The Digestive System

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The Digestive System

• Chapter 11

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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
  bodys 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.

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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?

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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.

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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.

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Function of GI tract

• 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?

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Terminology

• Gastro- refers to the stomach.
• Entero- refers to the intestine.

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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.

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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

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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.

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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.

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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.

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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

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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?

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Parotid salivary glands Mandibular salivary
glands Sublingual salivary glands Buccal salivary
glands
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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.

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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

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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

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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

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Dental Formula

• Typical number of each type of tooth found in
  upper/lower arcades
• Tooth type designate
• Iincisor,
• Ccanine,
• P premolar,
• Mmolar
• 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.

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• Species Dental Formula Total Canine
  - puppy i3/3 c1/1 p3/3 28
• Canine - adult I3/3 C1/1 P4/4 M2/3 42
• Feline - kitten i3/3 c1/1 p3/2 26
• Feline - adult I3/3 C1/1 P3/2 M1/1 30
• Equine - adult I3/3 C1/1 P3-4/3 M3/3 40 or 42
• Porcine - adult I3/3 C1/1 P4/4 M3/3
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• Bovine - adult I0/3 C0/1 P3/3 M3/3 32

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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.

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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.

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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

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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

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Dental Prophylaxis

• The scaling away of tartar from the teeth.
• Horses get teeth floated-this reduces points on
  buccal edge of teeth.

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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.

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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.

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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.

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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?

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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.

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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.

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Monogastric (Simple) Stomach

• Divided into five different areas.
• 1. Cardia
• 2. Fundus
• 3. Body
• 4. Pyloric antrum
• 5. Pylorus

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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.

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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?

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Pyloric Antrum glands

• G- cells
• Endocrine cells
• Secretes gastrin hormone
• Presence of food stimulates G-cells to dump
  gastrin into the blood.

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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.

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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.

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Empty vs. Full Stomach
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Stomach Structure

• Contains
• Inner mucosal layer
• Submucosa
• Muscular layer
• Circular
• Longitudinal
• Outer serosal layer

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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?

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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.

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Control of stomach

• Vagus Nerve
• Remember I told you it was important.
• Gastric atony- decreased muscle tone in the
  stomach.
• What can cause this?

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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.

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Stomach Secretions continued

• Hydrochloric acid
• Produced by parietal cells.
• Produce hydrogen and chloride separately which
  combine in stomach.
• Accounts for stomachs low pH
• Uses an active transport mechanism so production
  requires energy.
• When enough acid is produced, then gastrin
  release is inhibited.

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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?

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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?

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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

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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.

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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 NSAIDs fit into this?
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Gastric Ulcers
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Ruminants and Complex Stomachs

• Only have one true stomach (abomasum), but have 3
  forestomachs (reticulum, rumen, and omasum).

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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?

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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.

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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.

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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

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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 (VFAS)-
• 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.

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Fermentative Digestion Continued

• Proteases- convert proteins to peptides
• Then can convert down farther to ammonia and
  VFAs.
• Ruminant is dependant on microbes to digest
  nutrients.
• Very delicate balance
• Changes in diet can upset this balance

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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 VFAs and water.

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Abomasum

• Functions very similar to that of mongastric
  stomach.
• Only glandular part of the stomach.

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Young Ruminant Digestive Tract

• Newborns stomach functions mainly as a
  monogastric digestive system.
• Rumen and reticulum are small and non-functional
• 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.

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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).

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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

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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?

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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.

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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.

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Small Intestine Digestion

• Absorbed intact into small intestinal wall are
• Sodium
• Chloride
• Postassium
• First three collectively known as electrolytes
• Vitamins

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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

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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.

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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

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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.

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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

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Megacolon

• Similar to megaesophagus
• When colon wall loses muscle tone.
• Results in constipation.

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Horse Hindgut

• Consists of 4 sections
• Cecum
• Ventral colon
• Right and left halves
• Dorsal colon
• Right and left halves
• Small colon

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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

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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
• VFAs absorbed from cecum and colon provide
  energy needs in animals with hindgut
  fermentation.
• Guinea pigs, rats, rabbits, swine.

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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

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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.

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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

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Gallbladder

• Storage compartment for bile.
• Will force bile down bile duct to duodenum.

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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.

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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

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