Chapter 24
The Digestive System
Structure
Gross Anatomy
Histology
Function
Mechanical
Chemical
Development
Disorders
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 and anus---defecation
Layers of the GI Tract
1. Mucosal layer
2. Submucosal layer
3. Muscularis layer
4. Serosa layer
Mucosa
Epithelium
stratified squamous(in
mouth,esophagus & anus) = tough
simple columnar in the rest
secretes enzymes and absorbs
nutrients
specialized cells (goblet) secrete
mucous onto cell surfaces
enteroendocrine
cells---secrete hormones controlling organ function
Lamina propria
thin layer of loose connective
tissue
contains BV and lymphatic tissue
Muscularis mucosae---thin
layer of smooth muscle
causes
folds to form in mucosal layer
increases local movements
increasing absorption with exposure to “new” nutrients
Submucosa
Loose connective tissue
containing
BV, glands and lymphatic tissue
Meissner’s plexus---
parasympathetic
innervation
vasoconstriction
local movement by
muscularis
mucosa
smooth muscle
Muscularis
Skeletal muscle = voluntary control
in mouth, pharynx , upper esophagus
and anus
control over swallowing and
defecation
Smooth muscle = involuntary control
inner circular fibers & outer
longitudinal fibers
mixes, crushes & propels food
along by peristalsis
Serosa
An example of a serous membrane
Covers all organs and walls of cavities not open to the
outside of the body
Secretes slippery fluid
Consists of connective tissue covered with simple squamous epithelium
Peritoneum
Peritoneum
visceral layer covers organs
parietal layer lines the walls of
body cavity
Peritoneal cavity
potential space containing a bit of
serous fluid
Parts of the Peritoneum
Mesentery
Mesocolon
Lesser omentum
Greater omentum
Peritonitis = inflammation
trauma
rupture of GI tract
appendicitis
perforated ulcer
Greater Omentum, Mesentery & Mesocolon
Lesser Omentum
Peritonitis
Acute inflammation of the peritoneum
Cause
contamination by infectious
microbes during surgery or from rupture of abdominal organs
Mouth
Lips and cheeks-----contains buccinator
muscle that keeps food between upper & lower teeth
Vestibule---area between cheeks and teeth
Oral cavity proper---the roof = hard, soft palate and uvula
floor = the tongue
Pharyngeal Arches
Two skeletal muscles
Palatoglossal muscle
extends from palate to tongue
forms the first arch
posterior limit of the mouth
Palatopharyngeal muscle
extends from palate to pharyngeal
wall
forms the second arch
behind the palatine tonsil
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
Bicarbonate ions buffer acidic foods
bulemia---vomiting
hurts the enamel on your teeth
Chemical digestion of starch begins with enzyme (salivary
amylase)
Enzyme (lysozyme) ---helps destroy
bacteria
Protects mouth from infection with its rinsing action---1 to
1 and 1/2qts/day
Salivation
Increase salivation
sight, smell, sounds, memory of
food, tongue stimulation---rock in mouth
Stop salivation
dry mouth when you are afraid
sympathetic nerves
Mumps
Myxovirus that
attacks the parotid gland
Symptoms
inflammation and enlargement of the
parotid
fever, malaise & sour throat
(especially swallowing sour foods)
swelling on one or both sides
Sterility rarely possible in males with testicular
involvement (only one side involved)
Vaccine available since 1967
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
Crown
Neck
Roots
Pulp cavity
Composition of Teeth
Enamel
hardest substance in body
calcium phosphate or carbonate
Dentin
calcified connective tissue
Cementum
bone-like
periodontal ligament penetrates it
Dentition
Primary or baby teeth
20 teeth that start erupting at 6 months
1 new pair of teeth per month
Permanent teeth
32 teeth that erupt between 6 and 12 years of age
differing structures indicate
function
incisors for biting
canines or cuspids
for tearing
premolars & molars for crushing
and grinding food
Primary and Secondary Dentition
Digestion in the Mouth
Mechanical digestion (mastication or chewing)
breaks into pieces
mixes with saliva so it forms a
bolus
Chemical digestion
amylase
begins starch digestion at pH of
6.5 or 7.0 found in mouth
when bolus & enzyme hit the 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 or swallowing is facilitated by saliva and mucus
starts when bolus is pushed into
the oropharynx
sensory nerves send signals to
deglutition center in brainstem
soft palate is lifted to close nasopharynx
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
Histology of the Esophagus
Mucosa = stratified squamous
Submucosa = large mucous glands
Muscularis = upper 1/3 is
skeletal, middle is mixed, lower 1/3 is smooth
upper
& lower esophageal sphincters are prominent circular muscle
Adventitia = connective tissue blending with surrounding
connective tissue--no peritoneum
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 is 4-8 seconds for solids and 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 esophageal sphincter fails to close
stomach acids enter esophagus &
cause heartburn (GERD)
for
a weak sphincter---don't eat a large meal and lay down in front of TV
smoking and alcohol make the
sphincter relax worsening the situation
Control the 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
Which side is it on?
Size when empty?
large sausage
stretches due to rugae
Parts of stomach
cardia
fundus---air
in x-ray
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--Mechanical Digestion
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 the pylorus
open it and squirt out 1-2
teaspoons full with each wave
Physiology--Chemical Digestion
Protein digestion begins
HCl denatures (unfolds) protein
molecules
HCl transforms pepsinogen
into pepsin that breaks peptides bonds between certain amino acids
Fat digestion continues
gastric lipase splits the
triglycerides in milk fat
most effective at pH
HCl kills microbes in food
Mucous cells protect stomach walls from being digested with
1-3mm thick layer of mucous
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 with higher blood alcohol levels with same intake of alcohol
Regulation of Gastric Emptying
Release of chyme is regulated by
neural and hormonal reflexes
Distention & stomach contents increase secretion of gastrin hormone & vagal nerve
impulses
stimulate contraction of esophageal
sphincter and stomach and relaxation of pyloric sphincter
Vomiting (emesis)
Forceful expulsion of contents of stomach & duodenum
through the mouth
Cause
irritation or distension of stomach
unpleasant sights, general
anesthesia, dizziness & certain drugs
Sensory input from medulla cause stomach contraction &
complete sphincter relaxation
Contents of stomach squeezed between abdominal muscles and
diaphragm and forced through open mouth
Serious because loss of acidic gastric juice can lead to
alkalosis
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
Opens 4" below pyloric sphincter
Composition and Functions of Pancreatic Juice
1 & 1/2 Quarts/day at pH of 7.1 to 8.2
Contains water, enzymes & sodium bicarbonate
Digestive enzymes
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 the pancreas & starts to digest himself
Regulation of Pancreatic Secretions
Secretin
acidity in intestine causes
increased sodium bicarbonate 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
Flow of Fluids Within the Liver
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
Blood Supply to the Liver
Hepatic portal vein
nutrient rich blood from stomach, spleen & intestines
Hepatic artery from branch off the aorta
Bile Production
One 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
Regulation of Bile Secretion
Liver Functions--Carbohydrate Metabolism
Turn proteins into glucose
Turn triglycerides into glucose
Turn excess glucose into glycogen & store in the liver
Turn glycogen back into glucose as needed
Liver Functions --Lipid Metabolism
Synthesize cholesterol
Synthesize lipoproteins----HDL and LDL(used
to transport fatty acids in bloodstream)
Stores some fat
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 the clotting
mechanism and immune system
Convert one amino acid into another
Other Liver Functions
Detoxifies the 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)
Summary of 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
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
Histology of Small Intestine
Histology
of the Small Intestine
Structures that increase surface area
plica circularis
permanent ˝ inch tall folds that
contain part of submucosal
layer
not found in lower ileum
can not stretch out like rugae in stomach
villi
1 Millimeter tall
Contains vascular capillaries and lacteals(lymphatic
capillaries)
microvilli
cell surface feature known as brush
border
Functions of Microvilli
Absorption and digestion
Digestive enzymes found at cell surface on microvilli
Digestion occurs at cell surfaces
Significant cell division within intestinal glands produces
new cells that move up
Once out of the way---rupturing and releasing their
digestive enzymes & proteins
Cells of Intestinal Glands
Absorptive cell
Goblet cell
Enteroendocrine
Paneth cells
secretes lysozyme
Goblet Cells of GI epithelium
Roles of Intestinal Juice & Brush-Border Enzymes
Submucosal layer has duodenal
glands
secretes alkaline mucus
Mucosal layer contains intestinal glands = Crypts of Lieberkuhn(deep
to surface)
secretes intestinal juice
1-2 qt./day------ at pH 7.6
brush border enzymes
paneth
cells secrete lysozyme kills bacteria
Mechanical Digestion in the 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
Lactose Intolerance
Mucosal cells of small intestine fail to produce lactase
essential for digestion of lactose
sugar in milk
undigested lactose retains fluid in
the feces
bacterial fermentation produces
gases
Symptoms
diarrhea, gas, bloating &
abdominal cramps
Dietary supplements are helpful
Digestion of Proteins
Stomach
HCl denatures or unfolds proteins
pepsin turns proteins into peptides
Pancreas
digestive enzymes---split peptide
bonds between different amino acids
brush border enzymes-----aminopeptidase or dipeptidase------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
Digestion of Nucleic Acids
Pancreatic juice contains 2 nucleases
ribonuclease
which digests RNA
deoxyribonuclease
which digests DNA
Absorbed by active transport
Absorption in Small Intestine
Where will the absorbed nutrients go?
Absorption of Lipids
Small fatty acids enter cells & then blood by simple
diffusion
Lipids enter cells by simple diffusion leaving bile salts
behind in gut
were within micelles
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
travel in micelles & are
absorbed by simple diffusion
Water-soluble vitamins
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 2˝ 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
Histology of Large Intestine
Muscular layer
internal circular layer is normal
outer longitudinal muscle
taeniae
coli = shorter bands
haustra
(pouches) formed
epiploic
appendages
Serosa = visceral peritoneum
Appendix
contains large amounts of lymphatic
tissue
Mechanical Digestion in Large Intestine
Smooth muscle = mechanical digestion
Peristaltic waves (
haustral
churning----relaxed pouches are filled from below by muscular contractions
(elevator)
gastroilial
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
Bacteria ferment
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 the 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, vegie 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
decreased 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
Cancer of the colon or rectum is common
Diseases of the GI Tract
Dental caries and periodontal disease
Peptic Ulcers
Diverticulitis
Colorectal cancer
Hepatitis
Anorexia nervosa