Digestion and Absorption

ANS 215

Anatomy and Physiology of Domestic Animals

 

  1. Digestive Tract
    1. Animals are classified, according to the diet in their natural state as:
      1. Carnivores
      2. Herbivores
      3. Omnivores
    2. Because of the diversity of diet, various parts of the digestive system

      3. Developed in different ways.

    3. Mouth
      1. Most cranial part of the digestive tract
        1. Also referred to as oral cavity
        2. Site of reduction of food particle size
        3. Teeth and tongue are structures that assist in digestion
        4. Salivary enzymes are also added to digesta
      2. Teeth
        1. Mechanically reduce the size of ingested food
          1. Increases surface area
        2. Teeth are also used for cutting food (incisors) and defense (canines)
        3. Types of teeth
          1. Identified by location and use
            1. Incisors
              1. most forward
              2. used for cutting
              3. also called "nippers

ii. Canines
    1. also called fangs, eyeteeth, tusks
    2. Used for tearing
    3. located posterior to incisors

iii. Premolars
    1. Used for grinding
    2. located posterior to canines

iv. Molars

    1. located caudal to premolars
    2. Used for grinding

      1. Dental Formula
        1. Indicates the number of incisors (I), canines (C), premolars (P) and molars (M) on one side of the mouth.
        2. Dental formula of the cow is IO/4C0/0P3/3M3/3
          1. Numerator represents upper jaw, denominator is lower jaw
          2. Cow has a firm dental pad instead of incisors in upper jaw
        3. Formula represents teeth on one side of the mouth
      2. Exposed surface of teeth
        1. Several terms are used to describe the exposed surface of a tooth.
          1. Table (grinding surface)- makes contact with surface of opposite jaw
          2. Lingual surface-side of tooth next to tongue
          3. Labial surface-outer surface next to lips
          4. Buccal surface-outer surface next to cheeks
          5. Contact surface-next to a neighboring tooth of the same arcade (row)
          6. Upper arcades of cheek teeth (molars and premolars) are slightly wider apart than the lower arcades of cheek teeth.
          7. Upper cheek teeth have a wider table surface than the lower cheek teeth.

        2. Uneven wear of tooth surface
          1. Can develop/ particularly in horses
          2. Points can develop on teeth that injure the buccal or lingual membranes causing pain
          3. Filing off points with rasp is called "floating teeth"
        3. Age of horses can be approximated by examining lower

          4. Incisors.

          1. determine if permanent incisors have erupted
            1. full mouth includes all 3 incisors
              1. IL1 =2 ½ years
              2. IL2 =3 ½ years
              3. IL3 =4 ½ years

        4. Wear Characteristics
          1. Mouth is in wear when two complete enamel rings are present on the table surface of each incisor
          2. Approximate ages for each pair of incisors to be in wear are 6,7, and 8 years for I1,I2 and I3.
          3. Judgment is made regarding the loss of the inner enamel ring and appearance of the pulp cavity (dental star).
            1. 11,12 and 13 years for I1, I2 and I3.
          4. A horse has a smooth mouth when these occur in

            5. All three pairs of incisors.

          5. Rough approximation of horse ages for
            1. Full mouth 5 years
            2. In wear 10 years
            3. Smooth mouth 15 years
    1. Tongue
      1. Muscular organ used to maneuvar the food mass
      2. Tongue can be differentiated from other muscle tissues because it has fibers oriented in three directions.
      3. Rough surface of the tongue is provided by numerous projections known as papillae
        1. Provide traction for moving the food within the mouth
        2. Help in grooming themselves and offspring
      4. Process of digestion is aided by discriminatory taste buds

      Located on the tongue surface within the vallate and fungiform papillae

    2. Pharynx
      1. Common passageway for food and air
      2. Opens into mouth , nasal cavities, Eustachian tubes, larynx and esophagus.
      3. During passage of food through pharynx it is prevented from entering larynx and nasal cavities because of reflex and mechanical factors.
    3. Esophagus
      1. Muscular tube extending from the pharynx to the stomach
      2. During its course to the stomach the esophagus traverses the thorax within the mediastinal space.

        3.  

      3. Esophagus then passes through the diaphragm into abdominal

        4. Cavity and enters the stomach

      4. Food and fluid are moved from the pharynx to the stomach by

        5. Muscular waves of contraction.

      5. The esophagus is normally closed at the pharyngeal end by

        6. Tonic activity of the cranio-esophageal sphincter.

      6. The esophagus remains closed at the opening to the stomach

        7. (cardia) because of a closure that is physiologic in nature.

      7. On its way to the stomach, the esophagus courses along the left

      Side of the trachea. Bolus transport can be observed by watching the left side of the neck (especially in ruminants)

    4. Stomach
      1. Food is received by the stomach for storage (pending further

        2. Digestion) and for continuation of digestion.

      2. Primary regions as viewed from outside:
        1. Cardia
          1. Located nearest the esophagus
          2. Continued by the fundus
        2. Fundus
          1. Dome-shaped part of the stomach
          2. Fundus is adjacent to the Corpus
        3. Corpus
          1. Rounded base or bottome
        4. Antrum
          1. Constricted part that joins duodenum

         

      3. Regions of stomach as defined by cell type (inner aspect)
        1. Esophageal gland region
        2. cardiac gland region
          1. Secretes mucous
        3. fundic gland region
          1. entire space between cardiac and pyloric gland regions.
          2. These glands sometimes called gastric glands
          3. Secrete hydrochloric acid and pepsinogen
        4. pyloric gland region
          1. secrete mucous and the hormone gastrin
      4. Ruminant Stomachs
        1. Foregut fermentation
        2. Includes the rumen, reticulum (omasum) and abomasum
        3. True stomach is the abomasums

    5. Small Intestine
      1. Stomach contents enter the small intestine after their preparation in the stomach.
      2. Small intestine comprised of three segments
        1. duodenum
          1. closely connected to the pancreas
          2. receives pancreatic secretions
          3. Also receives bile
        2. jejunum
        3. ilium

      3. Inner layer of the small intestine having intimate contact with the contents of the lumen is composed of an epithelial cell layer known as the mucosa.
      4. The submucosa is a connective tissue layer that provides space for blood vessels, lymph vessels and nerve fibers.
        1. There is also a sparse layer of smooth muscle fibers known as the muscularis mucosa
          1. Individual fibers from muscularis mucosa attach to villi and cause movement when contracting.
        2. A nerve network (Meissner’s plexus) in the submucosa is important in controlling secretions of the epithelial cells and blood flow.
          1. This network also serves a sensory function via stretch receptors
          2. Another nerve plexus (Auerbach’s plexus), between the inner circular and outer longitudinal muscle layers is important in controlling gastrointestinal movement.
            1. These two nerve plexus’s are referred to as the enteric nervous system and it extends from the esophagus to the anus.
        3. Outer layer of the intestine is the serosa. It covers the

          4. Intestine and is continuous with the mesentery which suspends the intestine within the abdominal cavity.

          1. The mesentery is in turn continuous with the

          Peritoneum.

        4. The surface area of the intestine is increased by long length and folding of the tissue within the intestine.
          1. Folds or placations are covered with villi and the individual cells that cover the villi have their own microvilli.
          2. Microvilli provide for the greatest amplification of surface area and constitute the brush border.
          3. This amplification increases the total surface area of the small intestine about 600 times that of

          A smooth cylinder of the same size.

        5. The crypts of liberkÜ hn are cloistered groups of

          6. Undifferentiated cells between adjacent villi.

          1. Only cells of villi to undergo division
          2. Renewal of cells in the villi is accomplished by migration of new cells away from the crypts.

    6. Large Intestine
      1. Contents from the terminal part of the ileum enter the large intestine at the cecum (ileocecal junction) in the horse, at the colon(ileocolic junction) in the dog or at the cecumand colon (ileocecolcolic junction) in the ruminant and pig.
      2. The large intestine consists of the cecum and colon.
        1. Development of the large intestine varies among animals according to diet.
        2. In ruminants bacterial and protozoal cells are digested here.
        3. In simple herbivores, (e.g. horse) enzymatic digestion preceeds fermentation and no microbes or protozoa are available for digestion here.
        4. Food requiring further digestion by fermentation enters or is diverted into the cecum unless it is developed poorly as in the dog.
      3. The colon continues from the cecum to its termination at the

      Anus.

       

    7. Accessory Glands
      1. Salivary Glands
        1. Three pairs of well-defined glands
        2. Glands connected to mouth by ducts
        3. Salivary glands are serous, mucous or mixed depending on secretion
        4. Blood vessels and nerves enter each gland where ducts exit
        5. Innervation is sympathetic and parasympathetic

      2. Pancreas
        1. has both endocrine and exocrine function
        2. Always located near the first part of the duodenum
        3. appears as elongated gland of loosely connected nodules
        4. Main pancreatic duct enters duodenum near common bile duct.
          1. In sheep and goats pancreatic duct enters bile duct
        5. Endocrine cells are scattered throughout pancreas
          1. Islets of langerhans
          2. Alpha cells produce glucagons
          3. Beta cells produce insulin
          4. Cells secrete directly into the bloodstream

      3. Liver
        1. Multipurpose organ
        2. Always located immediately behind the diaphragm
        3. In ruminants it tends to be on the right side
        4. Lobules of the liver are clearly demarcated
        5. Liver receives arterial blood through hepatic artery and venous blood through portal vein.
          1. Portal vein drains stomach, spleen, pancreas and intestines.
          2. Blood from both sources is circulated through the sinusoids
            1. Here it is detoxified and modified before returning via the hepatic vein to the vena cava

  1. Physical and Mechanical Factors
    1. Prehension
      1. Seizing and conveying food into the mouth
        1. mouth, tongue and lips
    2. Mastication
      1. Mechanical breakdown of food in the mouth
      2. A bolus of food is formed by the mastication process.
      3. Bolus is mixed with saliva which provides adhesiveness
    3. Deglutition
      1. Act of swallowing
      2. Conveys the food mass from the mouth to the stomach
      3. Some degree of conciousness is required because activity is voluntary
        1. inhaling food and vomitus is possible when unconcious
    4. Smooth Muscle Activity
      1. Once food has reached the stomach its movement is controlled by activity of the smooth muscle of the stomach and intestine.
      2. Muscle activity is spontaneous and controlled by autonomic nervous system.
      3. Segmentation is represented by contractile waves that travel short distances
      4. Peristaltic waves travel longer distances and usually move in an arboreal direction.
      5. An important intrinsic reflex for the small intestine is the peristaltic reflex.
        1. initiated by distention of the bowel
        2. Hormones gastrin and cholecystokinin are known to stimulate gastrointestinal smooth muscle and affect the rate of passage.

    5. Physical functions of the stomach and intestine
      1. Mixing food with secretions
      2. Control of emptying of its contents
        1. Delay of gastric emptying
          1. Neural mechanism (enterogastric reflex)
            1. Osmoreceptors detect hypertonicity
            2. Excessive protein or carbohydrate is also effective in delaying gastric emptying
            3. Other receptors respond to high hydrogen ion concentrations
            4. Cholecystokinin slows gastric emptying to aid in fat digestion
            5. Gastric inhibitory polypeptide (GIP) is secreted by the jejunal mucosa in response to high lipid and carbohydrate content of the diet.
  2. Composition of Foodstuffs
    1. Six basic foodstuffs are classified as:
      1. Carbohydrates
        1. Classified as monosaccharides, disaccharides or polysaccharides depending on number of pentose or hexose carbon units they contain.
        2. Primary food

          3.  

           

           

           

           

           

           

        3. Glycogen represents principal carbohydrate reserve

      2. Proteins
        1. Complex, high molecular weight, large, colloidal molecules that contain a high percentage of amino acids. Important source of nitrogen-primary food
        2. Essential amino acids are those that cannot be synthesized at all or rapidly enough to permit normal growth. They must be provided in the diet.
        3. Non-essential amino acids are those the animal can synthesize.

      3. Lipids
        1. Include fat and related substances- primary food

      4. Water
        1. Considered an accessory food but essential to life
      5. Inorganic Salts
        1. minerals are required for skeletal development and many chemical reactions that occur during metabolism
        2. minerals only required in trace amounts are called trace minerals
        3. Combined amount of mineral in the diet is determined by ashing (burning) the food source.
        4. Considered an accessory food.
      6. Vitamins
        1. Group of chemically unrelated organic compounds
        2. Generally function as metabolic catalysts usually in the form of coenzymes
        3. Considered an accessory food
  3. Digestive Secretions and Their Functions
    1. Saliva-facilitates mastication and deglutition
      1. Volume varies considerably among species.
      2. Cow can produce 25-50 gallons per day
      3. Can perform cooling function
      4. Contains amylase for starch digestion
    2. Gastric Secretions
      1. Mucous-protective function
      2. Pepsinogen-converted to pepsin by HCL
      3. Gastrin-stimulates secretion of HCL
      4. Hydrochloric Acid
      5. Rennin-in young ruminants-coagulates milk protein
    3. Pancreatic Secretions
      1. HCO3- neutralizes HCL content of stomach contents entering duodenum.
      2. Enzymes include all those needed for digestion of fat, protein and carbohydrates
        1. Amylase
        2. trypsinogen
        3. chymotrypsinogen
        4. elastase
        5. carboxypeptidase
        6. pancreatic lipase

    4. Biliary Secretion
      1. Bile is a greenish-yellow solution of bile salts, bilirubin, cholesterol, lecithin and electrolytes.
      2. Biles salts are produced continuously by the liver but the amount required for digestion far exceeds production therefore they are recirculated from the intestine to the hepatic cells (portal circulation).
      3. Bile salts are synthesized from cholesterol
      4. The only domestic animal without a gallbladder is the horse.
      5. Fat in the intestine is emulsified by bile salts and lecithin.
  4. Breakdown and Absorption of Carbohydrates, Proteins and Fats
    1. Most digestion of carbohydrates, proteins and fats occurs in the small intestine (except in ruminants).
        1. Carbohydrate
          1. Amylase breaks down starch to maltose
          2. Maltose is further degraded by maltase to monosaccharides.
          3. Fructose, glucose and sucrose all absorbed at brush border

        2. Protein
          1. Gastric and pancreatic proteases hydrolyze proteins into smaller units
          2. Oligopeptidases at the brush border break down oligopeptides to individual amino acids for absorption.
          3. Amino acids are actively transported into blood
        3. Fat
          1. Dietary triglycerides are emulsified by bile salts
          2. Triglycerides broken down into glycerol and fatty acids by lipase

     

     

     

     

  5. Microbial Digestion in the Large Intestine
    1. No enzymatic digestion occurs in the large intestine of mammals
    2. Digestion that occurs results from microbial action which is significant for non-ruminant herbivores and omnivores
    3. End products of digestion are volatile fatty acids (VFA’s)
      1. Acetate
      2. Proprionate
      3. Butyrate
    4. Horses obtain as much as 75% of their energy requirement from large

      5. Intestinal absorption of VFA’s.

    5. Large intestine fermentation salvages otherwise lost calories as VFA’s

    And decreases the effective osmotic pressur of large intestine contents so that water can be reabsorbed.

  6. The Ruminant Stomach
    1. Animals thtat regurgitate and remasticate their food are called ruminants.
    2. Ruminant stomach is adapted for fermentation of ingested food by bacterial and protozoan microorganisms.
    3. Mammalian digestive enzymes cannot digest cellulose. However, microbial enzymes CAN digest cellulose

    4. Ruminant stomach has four compartments
      1. Rumen (paunch) allows for soaking and fermentation of bulk fibroud food, contents are continually mixed
      2. Reticulum (honeycomb) Serves as pump that causes liquid to flow in and out of the rumen.
      3. Omasum (many plies) Provides for continued fermentation and absorption
      4. Abomasums (true stomach)

    5. Process of rumination
      1. Regurgitation
      2. Remastication
      3. Reinsalivation
      4. Redeglutition
    6. Byproducts of Fermentation
      1. Methane (30-40% of rumen gas)
      2. Carbon Dioxide (60-70% of rumen gas)
        1. gases mostly eliminated by eructation
      3. Volatile Fatty Acids
      4. Protozoa
      5. Bacteria
  7. Avian Digestion

    1. Birds have not teeth therefore the mechanical breakdown of their ingested food is accomplished by the beak and gizzard.
    2. Sailivary glands are present in birds and well developed in species which consume dry food.
    3. The esophagus is divided into precrop and postcrop segments
    4. The crop is a dilatation of the esophagus and has a food storage function.
    5. The proventriculus is located between the postcrop esophagus and the gizzard
    6. The gastric secretions HCL and pepsinogen as well as mucus are secreted by the proventriculus.
    7. The small intestine has a well-defined duodenum with the pancreas located between its loops but there is no distinction between the jejunum and ileum.
    8. The ceca, which are paired structures, are located at the junction of the small and large intestine. Here microbial digestion of cellulose occurs.
    9. Digestive tract ends with the cloaca, the site that is common to the digestive, urinary and reproductive tracts in birds.