• oral cavity proper: space that extends from
gums & teeth  fauces: opening between
oral cavity & pharynx
• hard palate: anterior portion of roof of
mouth
– maxillae & palatine bones form bony partition
between oral & nasal cavities
– covered by mucous membrane
MOUTH - 3
• soft palate: forms posterior portion of roof
of mouth
– muscular partition between oropharynx &
nasopharynx
– uvula : hangs from free border of soft palate
– when swallowing soft palate & uvula drawn
superiorly preventing food & liquids from
entering nasal cavity
Salivary Glands
• release saliva into oral cavity
• 4 sets:
1. Parotid glands (“near ear”)
– between masseter & skin
– parotid duct secretes saliva into vestibule
opposite 2nd molar
2. Submandibular glands
– floor of mouth/ enter just lateral to lingual
frenulum
3. Sublingual glands
– under tongue
4. lesser Sublingual glands: floor of mouth
Salivary Glands
Saliva
• 99.5% water
• 0.5% solutes:
– ions
– urea & uric acid
– mucus
– Ig A
– lysozyme (bacteriostatic enzyme)
– salivary amylase: digestive enzyme acts on
starch
Salivation
• controlled by ANS
• average adult secretes 1000 – 1500 mL/day
• parasympathetic stimulation promotes
continuous secretion  keeps mouth moist &
lubricates tongue & lips during speech
• saliva is then swallowed moistening
esophagus  most water is reabsorbed
• sympathetic stimulation dominates if
stressed  mouth dry
• dehydration: secretion stops to conserve
water
Mumps
• inflammation &
enlargement of
parotid glands
• pain, malaise, fever
• swelling on affected
side
Tongue
• skeletal muscles covered by mucous
membrane forming floor of oral cavity
• median septum separates tongue into
symmetric ½ s (attaches to hyoid bone)
• lingulum frenulum
– limits movement posteriorly
– if abnl short: “tongue-tied”
• each ½ composed of extrinsic & intrinsic
muscles
– extrinsic: origins out of tongue/ insert to CT in
tongue: move tongue side-to-side/ anchor tongue
– intrinsic: origin & insertion in tongue: alter shape
& size of tongue for speech & swallowing
Tongue - 2
• dorsum (upper surface) & lateral surfaces
covered with papillae
– projections of lamina propria covered with
keratinized epithelium
– some contain taste buds
– others touch receptors
– all increase friction between tongue/food
– lingual glands secrete mucous & a watery
serous fluid that contains enzyme lingual
lipase: acts on triglycerides
Taste Buds
• most on tongue, few on soft palate,
pharynx, & epiglottis
• each taste bud has 3 types epith cells:
1. supporting cells
– surround ~50 receptor cells
2. gustatory receptor cells
– single microvillus
3. basal cells
Taste Buds - 2
• each taste bud has 3 types epith cells:
1. supporting cells
– surround ~50 receptor cells
2. gustatory receptor cells
– single microvillus from each = gustatory hair
extends thru a taste pore (opening in taste bud)
3. basal cells
– stem cells @ edge of taste bud
– produce supporting cells that then develop into
gustatory cells (each lasts ~10 days)
Teeth
• dentes
• in alveolar processes of mandible &
maxillae
– covered by gingivae: “gums”
– lined by peridontal ligament: anchors tooth to
socket
• parts of a tooth:
1. Crown
2. Root
3. Neck
Crown of Tooth
• visible portion, above level of gums
• interior made of dentin: calcified CT
– gives shape & rigidity to tooth
– harder than bone
• covered by enamel
– Ca++ phosphate & carbonate
– hardest substance in body
– protects tooth from:
• wear & tear of chewing
• acids that could dissolve dentin
Pulp Cavity
• w/in dentin
• pulp: CT with blood, lymph & nerve
supply to tooth
• extension thru roots = root canals 
opening @ base for vessels/nerve to
enter/exit tooth
Root of Tooth
• below gums
• covered by cementum
2 Dentitions
1. Deciduous teeth
– baby or primary teeth
– ~6 mos  24/mo  20 total
– lost age 6 - 12
2. Secondary teeth
– 32
– begin to erupt ~ 6  adult
Digestion in the Mouth
• mastication: chewing
MECHANICAL
– food manipulated by tongue/ground by
teeth & mixed with saliva
– bite of food reduced to soft, flexible,
easily swallowed mass = bolus
1. Salivary amylase
• initiates breakdown of starch
–
–
into di- & trisaccharides, shorter polymers
only monosaccharides can be absorbed
2. Lingual lipase
• becomes activated in acid pH of stomach
CHEMICAL
Pharynx
• funnel-shaped tube extends from internal
nares  esophagus posteriorly & to the
larynx anteriorly
• skeletal muscle covered by mucous
membranes
• 3 parts:
1. Nasopharynx: functions only in
respiration
2. Oropharynx: digestive + respiratory
functions
3. Laryngopharynx: digestive & respiratory
Esophagus
• collapsable muscular tube
• posterior to trachea
• begins @ inferior end of laryngopharynx
 passes thru mediastinum  pierces
diaphragm (opening called esophageal
hiatus)  ends in superior portion of
stomach
Histology of the Esophagus
• mucosa:
– nonkeratinized stratified sq epith
– lamina propria
– Muscularis mucosae (smooth muscle)
• submucosa: areolar CT
• muscularis:
– upper 1/3 skeletal
– mid 1/3 skeletal & smooth
– lower 1/3 smooth
Ends of Esophagus
• muscularis thickens forming:
1. upper esophageal sphincter (UES)
– skeletal
– regulates movement of food from pharynx
 esophagus
2. lower esophageal sphincter (LES)
– smooth
– regulates movement of food from esophagus
 stomach
Physiology of the Esophagus
• secretes mucus & transports food 
stomach
• No enzymes produced or secreted
• No absorption
Deglutition
• swallowing
• facilitated by secretion of saliva & mucus
• involves mouth, pharynx, esophagus
1. Voluntary stage
– bolus of food from oral cavity to oropharynx
– stimulates receptors in oropharynx 
deglutition center in medulla & lower pons
 effector fibers cause soft palate & uvula to
move up to close off nasopharynx AND
epiglottis closes off opening of larynx
Deglutition Involuntary Stage
2. Esophageal stage
• bolus enters esophagus
• peristalsis: progression of coordinated
contractions & relaxations of circular &
longitudinal layers of muscularis, pushes
bolus onward
Stomach
• J-shaped enlargement of GI tract
• just inferior to diaphragm
• connects esophagus  duodenum
– most distensible part of GI tract
• serves as a
1. mixing chamber
2. holding reservoir
Stomach Adaptations for
Digestion
1.
2.
3.
4.
5.
6.
7.
rugae
mucus glands:
secretion of H+ & Clpepsin
gastric lipase
intrinsic factor
3-layered muscularis
Anatomy of the Stomach
Histology of the Stomach
• 4 basic layers in stomach wall:
– (stomach wall is impermeable to most
substances)
1. surface mucosa = simple columnar epith
that extend down into lamina propria
where they form columns of secretory
cells called gastric glands, channels
between columns called gastric pits
Mechanical Digestion in
Stomach
• few minutes after food bolus enters
stomach: gentle, rippling, peristaltic
movements called mixing waves pass thru
stomach q15 – 25 s
– macerate food
– mix with mucus secretions
– results: chyme soupy liquid  pylorus
Pyloric Sphincter
• slightly open
• when chyme down
to lower pylorus,
each mixing wave
forces ~ 3 mL
chyme into
duodenum = gastric
emptying
Chemical Digestion in the
Stomach
• salivary amylase:
– continues to function while food in fundus
– when churning forces bolus further into
stomach the acid pH inactivates it
• lingual lipase:
– acid pH activates
– triglycerides  fatty acids & diglycerides
Chemical Digestion - 2
• H+ & Cl- ions secreted separately by
parietal cells
• secretion stimulated by:
– parasympathetic neurons
– gastrin (from G cells)
– histamine (from mast cells in lamina propria):
receptors on parietal cells = H2 receptors
Stomach Acid
• kills many microbes in food
• partially denatures proteins
• stimulates secretion of hormones that
promote flow of bile & pancreatic juice
Pepsin
• secreted by chief cells
• greatest activity in low pH / inactivates in
higher pH of small intestine
• secreted as pepsinogen: inactive form of
pepsin (so will not break down proteins in
chief cells
• severs peptide bonds breaking protein 
smaller peptide fragments
• stomach wall protected from pepsin by
alkaline mucus secreted by surface cells
Gastric Lipase
• splits short-chain triglycerides  fatty
acids & monoglycerides
• most effective @ pH 5-6 (limited role in
stomach)
Absorption in Stomach
• very little in stomach (epithelial cells
impermeable to most substances)
• mucous cells do absorb some:
– water
– ions
– short-chain fatty acids
– aspirin
– alcohol
Stomach
•
•
•
•
2 – 4 hrs for food to exit
meal mostly carbs: shortest time
protein – rich meal longer
fat-laden meal longest
Emesis (vomiting)
• forcible expulsion of contents of upper GI
tract (stomach +/- duodenum)
• strongest stimuli: irritation & distension of
stomach
• other irritants:
– unpleasant sites
– general anesthesia
– dizziness
– drugs: morphine, digitalis derivatives
Pancreas
• retroperitoneal gland: lies posterior to greater
curvature of stomach
• 3 parts:
1. Head: expanded portion near curve of
duodenum
2. Body:
3. Tail: tapering portion
• Ducts:
1. Pancreatic: runs length of pancreas, joins
bile duct
2. Accessory: branch of pancreatic
Histology of the Pancreas
• exocrine portion made up of small cluster
of glandular cells = acini
– about 99% of pancreas
– secrete mixture of fluid & digestive enzymes
= pancreatic juice
Pancreatic Juice
• 1200 – 1500 mL/d
• mostly: water, salts, sodium bicarbonate
(makes pH 7.1 – 8.2), & enzymes (secreted in
an inactive form):
1.
2.
3.
4.
5.
6.
pancreatic amylase
trypsin
chymotrypsin
carboxypeptidase
elastase
pancreatic lipase: #1 triglyceride-digeting
enzyme
7. ribonuclease & deoxyribonuclease
Liver
• heaviest gland in body (~1.4 kg or 3 lb)
• 2nd largest organ in body
• found inferior to diaphragm taking up
most of RUQ
Anatomy of the Liver
• Ligaments:
1. falciform: attaches liver to anterior
abdominal wall
2. coronary: attaches liver to diaphragm
3. ligamentum teres: remnant of umbilical
vein
Anatomy of the Liver
• Lobes:
1. left: smaller
– includes inferior lobe: quadrate
– and a posterior lobe: caudate
2. right: larger
Histology of the Liver
• lobules:
• functional unit of liver
• 6-sided structure made of specialized
epith cells called hepatocytes
– arranged in branching, interconnected plates
around a central vein
– highly permeable capillaries called sinusoids
– fixed macrophages in sinusoids called Kupffer
cells destroy worn out RBCs, bacteria or other
foreign material in venous blood that just
arrived from small intestine
Bile
• secreted by hepatocytes  bile canaliculi 
bile ductiles  bile ducts @ periphery of
lobules
• eventually, bile flows into larger right & left
hepatic ducts  combine to form & exit liver
as common hepatic duct
• made by hepatocytes  stored in gallbladder
 small intestine
• function: emulsification of fats (large lipid
globules  suspension of small lipid
globules so can be absorbed
Jaundice
• yellowish coloration of sclera & mucous
membranes due to a buildup of bilirubin
– formed as product of breaking down heme
pigment in worn out RBCs
– excreted in bile
– 3 categories:
1. Prehepatic
•
excess production of bilirubin
2. Hepatic
•
due to congenital liver disease, cirrhosis, hepatitis
3. Extrahepatic:
•
blockage bile drainage by gallstones or CA of bowel
or pancreas
Neonatal Jaundice
• aka physiologic jaundice
• liver enzymes not always mature @ birth
– generally resolves in <10 -12 days
– treated with blue lights: changes bilirubin into
products that can be cleared by kidneys
Functions of the Liver #1
• Carbohydrate Metabolism
• liver has important role in maintaining a
normal blood glucose level
– if low: glycogen  glucose  increases blood
levels
– enzymes can convert a.a. or lactic acid glucose
– if high: glucose  glycogen or triglycerides for
short-term or long-term storage
Functions of the Liver #2
• Lipid Metabolism
• Hepatocytes:
– store some triglycerides
– fatty acids broken down  ATP
– synthesize lipoproteins (transport fatty acids,
triglycerides, steroids)
– synthesize cholesterol & use some to make
bile salts
Functions of the Liver #3
• Protein Metabolism
• Hepatocytes:
– deaminate a.a. (remove amine group) (rest of
a.a. then used to make ATP or convert to
carbs or fats)
• amine group  urea  excreted in kidneys
– synthesize most plasma proteins
• α & β globulins
• albumin
• prothrombin
• fibrinogen
Functions of the Liver #4
• Processing of Drugs & Hormones
• detoxifies alcohol & drugs
• chemically alters & excretes steroid
hormones
– thyroid
– estrogens
– aldosterone
Functions of the Liver #5
• Excretion of Bilirubin
• absorbed by hepatocytes from aged RBCs
then secreted into bile
– most bilirubin in bile metabolized in small
intestines by bacteria  eliminated in feces
Functions of the Liver #6
• Synthesis of Bile Salts
• used in small intestine for emulsification
& absorption of lipids
Functions of the Liver #7
•
•
•
•
Storage
glycogen
Vitamins A, B12, D, E, & K
Fe, Cu
Functions of the Liver # 8
• Phagocytosis
• Kupffer cells eat aged RBCs, WBCs, &
some bacteria
Functions of the Liver # 9
• Activation of Vit D
• skin, liver, & kidneys
all have role in
synthesizing the
active form of Vit D
Gallbladder
• pear-shaped sac found in a depression
tucked under liver
• function: store bile until fatty chyme
enters duodenum
– bile  cystic duct  common bile duct 
ampulla of Vater  duodenum
• parts:
1. fundus
2. body
3. neck
Gallstones
• If bile contains insufficient bile salts or
lecithin or excessive cholesterol, the
cholesterol crystallizes to form stones
• passing small ones can cause intermittent
pain but larger ones can obstruct  severe
pain/infection
Small Intestine
• site of most major events of digestion &
absorption
• adaptations
1.
2.
–
–
–
long: ~ 10 ft in living adult/ 21 ft in dead
surface area increased by
circular folds
villi
microvilli
Functions of the Small Intestine
1. segmentations mix chyme with digestive
juices & bring food particles into contact
with the mucosa for absorption;
peristalsis propels chyme thru
2. completes digestion of carbs, proteins,
lipids; begins & completes digestion of
nucleic acids
3. absorbs 90% of nutrients & water that
pass thru
Anatomy of Small Intestine
• 3 regions
1. Duodenum
–
“12” width of 12
fingers
2. Jejunum
–
–
1 m in length
“empty”
3. Ileum
–
–
2m
joins large intestine
@ ileocecal valve
Histology of Small Intestine
• same 4 layers of GI tract
• mucosa: simple columnar epith with 6
types of cells
1. absorptive cells
2. goblet cells
3. intestinal glands (crypts of Lieberkϋhn)
4. Paneth cells
5. Enteroendocrine cells: S, CCK, K
Absorptive Cells
• digest & absorb
nutrients from chyme
• apical membranes
form microvilli:
projections that
increase surface area:
form fuzzy line called
brush border
Crypts of Lieberkϋhn
• intestinal glands w/in deep crevices of
small intestine mucosa
• secrete intestinal juices
Paneth Cells
• enteroendocrine cells
• secrete lysozyme:
bactericidal enzyme
• also can act as
phagocytes
Enteroendocrine Cells
• S cells: secrete hormone secretin
• CCK cells: secrete hormone
cholecystokinin (CCK)
• K cells: secrete hormone glucosedependent insulinotropic peptide (GIP)
Histology of Small Intestine
• Lamina Propria:
• areolar CT with:
– abundance of MALT (mucosa-associated
lymphoid tissue)
– solitary lymphatic nodules in distal ileum
– Peyer’s Patches: aggregated lymphatic
follicles also found in ileum
Histology of Small Intestine
• Submucosa:
• Bruner’s glands in duodenum: secrete
alkaline mucus that neutralizes the gastric
acid in chyme
Structural Features that Facilitate
Digestion & Absorption:
1. Circular folds:
– folds of mucosa & submucosa (~1 cm long)
– proximal duodenum  mid-ileum
– increase surface area for absorption
2. Villi:
– fingerlike projections of mucosa (~,5 – 1 mm
long)
– gives mucosa velvety appearance
3. Microvilli:
– brush border
Lacteals
• the arteiole, venule, capillary bed & a
lymphatic capillary (the lacteal) found
w/in a villus
Intestinal Juice
• clear, yellow fluid
• + pancreatic juice makes a liquid
environment for chemical digestion &
absorption
• contains: water, alkaline mucus (pH 7.6)
Brush-Border Enzymes
• absorptive cells secrete several digestive
enzymes  inserting them into the plasma
membranes of microvilli
– lumen enzymes (from pancreatic secretions)
break larger molecules into smaller ones)
–
– brush- border enzymes break them down
even smaller (small enough to be absorbed)
– cytoplasmic enzymes finish digestion
Mechanical Digestion in Small
Intestine
• Segmentations: localized, mixing
contractions, occur in portions of sm
intestine distended by large vol of chyme
– mix chyme with digestive juices
– bring particles of food into contact with
mucosa for absorption
– Do Not push chyme onward
Brush-Border Enzymes
•
•
•
•
•
•
•
digest:
α-dextrins
maltose
sucrose
lactose
peptides
nucleotides
Absorption in Small Intestine
Absorption of Alcohol
• lipid-soluble – so begins absorption in
stomach where surface area for absorption
much less than in small intestine
– longer stays in stomach the slower blood
alcohol levels rise
– fatty acids in chyme slow gastric emptying
– gastric juices have enzyme alcohol
dehydrogenase (breaks down alcohol): ♀ 60%
less than ♂ so become intoxicated on less
• alcohol  chyme into small intestine
absorption more rapid
– greater surface area
Large Intestine
terminal portion of GI tract
~1.5 m long & 6.5 cm diameter
ileocecal sphincter  anus
attached to posterior abdominal wall by its
mesocolon
• Regions:
1. Cecum
2. Colon
3. Rectum
4. Anal Canal
•
•
•
•
Functions of Large Intestine
1. haustral churning, peristalsis, & mass
peristalsis
2. bacteria convert proteins to a.a.,
breakdown a.a., & produce some B
vitamins and vitamin K
3. absorption of some: water, ions, vitamins
4. forming feces
5. defecation (emptying rectum)
Large Intestine: Layers
• Mucosa:
– many mucous cells
– mucus only secretion
– absorptive cells that absorb water
• Muscularis:
– external layer of longitudinal smooth muscle &
internal layer of circular but here longitudinal
layer in bands = teniae coli that run most of
length
– contraction of teniae cause series of pouches =
haustra (singular: haustrum) which give organ a
puckered appearance
Cecum
• opening from ileum has fold of mucous
membrane = ileocecal valve
– hanging inferior to it is cecum (small pouch)
– vermiform appendix attached to cecum which
is attached to mesocolon by mesoappendix
Appendicitis
• inflammation of appendix
• starts with obstruction to entry of appendix
– chyme, tumor, foreign body
• Symptoms: high fever, elevated WBC’s
(neutrophils), anorexia, pain McBurney’s
point
– appendix can become edematous, gangrenous,
rupture w/in 24 hrs
– often patient presents after rupture (much higher
mortality rate)
Colon
Parts of Colon
• ascending colon  rt hepatic flexure 
transverse colon  lt splenic flexure 
descending colon  level of iliac crest
where it becomes sigmoid colon (Sshaped)  projects medially to midline 
terminates as rectum
Rectum
• last 20 cm of GI tract
• anterior to sacrum & coccyx
• anal canal: last 2 – 3 cm
– mucous membranes arranged in longitudinal
folds called anal columns: contain network of
arteries & veins
– opening to outside body = anus: guarded by
an internal anal sphincter (smooth muscle) &
an external anal sphincter (skeletal muscle)
– normally, both closed except during
elimination of feces
Mechanical Digestion in Large
Intestine
• ileocecal sphincter: controls chyme
entering cecum
– relaxes from hormone gastrin
• gastroileal reflex:
– starts immediately after meal
– intensifies peristalsis in ileum
– which forces any chyme there  cecum: when
distended ileocecal sphincter tightens
Mechanical Digestion in Large
Intestine - 2
• Haustral churning: characteristic movement
in large intestine
– Haustra remain relaxed & become distended
while they fill up
• Peristalsis:
– slower rate (3 – 12 contractions/min) than in
proximal GI tract
• Mass Peristalsis:
– strong peristaltic wave begins mid-transverse
colon & quickly drives forward contents 
rectum
Chemical Digestion in
Large Intestine
• done by bacteria acting on chyme
1. use fermentation on remaining carbs 
gases (H2, C2, CH4)
• Flatulence: excessive gas
2. convert remaining proteins  a.a. 
skatole, indole (contributes to odor of feces)
3. decompose bilirubin  simpler pigments
(gives brown color to feces)
4. produce Vitamin K & some B Vitamins
Occult Blood
• “hidden blood” (not visible)
• screen for colorectal cancer
GI Tract Homeostatic
Imbalances
• Colorectal Cancer:
– 2nd to lung ca in males & 3rd after lung &
breast in females for deaths due to ca
– >1/4th have family hx
– 5 – 6% have known gene
– ~95% adenocarcinomas:
• many start as colon polyp
• flat, depressed lesions more likely to be malignant
GI Tract Homeostatic
Imbalances
• Hepatits
–
–
–
–
inflammation of liver
Causes: viruses, drugs, alcohol, chemicals
Viral Hepatitis:
A: hep A virus
• spread by fecal contamination  orally
• mild in children & young adults
• jaundice, malaise , anorexia, nausea, diarrhea, fever,
chills
• resolves 4 – 6 wks
• NO lasting damage
GI Tract Homeostatic
Imbalances
• Hepatitis B
– Hep B virus
– spread thru body fluids:
• sexual contact, contaminated needles or transfusion
equipment
• tears, saliva,
– can be present a lifetime  cirrhosis or ca of liver
– vaccines available
• Hepatitis C
hep C virus
similar to hep B
GI Tract Homeostatic
Imbalances
• Hepatitis D
• Hepatitis E
– Hep E spread like Hep
• hep D virus
A
• person must already
– No liver damage
be infected with hep B
– has high mortality in
to get hepD
pregnant women
• severe liver damage
• higher fatality rate
than hep B alone
Medial Terminology
• Cirrhosis:
• scarred liver due to
chronic inflammation
due to hepatitis,
chemicals, parasites,
or alcoholism
• jaundice, swelling of
lower limbs,
uncontrolled
bleeding, increased
sensitivity to drugs
• Dysphagia:
• difficulty swallowing
due to inflammation,
paralysis, obstruction,
trauma
Medical Terminology
• Halitosis:
• bad breath
• Hernia:
• protrusion of all or
part of an organ thru
a membrane or cavity
wall
• hiatal hernia
• umbilical hernia
• inguinal hernia
Medical Terminology
• Irritable Bowel
Syndrome (IBS):
• involves entire GI
tract
• stress-related abd
pain or cramping
ass’c with alternating
diarrhea/constipation
• Nausea:
• discomfort
characterized by loss
of appetitie &
sensation of
impending vomiting
• caused by local
irritation of GI tract,
some other systemic
disease, brain
trauma, overexertion,
or 2° to meds/drugs