Monogastric Digestive System

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Monogastric Digestive System
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Matching

• Species

• Digestive System

• Cow
• Pig
• Kangaroo
• Sheep
• Horse
• Dog
• Chicken

• Ruminant
• Monogastric
• Pre-gastric Fermentation
• Post-gastric Fermentation
• Herbivore
• Carnivore
• Omnivore

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Answers

• Cow- Ruminant, Pre-gastric, Herbivore
• Pig- Monogastric, Post-gastric, Omnivore
• Kangaroo- Monogastric, Pre-gastric, Herbivore
• Sheep- Ruminant, Pre-gastric, Herbivore
• Horse- Monogastric, post-gastric, herbivore
• Dog- Monogastric, post-gastric, carnivore
• Chicken- Monogastric, Post-gastric, Omnivore

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Basic Organization

• Mouth
• Esophagus
• Stomach
• Small intestine
• Large intestine
• Anus

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Associated Structures

• Pancreas
• Liver
• Gallbladder
• Salivary glands

Contribute to small intestinal digestion
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Structures in Mouth

• Lips
• Teeth
• Tongue
• Salivary glands

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Monogastric Teeth

• Function
• Mechanically reduce particle size
• Increase surface area
• Four types
• Incisors are used for cutting

• Canine (fangs, eye teeth, tusks) are tearing teeth

• Premolars and molars (cheek teeth) grind the food

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Monogastric Tongue

• Function
• Comprised of three muscles
• Maneuvers food in the mouth
• Moves feed to teeth for grinding and to the back
  of the mouth for swallowing
• Can distinguish between feed and toxins by
  papillae or taste buds

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Monogastric Salivary Glands

• Types of Glands

Zygomatic
Parotid
Sublingual
Mandibular
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Salivary Glands
Gland Type of secretion Main constituents
Parotid Serous Water, enzymes, ions
Submaxillary Mucous or mixed Mucin (mucous), mucin plus enzymes (mixed), water
Sublingual Mucous or mixed Mucin (mucous), mucin plus enzymes (mixed), water
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Functions of Saliva

• Moisten feed (salt and water)
• Lubrication (aids swallowing)
• Starch and(or) lipid digestion (amylase and(or)
  lipase)

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Monogastric Salivary Glands

• Flow rate affected by
• Parasympathetic nervous system
• Increased tone Increased flow
• Increased flow Increased dilution
• Sympathetic nervous system
• Increased tone Decreased flow
• Decreased flow Increased concentration
• Volume of saliva
• 1 - 1.5 L/d man and pig
• 7 - 10 L/d horse

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Monogastric Esophagus

• Transport of food from mouth to stomach
• Uses peristaltic contractions (wave contractions)
• Horse/Pig
• Striated muscles for first 2/3
• Smooth muscles for last 1/3
• In horse, esophagus joins stomach at an oblique
  angle and cardiac sphincter (the valve between
  the stomach and esophagus) only allows one-way
  flow
• MOST horses cannot belch out gas or vomit
• Dog
• Striated muscles throughout allow GREAT control
  of digesta movement both directions

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Deglutition (Swallowing)

• Reflex initiated by presence of food in pharnyx
• Propulsion of food to stomach by esophageal
  peristalsis

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Gastric Digestion

• Functions
• Reservoir for controlled release of digesta to
  small intestine
• Horse has small capacity requires increased
  number of smaller sized meals
• Mixing food
• Mechanical breakdown of feed
• Hydrolytic digestion by acid and enzymes
• Mainly protein
• Kill bacteria
• Secrete intrinsic factor needed for vitamin B12
  absorption
• Hormone production

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Stomach Regions

• Esophageal
• Non-glandular
• Cardiac
• Secretes mucus
• Fundic
• Parietal cells
• Chief cells
• Pyloric
• Mucus

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Gastric Pits

• Formed by numerous folds in the epithelium
• Glands empty into the gastric pit
• Many types of glands may empty into one gastric
  pit

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Gastric Glands
Gland Type of secretion Main constituents
Cardia Mucous Mucin
Pylorus or Antrum Mucous Mucin
Fundus Chief cells Parietal cells Enzyme Acid acid Pepsinogen Pepsin HCl, intrinsic factor
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Stomach Secretions

• HCl
• Decreases pH (2-3)
• Denatures protein
• Kills bacteria
• Activates pepsinogen
• Mucus
• Protects lining from acid and enzymes
• No autodigestion
• Lubricant

• Pepsinogen
• Activated form is pepsin
• Hydrolyzes protein
• Rennin (abomasum)
• Clots milk
• Lipase
• Some species

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Gastric Motility and Emptying

• Motility aids mixing, mechanical and hydrolytic
  reduction of feed to chyme
• acid pulp
• Emptying is stimulated by distension of antral
  wall and presence of liquid chyme

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Control of Gastric Secretions and Gastric Motility

• Cephalic phase
• Gastric phase
• Intestinal phase

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Cephalic Phase

• Vagal reflex
• Parasympathetic innervation
• Increases gastric motility, enzyme secretion
• Small increase in HCl secretion

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Gastric Phase

• Local reflex, depends on presence of feed in
  stomach
• Mainly mediated by gastrin
• Increases HCl secretion

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Intestinal Phase

• Stimulated by duodenal distension, pH,
  osmolarity, nutrients (fat)
• Pancreozymin-cholecystokinin (PZ-CCK) is released
  by the small intestine
• Decreases HCl secretion and gastric motility

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Gastrointestinal Hormones

• Gastrin
• Origin Stomach, Abomasum
• Stimulus Food in stomach
• Function Stimulates HCl pepsinogen secretion,
  increases stomach motility
• Secretin
• Origin Duodenum
• Stimulus Acid
• Function Stimulates pancreatic secretions.
  Slows stomach motility and acid production

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Gastrointestinal Hormones

• Cholecystokinin (CCK)
• Origin Duodenum
• Stimulus Fat protein in duodenum
• Function Stimulates bile and pancreatic
  secretions
• Also regulates appetite and feed intake
• Gastric Inhibitory Protein (GIP)
• Origin Duodenum
• Stimulus Fats and bile
• Function Inhibit stomach motility and secretion
  of acid and enzymes

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Small Intestine

• Composed of 3 segments (proximal to distal)
• Duodenum
• Releases bile and pancreatic secretions
• Active site of digestion
• Jejunum
• Active site of nutrient absorption
• Ileum
• Active site of nutrient absorption
• Most water, vitamins minerals
• Some bacterial presence
• Fermentation
• ?The pH of the small intestine increases towards
  7.0 as food moves from the duodenum to the ileum

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Intestinal Epithelial Cell
Brush border
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Specialized Cells Lining Villi
Nutrients
Mucus

• Absorptive epithelial cell
• Contain brush border on lumen/apical side
• Brush border
• Enzymes
• Nutrient transport molecules
• Goblet cell
• Secretes mucus

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Specialized Cells Lining Villi
Anti-microbial compounds

• Endocrine cell
• Secrete hormones into bloodstream or local cells
• Paneth cell
• Secretory granules with anti-microbial properties

CCK, Secretin, etc.
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Small Intestine Absorptive Surface

• Villi
• Enterocyte
• Brush border
• Cell migration from crypts to tips of villus
• 2-3 days

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Small Intestine - Structure

• Lumen
• Mucosa
• Villi
• Crypts
• Lacteal
• Enterocyte
• Brush border

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Intestinal Wall
Villi
Mucosa
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Enhanced Surface Area for Increased Nutrient
Absorption
Intestinal villi
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Increased Surface Area in Small Intestine for
Absorption
Structure Description Increase in surface area
Plicae circularis Regular ridges in small intestine 3x
Villi Finger-like projections on mucosal (inner) surface 10x
Microvilli Brush Border 1 um projections on surface of epithelium 20x
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Nutrient Absorption in the Small Intestine

• Principal site of absorption of amino acids,
  vitamins, minerals and lipids
• Glucose and other sugars in monogastrics
• Generally, most absorption occurs in the proximal
  (upper) part of the small intestine but some
  absorption occurs in all segments
• Duodenum, jejunum and ileum
• Digestion and absorption within SI is rapid
• Within 30 minutes of entering SI

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Nutrient Absorption

• Variety of mechanisms
• Diffusion
• Facilitated diffusion
• Active transport
• Pinocytosis or endocytosis
• Dependent upon
• Solubility of the nutrient (fat vs. water)
• Concentration or electrical gradient
• Size of the molecule to be absorbed

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Diffusion

• Water and small lipid molecules pass freely
  through membrane
• Move down concentration gradient to equalize
  concentrations

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Facilitated Diffusion

• Carrier loads particle on outside of cell
• Carrier releases particle on inside of cell
• Reverse
• Allows equalization of concentrations across
  membrane

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Active Transport

1. Carrier loads particle on outside of cell
2. Carrier releases particle on inside of cell
3. Carrier returns to outside to pick up another
   particle

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Active Transport

• Unidirectional movement
• Transports nutrients against concentration
  gradient

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Pinocytosis or Endocytosis

• Substance contacts cell membrane
• Membrane wraps around or engulfs substance into
  sac
• Sac formed separates from the membrane and moves
  into cell

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Transporters
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Secretions Entering SI
Secreted from within SI

• Intestinal mucus
• Brush border enzymes
• Pancreatic juices
• Produced stored in pancreas
• Bile
• Produced in liver
• Stored in gallbladder
• Horse has no gallbladder
• Direct bile secretion into duodenum
• Cannot store bilecontinuous intake of food

Enters from ducts into SI
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Intestinal Mucus

• Secreted by glands in wall of duodenum
• Brunners glands
• Acts as lubricant and buffer to protect duodenal
  wall

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Primary Enzymes for Carbohydrates
Nutrient Enzyme Origin Product
Starch, glycogen, dextrin Maltose Lactose Sucrose Amylase Maltase Lactase Sucrase Saliva pancreas SI SI SI Maltose Glucose Glucose Glucose galactose Glucose fructose
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Primary Enzymes for Proteins
Nutrient Enzyme Origin Product
Milk protein Proteins Polypeptides Peptides Rennin Pepsin Trypsin Chymotrypsin Carboxypeptidase Aminopeptidase Gastric mucosa Gastric mucosa Pancreas Pancreas Pancreas Small intestine Curd Polypeptide Peptides Peptides Peptides amino acids
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Primary Enzymes for Lipids
Nutrient Enzyme Origin Product
Lipids Lipase colipase Pancreas Monoglycerides free fatty acids
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Bile

• Green, viscous liquid
• Alkaline ph (neutralize acidic chyme)
• Secreted by liver via bile duct to duodenum
• Stored in gall bladder (except in horses)
• Functions to emulsify fats
• Composition
• Bile salts (glycocholic and taurocholic acids)
• Bile pigments (bilirubin and biliverdin)
• Cholesterol
• 95 reabsorbed and returned to liver
• NOT AN ENZYME

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Nutrient Digestion - Lipids
Large Lipid Droplet
Action of bile salts Lipid emulsion
Small
Bile salts pancreatic lipase and colipase
Water soluble micelles
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Pancreatic Juice

• Contains
• HCO3-
• Trypsinogen
• Chymotrypsinogen
• Procarboxypeptidase
• Amylase
• Lipase
• Nuclease

• Clear, watery juice
• Enters duodenum via pancreatic duct
• Aids in fat, starch, and protein digestion

Pro-enzymes
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Importance of Pancreas for Digestion

• Produces enzymes responsible for
• 50 of carbohydrate digestion
• 50 of protein digestion
• 90 of lipid digestion
• Produces sodium bicarbonate for neutralization of
  chyme in duodenum

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Activation of Pancreatic Enzymes

• Enterokinase
• Secreted from crypts in duodenum
• Trypsinogen trypsin
• Trypsin then converts
• Trypsinogen trypsin
• Chymotrypsinogen chymotrypsin
• Procarboxypeptidase carboxypeptidase

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Overview of Digestive Enzymes

• Stomach
• Pepsinogen
• Chymosin (rennin)
• Pancreas
• Trypsinogen
• Chymotrypsinogen
• Procarboxypeptidase
• Amylase
• Lipase
• Nuclease

• Brush Border (SI)
• Sucrase
• Maltase
• Lactase
• Aminopeptidase
• Dipeptidase
• Enterokinase

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Large Intestine

• Composed of three segments
• Cecum
• Colon
• Rectum
• Function
• Fermentative digestion
• No enzyme secretion
• Relies on microbes or secretions washed out of
  the SI
• Absorption of remaining water, volatile fatty
  acids (VFAs) from microbial fermentation and
  minerals
• Digesta storage
• Degree of development is species dependent

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Monogastric Cecum

• Located at junction of small and large intestine
• Function similar to rumen in ruminants
• Microbial activity and digestion of feeds
• Contains a microbial population similar to the
  rumen
• Cellulolytic hemicelluloytic bacteria
• Since cecum is located AFTER major site of
  nutrient absorption (small intestine), then
  microbial cell proteins are not available to the
  animal
• Fecal loss

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Monogastric Large Intestine

• Function
• Absorption of liquid
• Mass movements move fecal matter to anus
• Usually only a few times a day
• Associated with defecation

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Bacteria

• Cellulolytic digest cellulose (forages)
• Amylolytic digest starches and sugars
  (concentrates or grains)
• Other types
• Proteolytic
• Clostridium
• Organic acid utilizers
• Methanogens
• Produce CO2, H2, formate, CH4

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Rectum

• Muscular area of large intestine used for storage
  of feces and ultimately for defecation
• Feces includes sloughed cells, undigested food
  and microbial matter

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Avians (Poultry)

• Mouth
• No teeth, rigid tongue
• Poorly developed salivary glands
• Saliva contains amylase
• Beak is adapted for prehension and mastication

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Avians (Poultry)

• Esophagus
• Enlarged area called crop
• Ingesta holding and moistening
• Location for breakdown of carbohydrate by amylase
• Fermentation
• Proventriculus (stomach)
• Release of HCl and pepsin (gastric juices)
• Ingesta passes through very quickly (14 seconds)

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Avians (Poultry)

• Gizzard (ventriculus)
• Muscular area with a hardened lining reduces
  particle size
• Muscular contractions every 20-30 seconds
• Includes action of grit
• HCl and pepsin secreted in proventriculus
• Small intestine
• Similar to other monogastrics
• No Lacteals

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Avians (Poultry)

• Ceca and large intestine
• Contain two ceca instead of one as in other
  monogastrics
• Large intestine is very short (2-4 in) and
  empties into cloaca where fecal material will be
  voided via the vent
• Water resorption
• Fiber fermentation by bacteria
• H2O soluble vitamin synthesis by bacteria