The Digestive System
Chapter 11
Introduction
• Takes complex food and breaks them down into
simple nutrient molecules through process of
digestion.
• After digestion takes place, nutrient molecules are
absorbed into the bloodstream for use by the
body’s cells.
• Consists of a tube that runs from the mouth to the
anus and accessory digestive organs that aid in the
process of digestion and absorption.
• Known as digestive tract, Gastrointestinal (GI)
tract, the alimentary canal, or gut.
Lumen
• The lumen is the opening in the middle of
the intestinal tract or any hollow organ.
• Anything in the lumen is considered
outside the body.
• Why?
• What must occur for things to enter the
body?
Animal Diets
•
The requirements for digestion and absorption of
foodstuffs vary considerably depending on the diet
of the animal.
• Each type of animal has different mechanisms to
handle the digesting and absorbing chores.
• Herbivores
• Plant eating animals
• Carnivores
• Meat eating animals
• Omnivores
• Animals tat eat both plants and meat.
Types of Stomachs
• Monogastric
• Animals that have simple, single
stomachs.
• Complex Stomachs
• Animals that have fermentation
compartments in addition to the
stomach.
• Example is ruminant animals.
Function of GI tract
1.
2.
3.
4.
5.
•
•
Prehension (grasping) of food with the lips or teeth
Mastication - mechanical grinding and breaking
down of food (chewing)
Chemical digestion of food
Absorption of nutrients and water
Elimination of wastes
If any function fails, then malnutrition may result.
Usually failure is followed by clinical signs.
• Examples of clinical signs?
Terminology
• Gastro- refers to the stomach.
• Entero- refers to the intestine.
GI tract structure
• Made of multiple layers.
• The mucosa-the lining layer and consists of the
lining epithelium and some loose connective tissue
• The submucosa-beneath the mucosa and contains
glands and dense connective tissue.
• The thick muscle layer is outside the submucosa.
• What type of muscle is this?
• The serosa- is the outermost layer that consists of
a thin, tough layer of connective tissue.
Digestive Tract Structure
• Mesentery - Sheets of connective tissue
• Suspends digestive tube from dorsal
body wall
• Contains blood vessels, lymph vessels,
and nerves that supply GI tract
Epithelium of the Digestive Tract
• 2 types
• Stratified Squamous
• Thick and tough.
• Lines mouth, pharynx, esophagus,
and anus.
• Simple Columnar
• Change occurs where esophagus
enters stomach.
• Allows for better absorption of
nutrients.
Muscle of the GI tract
• 2 types
• Skeletal Muscle
• Under voluntary control
• Mouth, pharynx, cranial esophagus, external anal
sphincter.
• Allows process of chewing, mixing saliva with food, and
swallowing to be conscious acts.
• Controls defecation.
• Smooth Muscle
• Present in remainder portion of digestive tract.
• Arranged in circular and longitudinal layers.
• Circular Layer- narrows segment of tube.
• Longitudinal Layer- shortens segment of tube.
How food is moved?
• Moved and mixed by two types of muscle
contractions.
• Peristalsis contractions
• Move contents along digestive tract.
• Circular muscle contractions that
move food along in waves.
• Propel food ahead of contractions.
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Food Movement continued
• Segmental contractions
• Cause back and forth mixing
movements of the digestive tract
contents.
• Consist of periodic, circular muscle
contractions that occur in different
adjacent sites.
• Aid in digestion and absorption by
mixing the digestive tract contents
and slowing their movement through
the tract.
Mouth or Oral Cavity
•
•
•
Also called the buccal cavity.
Where food is initially taken in and where digestion
actually begins.
Structures include:
• Lips
• Prehensile organ
• Labial- term for referring to the lips.
• Tongue
• Teeth
• Salivary glands
• Produce saliva, which performs digestive and
lubrication functions.
• Also involved in evaporative cooling.
• Hard palate
• Soft palate
• Oropharynx
Salivary glands continued
• Most animals have 3 matching pairs of glands.
• Parotid salivary glands
• Located just ventral to ear canals
• Mandibular salivary glands
• Located ventral to parotid glands at the
caudal angle of the mandible
• Sublingual salivary glands
• Located medial to the shafts of the mandible
just under the base of the tongue.
• All have ducts that carry saliva to oral cavity.
• Salivary glands are controlled by autonomic
nervous system
• What happens during fight or flight?
Parotid salivary glands
Mandibular salivary
glands
Sublingual salivary
glands
Buccal salivary glands
Teeth
• Responsible for breaking down food into smaller pieces by
process of mastication.
• Increases surface area of the food that is exposed to
digestive processes.
• Maxilla contains upper arcade
• Mandible contains lower arcade.
Teeth Surfaces
• Lingual (tongue)- inner surface of lower
arcade
• Palatal (hard palate)- inner surface of
upper arcade
• Labial (lips)- outer surface of upper/lower
arcades (rostral)
• Buccal (cheek) - outer surface of teeth
(caudal)
• Occlusal – surface that grinds with other
teeth
Teeth shape
• Carnivore teeth - pointed on occlusal
surface; slightly curved toward back of
mouth
• Good for holding prey, tearing, cutting,
shredding
• Herbivore teeth - flat occlusal surfaces
• Good for grinding plant and grain
material
Types of Teeth
•
•
•
•
Incisors
• Grasping teeth
• Most rostral teeth of upper
and lower arcade
Canines
• Tearing teeth
• Located at corners of incisors
• Longer than other teeth
• Pointed at tip
Premolars
• Cutting teeth
• Rostral cheek teeth
• Sharp points and surfaces
in carnivores
Molars
• Grinding teeth
• Caudal cheek teeth
• Larger, flatter occlusal surfaces
• Used for grinding
Dental Formula
• Typical number of each type of tooth found in
upper/lower arcades
• Tooth type designate
• I=incisor,
• C=canine,
• P= premolar,
• M=molar
• Upper case: adult teeth
• Lower case: deciduous teeth
• Ruminants have no upper incisors or canine teeth.
Instead have a dental pad- a flat, connective tissue
structure of the maxilla opposite the lower incisors
and canine teeth.
• Species
Canine - puppy
• Canine - adult
• Feline - kitten
• Feline - adult
• Equine - adult
• Porcine - adult
• Bovine - adult
Dental Formula
i3/3 c1/1 p3/3
I3/3 C1/1 P4/4 M2/3
i3/3 c1/1 p3/2
I3/3 C1/1 P3/2 M1/1
I3/3 C1/1 P3-4/3 M3/3
I3/3 C1/1 P4/4 M3/3
I0/3 C0/1 P3/3 M3/3
Total
28
42
26
30
40 or 42
44
32
Numerical Dental Charting
• Each quadrant of the dental arcade is
assigned a particular number.
• Upper right quadrant is 100
• Upper left quadrant is 200
• Lower left quadrant is 300
• Lower right quadrant is 400
• Each tooth is assigned a number
according to position.
Teeth continued
• The fourth premolar is also referred to as
the carnassial tooth.
• If this tooth abscesses may cause
drainage through the skin under the
eye.
• Because of root system and length of
roots, removal is hard work.
Structure of Teeth
• Living structures.
• Have nerve, blood vessel and lymph
supply.
• Are susceptible to damage and pain.
• Consist of:
• Apex
• Pulp
• Dentin
• Cementum
• Enamel
• Gingiva
Teeth Structure continued
•
•
•
•
•
•
Apex
• Remember the pointy part
• Where blood and nerve supply enter the tooth
Pulp
• The center of the tooth
Dentin
• Surrounds the tooth pulp.
• More dense than bone but less dense than enamel.
Cementum
• Hard connective tissue that helps to fasten tooth in
bony socket.
Enamel
• Hardest, toughest tissue in body.
• Outer cover of crown of tooth
Gingiva
• Epithelial tissue that forms the gums around the teeth
Dental Prophylaxis
• The scaling away of tartar from the teeth.
• Horses get teeth “floated”-this reduces
points on buccal edge of teeth.
Functions of the Oral Cavity
• Prehend food
• Initiate mastication
• Also referred to as mechanical
digestion
• Initiate chemical digestion
• Saliva has enzymes that aid in
breakdown of food.
• Prepare food for swallowing.
Digestive Enzymes
•
•
•
Proteins that promote the chemical reactions that split
complex food molecules up into simpler compounds.
Secreted in the digestive system to break down different
components of food.
• How do we recognize?
Two common Digestive enzymes:
• Amylase
• Found in saliva of omnivores but absent in
carnivores
• Breaks down amylose- a sugar component of
starch.
• Lipase
• Digests lipids
• Found in saliva of young animals while nursing or
on high milk diet.
Digestive Enzymes Continued
• Cattle use sodium bicarbonate and
phosphate buffers in saliva to neutralize
acids that form in rumen.
• Cattle can produce 25 to 50 gallons of
saliva a day.
Esophagus
• Tube that extends from pharynx to the stomach.
• Function is to take swallowed material to the
stomach.
• No digestion takes place, serves only for transport.
• Enters the cardia of the stomach.
• The cardiac sphincter surrounds the cardia.
• Esophagus enters stomach at an angle.
• This allows that as the stomach fills with food, can
close off against esophagus decreasing chance of
reflux.
• What is reflux?
• Which animals can not vomit?
• Why?
Swallowed Corrosive Material
• If a dog or cat swallows a corrosive
material, why do we not initiate vomiting?
• If corrosion is significant, then can cause
perforation- a tear or hole within the
esophagus.
• Perforation can cause food to enter
thoracic cavity.
Megaesophagus
• When esophagus loses muscle tone.
• Food accumulates in the esophagus
instead of being transported to stomach.
• How do we treat?
• Feed liquid based diet
• Feed on an elevated surface
• Holding animal up and allowing gravity
to pull food down to stomach.
• Can lead to aspiration pneumonia.
Monogastric (Simple) Stomach
• Divided into five different areas.
• 1. Cardia
• 2. Fundus
• 3. Body
• 4. Pyloric antrum
• 5. Pylorus
Stomach continued
•
•
•
•
Cardia
• Where esophagus enters stomach.
• Helps reduce reflux
Fundus & Body
• Forms a pouch that can distend as more food is
swallowed
• Rich with glands
Pyloric Antrum
• Distal part of the stomach that grinds food.
• Regulates hydrochloric acid
Pylorus
• Sphincter that regulates digested stomach contents
(Chyme) from stomach to duodenum (first part of
intestine).
• Prevents backflow of duodenal contents into the
stomach.
Gastric Glands
• Contain 3 key cells:
• Parietal cells (oxyntic cells)
• Produces hydrochloric acid
• Chief cells
• Produce precursor to pepsinogen
• Mucous cells
• Produce mucus that is protective in
nature.
• What else are mucous cells called?
Pyloric Antrum glands
• G- cells
• Endocrine cells
• Secretes gastrin hormone
• Presence of food stimulates G-cells to
dump gastrin into the blood.
Stomach shape
• C-shaped
• Inside called lesser curvature of the
stomach.
• Outside called greater curvature of the
stomach.
• Rugae- long folds in the stomach.
• Gastritis- inflammation of the stomach.
• Gastric ulcers- erosions of epithelium of
stomach.
Gastric Motility
• Each part of the stomach has different
muscle functions.
• Fundus and body relax with swallowing,
allowing stomach to distend and fill with
food.
• Body of stomach contracts to help mix
food within the stomach.
• Pyloric Antrum increases contractions with
the swallowing of food.
• Grinds food and propels food toward
pylorus.
Empty vs. Full Stomach
Stomach Structure
• Contains:
• Inner mucosal
layer
• Submucosa
• Muscular layer
• Circular
• Longitudinal
• Outer serosal layer
Stomach Continued
• Pylorus
• Circular muscle fibers
• Maintains constant tone
• Chyme is forced by contractions into
antrum of stomach.
• Remains partially open so contents can
move from stomach to duodenum.
• What about liquids?
Stomach continued
• The more full the intestines become, the
less the stomach will fill.
• Enterogastric reflex- prevents stomach
from emptying until intestines are able to
handle incoming contents.
Control of stomach
• Vagus Nerve
• Remember I told you it was important.
• Gastric atony- decreased muscle tone in
the stomach.
• What can cause this?
Stomach secretions
• Gastrin
• Produced by which cells?
• Produced where?
• Helps to increase hydrochloric acid
production.
• Inhibits muscle activity of fundus.
• Causes relaxation and greater filling of
the stomach.
Stomach Secretions continued
• Hydrochloric acid
• Produced by parietal cells.
• Produce hydrogen and chloride
separately which combine in stomach.
• Accounts for stomach’s low pH
• Uses an active transport mechanism so
production requires energy.
• When enough acid is produced, then
gastrin release is inhibited.
Stomach secretions continued
• Secretin
• Hormone released by intestines
• Can inhibit peristalsis which slows
gastric emptying.
• Cholecystokinin
• Nutrients high in fat or protein stimulate
release of this hormone.
• Decreases contraction of antrum, body
and fundus.
• Which other hormone is produced in
the antrum?
Stomach secretions continued
• Intrinsic factor- protein that combines with vitamin
B12 that aids in absorption of this vitamin.
• Pepsinogen- precursor to pepsin which breaks
down proteins to chains of amino acids.
• Peptides (chains of amino acids) stimulates
release of gastrin (from which cells?) which
increases hydrochloric acid production and
pepsinogen release.
• Pepsin is inactivated from pH change from
stomach to intestine.
• Which area has a higher pH?
• Which area has a lower pH?
More stomach secretions..
• Mucus
• Produced by gastric glands
• Complex of many substances
• Mucins- produced by goblet cells
• Main constituent of mucus
• Bicarbonate Ion- makes mucous coat more
alkaline in nature
• This helps to neutralize hydrochloric
acid.
• Help to coat the stomach
• pH of stomach is generally about 2-3
• Must be secreted continuously
Parietal cells and receptors
• 3 receptors on “blood side” that regulate
acid production
• Receptors are for:
• Acetylcholine- neurotransmitter
• Gastrin- increases hydrochloric acid
production
• Histamine- chemical associated with
inflammatory response
• Inhibiting these receptors decreases
hydrocholoric acid production.
• How antacids work.
Prostaglandins
• Small molecular structures released by the body
that have a wide variety of effects.
• Involved in inflammation.
• Some are beneficial to body.
• PGE & PGI
• Reduce hydrocholric acid production by
inhibiting gastrin release from G cells.
• Directly inhibit Parietal cells
• Stimulate bicarbonate ion production
• Enhance blood flow to stomach
• Help stomach to repair quickly.
How do NSAID’s fit into this?
Gastric Ulcers
Ruminants and Complex
Stomachs
• Only have one true stomach (abomasum),
but have 3 forestomachs (reticulum, rumen,
and omasum).
Rumination
•
The act of swallowing food, then regurgitating
it to chew it some more then swallow it again.
• What is regurgitation?
• How does this differ from vomiting?
Reticulum
• Smallest and most cranial
compartment.
• Separated from rumen by
ruminoreticular fold.
• Interior is a honeycomb type
arrangement to increase
absorption.
• Muscle wall is continuous with
the rumen
• Rumen and Reticulum work
together and produce
Reticulorumen contractions.
• Hardware disease is associated
with the reticulum.
Rumen
• Series of muscular sacs separated by
pillars (long, muscular folds of rumen wall).
• Sacs can close off to allow more mixing
to take place
• Fermentation takes place here
• Reticuloruminal contractions allow:
• Regurgitation to take place
• Eructation-Expulsion of built up carbon
dioxide or methane gas
• If build up continues, may cause
bloat.
Rumen Continued
• Gastric motility is controlled by:
• Vagus nerve
• pH (ideal is 5.8-6.4)
• Presence of fatty acids
• Consistency of foodstuffs
• Stretch receptors
Fermentative Digestion
• Enzymes are produced by bacteria and protozoa.
• How does this differ from monogastric animals?
• Produce cellulase
• Allows for the breakdown of plant cell walls.
• Volatile Fatty Acids (VFA’S)• Produced when glucose is absorbed into
microbes and converted.
• By products of anaerobic fermentation process
• Are rapidly absorbed
• Liver converts propionic acid into usable
glucose.
• Can also produce fat.
Fermentative Digestion
Continued
• Proteases- convert proteins to peptides
• Then can convert down farther to
ammonia and VFA’s.
• Ruminant is dependant on microbes to
digest nutrients.
• Very delicate balance
• Changes in diet can upset this balance
Omasum
• When reticulorumen
contractions occur,
move ingesta to
omasum.
• Muscular with interior
full of parallel,
longitudinal muscular
folds.
• Breaks down food
particles and carry
these particles to the
abomasum.
• Absorbs VFA’s and
water.
Abomasum
• Functions very similar to that of mongastric
stomach.
• Only glandular part of the stomach.
Young Ruminant Digestive Tract
•
•
•
•
Newborn’s stomach functions mainly as a
monogastric digestive system.
Rumen and reticulum are small and nonfunctional
Development of rumen and reticulum is
dependant on diet
Milk in rumen can disrupt fermentation
process so is carried directly to omasum.
• Reticular groove (Esophageal
groove)- allows liquids to be carried
from esophagus to omasum.
• Initiated by suckling, can not drink
from pail
• As ages, groove disappears.
Small Intestines
• Where majority of nutrients are absorbed into the
bloodstream.
• Divided into:
• Duodenum
• Short, first segment that leaves stomach.
• Jejunum
• Longest portion, makes up majority of small
intestines.
• Ileum
• Short section that enters the colon (large
intestine).
• Separated from colon by ileocecal
sphincter- muscle that regulates movement
of materials from small intestine into colon
or the cecum (blind pouch of the large
intestine).
Small Intestine Structure
• Similar to other segments of GI tract.
• Thickness changes depending of area of
GI tract.
• Inner mucosal layer
• Submucosal layer
• Muscular layer
• Serosal layer
Mucosa of Small Intestines
• Provides large surface area for absorbing
nutrients.
• Folds in intestinal wall
• Villi- tiny, cylindrical, fingerlike
projections that help with absorption.
• Each villus contains very small villi
called microvilli (brush border).
• Crypts- small invaginations in the
intestinal mucosa that help with
replenishing cells of villus.
• What cells produce mucus?
• Diseases that affect villi?
Small Intestine Motility
• Vagus nerve and some sacral-vertebral
nerves of parasympathetic system provide
stimulation for motility, secretions, and
blood flow.
• Intestinal tract is constantly functioning
and is never at rest.
• Has peristalsis
• What type of contractions are
associated with peristalsis?
• How does diarrhea occur?
• Antidiarrheal meds decrease secretion
and increase segmental contractions.
Small intestine motility continued
• Ileus- decreased movement of ingesta.
• May be caused by disease or stress.
• Cholecystokinin (CCK)
• Stimulates intestinal motility
• Prostaglandins
• Increase GI motility and secretions.
Small Intestine Digestion
• Absorbed intact into small intestinal wall
are:
• Sodium
• Chloride
• Postassium
• First three collectively known as
electrolytes
• Vitamins
Small Intestine Digestion
• Must be chemically broken down to absorb
in small intestine are:
• Carbohydrates
• Proteins
• Fats
• Chemically digested by
• Enzymes in lumen of intestine
• Enzymes of microvilli
• Once digested “unit” of foodstuff can be
transported or diffused across intestinal
tract membrane
Carbohydrate Digestion
• Amylase- converts starch into smaller sugar
segments
• Then sucrase, maltase, isomaltase and lactase
found in cell membrane of microvilli convert sugar
segments down farther.
• Finally are transported across microvilli into the
body
• Enzymes present is dependant on diet being eaten.
• Changes in diet produce diarrhea because
enzymes of new diet may not be present in
sufficient quantities for new diet.
Protein Digestion
•
•
•
Pepsin can help break apart protein chains.
Many proteases are present due to different types of
amino acids that are present.
Pancreas secreted 5 basic Proteases:
• 1. Trypsin
• Precursor is trypsinogen
• Trypsin activates other enzyme precursors
• Break apart bonds in middle of protein
• 2. Chymotrypsin
• Break apart bonds in middle of protein
• 3. Elastase
• Break apart bonds in middle of protein
• 4. Aminopeptidase
• Break apart proteins at amino `ends
• 5. Carboxypeptidase
• Break apart proteins at carboxyl ends
Fat Digestion
• Fats in water environment form globules.
• Globules are broken down by emulsificaton (mixing
of fat and water by agitation or shaking) and micelle
(fat surrounded by hydrophilic moleculest that allow
it to readily move in small intestine) formation.
• Bile acids (secreted by liver to make fat more water
soluble) combine with droplets to keep from
returning to globular state.
• Have a hydrophobic and hydrophilic end
• Pancreatic lipases penetrate and digest fat
• Fat soluble vitamins are A, D, E, and K.
Large Intestine
• General functions are to recover fluid and
electrolytes and to store feces until it can
be eliminated.
• Can vary depending on species
• Consists of:
• Cecum
• Blind sac at ileocecal junction
• Fermentation in horses
• Poorly developed in carnivores
• Colon
• Rectum
Megacolon
• Similar to megaesophagus
• When colon wall loses muscle tone.
• Results in constipation.
Horse Hindgut
• Consists of 4 sections:
• Cecum
• Ventral colon
• Right and left halves
• Dorsal colon
• Right and left halves
• Small colon
Horse Hindgut continued
• Ingesta from small intestine passes through
ileocecal sphincter and enters cecum.
• Cecum is composed of:
• Base
• Main body
• Apex
• Cecum and dorsal and ventral colons have
longitudinal bands that separate the structure into a
series of sacs called haustra
• Cecum is separated from colon by cecocolic orifice
Ingesta flow in horse hindgut
• Ileocecal sphincter►Cecum►Right
ventral colon►sternal flexure►left ventral
colon►Pelvic flexure►left dorsal
colon►diaphragmatic flexure►Right
dorsal colon►small colon.
• Colonic impaction is most common form of
colic in horse
• VFA’s absorbed from cecum and colon
provide energy needs in animals with
hindgut fermentation.
• Guinea pigs, rats, rabbits, swine.
Rectum
• Terminal portion of the large intestine
• Extension of colon
• Secretions similar to colon to help lubricate
feces as they pass through colon.
• Capable of more expansion than colon
• Has sensory receptors that detect
stretching or distention and stimulates
defecation response.
• Defecation- process of expelling feces
Anus
• Composed of:
• internal sphincter
• Under autonomic control
• Parasympathetic system causes relaxation
• Sympathetic system causes constriction
• external sphincters.
• Under voluntary control
• When material make contact with anal mucosa of
internal sphincter, increases conscious need for
defecation.
• Anal abnormalities can result in fecal incontinence.
Liver
• Produces bile acids that help to digest fat.
• Helps to filter materials absorbed from GI
tract before they reach blood.
• Hepatic portal system- is blood vessel
system that transports blood from
capillaries in the intestine to liver.
• Some nutrients are stored or metabolized
by the liver.
• Bile is produced in liver and leads to
gallbladder.
• Source of blood proteins
• Albumin
Gallbladder
• Storage compartment for bile.
• Will force bile down bile duct to duodenum.
Pancreas
• Exocrine (secretes substances to outside of body
through a duct) gland as well as Endocrine
(secretes hormones directly into the blood without
going through a duct) gland.
• Produces enzymes
• Secretes bicarbonate into the duodenum
• What does this do?
• Insulin and glucagon help regulate blood glucose
• Beta cells (located in pancreatic islets or islets
of langerhans) produce insulin which lowers
glucose levels in blood
• Lack of insulin leads to Diabetes mellitus
• Glucagon produced by alpha cells in pancreas
and antagonizes insulin.
Exocrine Pancreatic Insuffiency
• (EPI)
• Disease in which pancreas digestive
enzyme secretion is greatly reduced
• Animals lose weight and have chronic
diarrhea.
• Can get powdered form of digestive
enzymes with food.
• Is tested by TLI (trypsin like
immunoreactivity) test.
• Looks for trypsin and trypsinogen in
blood
• http://www.youtube.com/watch?v=Z7xKYN
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