Chapter 24 by dffhrtcv3

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									Chapter 24


The Digestive System
           Digestive Processes
 Ingestion
 Movement of food
 Digestion
   Mechanical digestion
   Chemical digestion
 Absorption
 Defecation
General Structure
          Digestive organs divided
           into 2 main groups
             GI (alimentary) tract
             Accessory structures
                teeth, tongue, salivary
                 glands
                liver, gallbladder, pancreas
     General Histology of GI Tract
 Same histological organization from inside to outside
  and from end to end
                   Salivary Glands




 3 pairs salivary glands
    Parotid glands
    Submandibular glands
    Sublingual glands
                Salivary Glands
 Composition of Saliva
   99.5% water, 0.5% solutes
      Na+, K+, Cl-, HCO3-, and PO4-, proteins, waste
       products
      lysozyme
      salivary amylase
   Saliva composition differs from each gland
      parotid - watery saliva, amylase
      submandibular - thicker mucous, amylase
      sublingual - mostly mucous, little amylase
              Salivary Glands
 Function of Saliva
   Water dissolves food for taste, digestion
   Mucous lubricates food
   Cl- activates amylase
   HCO3-, and PO4- buffer mouth acids
   Wastes are removed
               Salivary Glands
 Secretion of Saliva - 1-1.5 L l day
   Primarily under nervous control
      PNS normal salivary secretions
         saliva swallowed
         most reabsorbed
      SNS, dehydration decrease flow
   Food (mechanically, chemically) stimulates saliva
      behavioral – memories from cortex
      starts digestion
      continues long after eating
   Irritating foods or nausea
  Physiology of Digestion in Mouth
 Mechanical digestion
   Chewing, mastication
   Food mixed w/ saliva
   Shaped in bolus
 Chemical digestion
   Salivary amylase
   Polysaccharide (starches) to disaccharides
    (maltose)
           Physiology of Deglutition
   Swallowing
     Moving bolus from mouth to stomach
     Facilitated by saliva, mucous
   Three phases
    1. Voluntary stage - bolus to oropharynx
    2. Pharyngeal stage - bolus through pharynx into
       esophagus
        involuntary
        receptors in oropharynx stimulate brain to close off
          airway
    3. Esophageal stage
        upper esophageal sphincter closes
        esophagus controls movement
                     Stomach
 Mucosa - simple columnar epithelium w/ gastric pits
                             Stomach
 Mucosa
   Gastric glands w/
    secretory cells
      chief cells –
       pepsionogen
       (pepsin)
      parietal cells – H+
       ions
      mucous cells -
       mucous
      G cells - gastrin
   Secretions = 2-3
    Ll day
                        Stomach
 Physiology of digestion - Chemical digestion
   Chief cells secrete pepsinogen
      cleaves proteins into peptides
      only effective at lo pH, activates itself and by acid
   Parietal cells secrete HCl-
      active transport w/ CA
      kills microbes, denatures proteins
      stimulates secretion of hormones for bile, pancreatic juice
   Mucous cells secrete mucous - 1-3 mm mucous
    layer in stomach prevents self-digestion
   Gastric lipase
      splits short chain triglycerides
      limited role in digestion since works best at pH 5-6
                        Stomach
 Absorption
   Impermeable to passage of most materials to blood
   Does absorb:
     water
     electrolytes
     drugs (aspirin)
     alcohol
         Stomach
   Regulation of gastric secretion
    and motility
     Regulated by combination of
        nervous and hormonal factors
       3 phases of secretion
        1. cephalic
        2. gastric
        3. intestinal
       Stomach
1. Cephalic phase
    Stimulus
       smell, taste of food
       thoughts of food
     Response
       cortex and hypothalamus
       parasympathetic impulses
       stimulate juice and peristalsis
        Stomach
2. Gastric phase
    Stimulus - Neural
        stretch receptors, chemoreceptors
        distension or increase in pH
          stimulate submucosal plexus
    Response
        increased peristalsis
        increase gastric juice production
      As food passed on, pH ,
       distension , stimulus
       disappears
       Stomach
2. Gastric phase (cont.)
    Stimulus - Endocrine
      distension, partly digested proteins,
       caffeine stimulate G cells
      gastrin secretion
          inhibited at pH < 2
          stim when pH rises
   Response
      enteroendocrine (look it up!)
      gastrin in blood to gastric glands
          stimulate secretion gastric juices
          contraction of lower esophageal
             sphincter
             motility
            relax pyloric ileocecal sphincter
        Stomach
2. Gastric phase (cont.)
    Control of HCl secreting cells
       stimulated by three chemicals
           gastrin
           acetylcholine
           histamine
       work in conjunction with
         carbonic anhydrase
    All three needed for strong
      secretion
       presence of one results in
        small production of HCl
       blockage of the histamine
        receptor decreases secretion –
        different from allergic response
           Tagamet
           Zantac
        Stomach
3. Intestinal phase
    Stimulus - stretch receptors,
       chemoreceptors
      Response
        inhibitory - Start enterogastric reflex
            stretch receptors, chemoreceptors
               inhibit gastric emptying
              slows chyme exit
              ensures proper intestinal
               digestion/absorption
        excitatory - Chemoreceptors
            detect FA’s, glucose in small intestine
            stimulate enteroendocrine cells in
               small intestine
                 cholecystokinin (CCK) - activity
                 secretin - secretions
      Stomach
 Hormonal/neuronal
 reflexes regulate gastric
 emptying




                          Stomach emptying
                             inhibited by
                             enterogastric reflex, CCK
Pancreas
        Accessory organ -
           aids small intestine
           with food digestion
Pancreas
     Histology - clusters of
      glandular epithelial cells
        1% of cells in pancreatic
         islets (islets of
         Langerhans)
            endocrine pancreas
            glucagon, insulin,
            somatostatin, pancreatic
            polypeptide
        99% in acini
           exocrine portion of
            pancreas
           secrete pancreatic juice
Pancreas
   2 large ducts collect juice
      Larger - pancreatic duct
         joins common bile duct from
          liver, gallbladder
         enter duodenum at
          hepatopancreatic ampulla with
          common duct
      Smaller accessory duct
       empties into small intestine
       upstream
                        Pancreas
 Pancreatic juice
   1.2-1.5 L/day
   Mostly water some salts, bicarbonate, enzymes
      alkaline, pH 7.1-8.2
      buffers acidic gastric juice, stops pepsin, creates proper
      pH for enzymes in intestine
   Enzymes include
      pancreatic amylase
      trypsin, chymotrypsin, carboxypeptidase - secreted in
       inactive protein forms
      pancreatic lipase
      ribonuclease and deoxyribonuclease
Pancreas
       Regulation of
        pancreatic secretions
          Nervous mechanisms -
          PNS
         Hormonal mechanisms
            secretin
            CCK
Liver
     Liver
         General
            heaviest body organ, 3
             lbs
            2nd largest organ of
             body
         Anatomy
            covered in peritoneum
            large R lobe, smaller L
              lobe separated by a
              ligament
Liver
   Histology
      Hepatic ducts join cystic duct
       from gallbladder form common
       bile duct
      Gallbladder
      Common bile, pancreatic duct
       form hepatopancreatic ampulla
      Empty into duodenum
Liver
    Blood Supply
         Two sources
            hepatic artery-
             oxygenated blood from
             aorta
            hepatic portal vein -
             deoxygenated blood from
             intestine w/ newly
             absorbed nutrients
         Both carry blood into
          sinusoids
            Kupffer's cells phagocytize
             microbes, bits of foreign
             matter
            nutrients, toxins stored
         Substances from liver
          cells secreted into blood
                       Liver
 Bile
   800-1000 ml/day
   Yellow, brownish, or olive-green liquid
   pH 7.6-8.6, mostly water, bile salts, bile acids,
    cholesterol, lecithin (phospholipid), bile pigments,
    ions
   Part excretory product, part digestive secretion
      bile salts help in emulsification
      bilirubin
Liver
 Regulation of bile secretion
   Nervous
   Hormonal
                           Liver
 Physiology of the liver - vital
   Carbohydrate metabolism - blood glucose levels
      glycogenesis
      glycogenolysis
      gluconeogenesis
   Lipid metabolism -
      stores, metabolizes some triglycerides
      cholesterol for bile salt production
   Protein metabolism
      deamination
          Remove, detoxify NH3 group from AA’s
          AA's used for ATP production or changed to carbos, fats
      synthesizes most plasma proteins
      can convert AA's from one to another (transamination)
                          Liver
 Physiology of the liver - vital
   Removal of drugs, hormones
      detoxify or secrete into bile several drugs
      alter thyroid, steroid hormones
   Excretion of bile - bilirubin
   Synthesis of bile salts
   Storage - stores vitamins, iron
   Phagocytosis
   Activation of Vitamin D (?)
Gall Bladder
           Pear shaped sac, 7-
            10 cm long
           Physiology
              stores, concentrates
               bile
              CCK stimulates release
              when small intestine
               empty sphincter closes
               forcing bile into
               gallbladder
  Summary: Digestive Hormones
 Gastrin
 Secretin
 Cholecystokinin
Small Intestine
         Site of digestion,
          absorption
         Anatomy - divided into 3
          segments
            duodenum
            jejunum
            Ileum
         Other parts
            Pyloric sphincter
            Ileocecal sphincter
            Plicae circulares (circular
             folds)
Small Intestine
                          Small Intestine
 Intestinal juice
      Secreted from epithelium
      1-2 L/day, pH 7.6
      mostly water and mucous
      w/ pancreatic juice provide
       neutralization and medium for
       absorption
 Brush border enzymes
    enzymes on surface of epithelial cells
    brush border enzymes digest some
       chyme
         Carbohydrates
         Peptides
         Nucleotides
    brush border cells rubbed off, break
       open releasing enzymes
                 Small Intestine
 Physiology of digestion in small intestine
   Mechanical digestion
      controlled by autonomic nervous system
      2 phases
         segmentation
             primary action of sm.int. where food is present
             alternate contraction, relaxation of circular muscle around
              intestine
         peristalsis
             propels chyme onward
             weak and only after most has been absorbed
                 Small Intestine
 Physiology of digestion (cont.)
   Chemical digestion
      Complete digestion a function of pancreatic juice, bile,
       intestinal juice
      Prior to small intestine
          mouth - amylase
          stomach
             pepsin
             lingual lipase
                 Small Intestine
 Physiology of digestion (cont.)
   Chemical digestion (cont.)
      In small intestine:
         carbohydrate digestion
              pancreatic amylase
              other disaccharides, monosaccharides broken down at
               brush border
         protein digestion
              pancreatic juices
              finished by proteases in brush border membrane
         lipid digestion
              bile salt emulsification
              pancreatic lipase
         nucleic acid digestion
              ribonuclease and deoxyribonuclease in pancreatic juices
              brush border enzymes digest nucleotides
              Small Intestine
 Regulation of intestinal secretion and motility
   Local reflexes in response to chyme (Ach)
   Vasoactive Intestinal Peptide (VIP) stimulates
    production of intestinal juice
   Segmentation movements initiated by distension
      pacemakers
      local reflexes and PNS reflex return  motility
      w/ absorption,  distension and  peristalsis
                    Small Intestine
 Physiology of Absorption
   About 90% of all
    absorption takes place in
    small intestine
   Occurs by
      diffusion
      facilitiated diffusion
      osmosis
      active transport
         secondary active transport
         driven by active transport
Small Intestine
Small Intestine
        Water absorption
          Total volume into small
           intestine/day - 9.3 L
              2.3 L from ingestion
              7 L from secretions
          Small intestine absorbs 
           8.3 L /day
              passive absorption following
               nutrients
              osmosis
          Rest passes to large
           intestine where most is
           reabsorbed (0.9 L)
              Small Intestine
 Vitamin absorption
   Fat-soluble vitamins (A, D, E, K) included w/
    micelles
   Water soluble (B, C) absorbed by diffusion - B12
    absorbed w/ intrinsic factor
Large Intestine
            Functions
              Completion of
               absorption
              Manufacture certain
               vitamins
              Formation/expulsion
               of feces
                 Large Intestine
 Physiology of digestion in the large intestine
   Mechanical digestion
      chyme passage regulated by sphincter
         valve generally closed - slow passage
         following meal gastroileal reflex - ileal motility, chyme to
          cecum
         when cecum full sphincter contracts
      colon movements start when chyme passes sphincter
         haustral churning
             haustra relaxed, distended until full
             then contract, squeeze contents into next haustra
         peristalsis - slow
         mass peristalsis (gastrocolic reflex)
             during or immediately following a meal, 3-4 times day
             strong peristaltic wave from middle of transverse colon
             pushes contents into rectum
                Large Intestine
 Physiology of digestion in the large intestine
   Chemical digestion
      mucous but no enzymes secreted
      digestion by bacteria in colon
         final breakdown of substances, mostly carbohydrates
         produce some vitamins, B and K
               Large Intestine
 Absorption and feces formation
   Chyme
     3-10 hours in large intestine becomes solid (due to
      water reabsorption) feces
     large intestine absorbs water, electrolytes and some
      vitamins
   Feces
     water, inorganic salts, sloughed off epithelial cells,
      bacteria, products of bacterial decomposition,
      undigested parts of food
     most water reabsorbed in small intestine but large
      intestine also important in water reabsorption
                Large Intestine
 Physiology of defecation
   Mass peristalsis
      pushes fecal matter into rectum
      distension stimulates stretch receptors initiating reflex
      for defecation
   PNS stimulated by stretch receptors
      stimulates contraction of rectum
      shortens and  pressure in it
      PNS stimulation opens internal sphincter
      feces expelled

								
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