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Skeletal System The Digestive System Chapter 22 The Digestive

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Skeletal System The Digestive System Chapter 22 The Digestive Powered By Docstoc
					The Digestive System


      Chapter 22
         The Digestive System
   The digestive system
    – Takes in food
    – Breaks it down into nutrient molecules
    – Absorbs the nutrient molecules into the
      bloodstream
    – Rids the body of indigestible remains
             The Digestive System




   The organs of the digestive system can be separated
    into two main groups; those of the alimentary canal
    and the accessory organs
             The Digestive System




   The alimentary canal or gastrointestinal (GI) tract is
    the continuous muscular digestive tube that winds
    through the body
          The Digestive System
   The organs of the alimentary canal are
    – Mouth, pharynx, esophagus, stomach, small
      intestine and large intestine
    – Food in this canal is technically out of the
      body
   The accessory digestive organs are
    – Teeth, tongue, gallbladder, salivary glands,
      liver and pancreas
    – The accessory organs produce saliva, bile and
      digestive enzymes that contribute to the
      breakdown of foodstuffs
Digestive
Processes
   The digestive tract
    can be viewed as a
    process by which
    food becomes less
    complex at each step
    of processing and
    nutrients become
    available to the
    body
                   Ingestion
   Ingestion is simply
    the process of
    taking food into
    the digestive tract
    via the mouth
                 Propulsion
   Propulsion is the
    process that moves
    food through the
    alimentary canal
   It includes
    swallowing
    (voluntary process)
    and peristalsis
    (involuntary
    process)
                    Propulsion
   Peristalsis involves
    alternate waves of
    contraction and
    relaxation of
    muscles in the
    organ walls
   Its main effect is to
    squeeze food from
    one organ to the
    next
   Some mixing occurs
    as well
         Mechanical Digestion
   Mechanical
    digestion physically
    prepares food for
    chemical digestion
    by enzymes
              Mechanical Digestion
   Mechanical processes
    include chewing,
    mixing of food with
    saliva by the tongue,
    churning of food by
    the stomach, and
    segmentation
   Segmentation mixes
    food with digestive
    juices and increases
    the rate of absorption
    by moving food over
    the intestinal wall
           Chemical Digestion
   Chemical digestion
    is a series of
    catabolic steps in
    which complex
    food molecules are
    broken down to
    their chemical
    building blocks
          Chemical Digestion
   Chemical digestion is accomplished by
    enzymes secreted by various glands into
    the lumen of the alimentary canal
   The enzymatic breakdown of foodstuffs
    begins in the mouth and is essentially
    complete in the small intestine
                 Absorption
   Absorption is the
    passage of digested
    end products (plus
    vitamins, mineral
    and water) from
    the lumen of the
    GI tract into the
    blood or lymph
    capillaries located
    in the wall of the
    canal
                Absorption
   For absorption to occur these substances
    must first enter the mucosal cells by
    active or passive transport processes
   The small intestine is the main absorption
    site
                  Defecation
   Defecation is the
    elimination of
    indigestible
    substances from
    the body as feces
       Basic Functional Concepts
   Most organ systems respond to changes in
    the internal environment either by
    attempting to restore some plasma variable
    or by changing their own function
   The digestive system creates an optimal
    environment for its functioning in the lumen
    of the GI tract
   Essentially all digestive tract regulatory
    mechanisms act to control luminal conditions
    so that digestion and absorption can occur
    there as effectively as possible
     Basic Functional Concepts
   Digestive activity is provoked by a range
    of mechanical and chemical stimuli
    – Receptors are located in the walls of the
      tract organs
    – These receptors respond to several stimuli
    – The most important being the stretching of
      the organ by food in the lumen, osmolarity
      (solute concentration) and pH of the contents
      and the presence of substrates and end
      products of digestion
          Basic Functional Concepts
   When appropriately
    stimulated, these
    receptors initiate
    reflexes that
    – Activate or inhibit
      glands that secrete
      digestive juices into the
      lumen or hormones
      into the blood
    – Mix lumen contents
      along the length of the
      tract by stimulating
      the smooth muscle of
      the GI tract walls
     Basic Functional Concepts
   Controls of digestive activity are both
    extrinsic and intrinsic
    – Another novel trait of the digestive tract is
      that many of its controlling systems are
      intrinsic - a product of in-house nerve
      plexuses or local hormone-producing cells
    – The walls of the alimentary canal contain
      nerve plexuses
    – These plexuses extend essentially the entire
      length of the GI tract and influence each
      other both in the same and in different
      organs
               Digestive Processes
   Two kinds of reflex
    activity occur
   Short reflexes are
    mediated entirely by
    the local enteric
    plexuses in response
    to GI tract stimuli
   Long reflexes are
    initiated by stimuli
    arising from within
    or outside of the GI
    tract and involve
    CNS centers and ANS
          Digestive Processes
   The stomach and small intestine also
    contain hormone-producing cells that,
    when stimulated by chemicals, nerve
    fibers, or local stretch, release their
    products to the extracellular space
   These hormones circulate in the blood
    and are distributed to their target cells
    within the same or different tract organs,
    which they prod into secretory or
    contractile activity
          Digestive System Organs
   Most of the digestive
    organs reside in the
    abdominal-pelvic
    cavity
   All ventral body
    cavities contain
    serous membranes
   The peritoneum of
    the abdominal cavity
    is the most extensive
    serous membrane of
    the body
          Digestive System Organs
   The visceral peritoneum
    covers the external surface
    of most digestive organs
    and is continuous with the
    parietal peritoneum that
    lines the walls of the
    abdomino-pelvic cavity
   Between the two layers is
    the peritoneal cavity, a
    slitlike potential space
    containing fluid secreted
    by the serous membranes
          Digestive System Organs
   The serous fluid
    lubricates the mobile
    digestive organs,
    allowing them to glide
    easily across one
    another as they carry
    out their digestive
    activities
          Digestive System Organs




   A mesentery is a double layer of peritoneum - a sheet
    of two serous membranes fused back to back - that
    extends to the digestive organ from the body wall
          Digestive System Organs




   Mesenteries provide routes for blood vessels,
    lymphatics and nerves to reach the digestive viscera
          Digestive System Organs




   Mesenteries also suspend the visceral organs in place
    as well as serving as a site for fat storage
               Digestive Processes




   Not all alimentary canal organs are suspended with
    the peritoneal cavity by a mesentery
   Some parts of the small intestine originate the cavity
    but then adhere to the dorsal abdominal wall
    (Figure 22.5) above
              Digestive Processes




   Organs that adhere to the dorsal abdominal wall lose
    their mesentery and lie posterior to the peritoneum
   These organs, which also include most of the pancreas
    and parts of the large intestine are called retro-
    peritoneal organs
              Digestive Processes




   Digestive organs like the stomach that keep their
    mesentery and remain in the peritoneal cavity are
    called interperitoneal or peritoneal organs
   It is not known why some digestive organs end up in
    the retroperitoneal position
              Blood Supply
   The splanchnic circulation includes those
    arteries that branch off the abdominal
    aorta to serve the digestive organs and
    the hepatic portal circulation
   The hepatic, splenic and left gastric
    branches of the celiac trunk serve the
    spleen, liver, and stomach
   The mesenteric arteries (superior and
    inferior) serve the small and large
    intestine
                Blood Supply
   The arterial supply to the abdominal organs
    is approximately one quarter of the cardiac
    output
   The hepatic portal circulation collects
    nutrient-rich venous blood draining from the
    digestive viscera and delivers it to the liver
   The liver collects the absorbed nutrients for
    metabolic processing or for storage before
    releasing them back to the bloodstream for
    general cellular use
Histology of the Alimentary Canal
    From the esophagus to the anal canal, the
     walls of every organ of the alimentary
     canal are made up of the same four basic
     layers or tunics
     –   Mucosa
     –   Submucosa
     –   Muscularis externa
     –   Serosa
    Each tunic contains a predominant tissue
     type that plays a specific role in food
     breakdown
    Histology of the Alimentary Canal
   From internal to
    external the four
    layers of the
    alimentary canal
    are
    – Mucosa
    – Submucosa
    – Muscularis
      Externa
    – Serosa
                 Histology: Mucosa
   The mucosa is the
    moist epithelial
    membrane that lines
    the length of the
    lumen of the
    alimentary canal
   Major functions are
    – Secretion of mucus,
      digestive enzymes
      and hormones
    – Absorption
    – Protection
                 Histology: Mucosa
   The mucosa is the
    moist epithelial
    membrane that lines
    the length of the
    lumen of the
    alimentary canal
   Major functions are
    – Secretion of mucus,
      digestive enzymes
      and hormones
    – Absorption
    – Protection
           Histology: Mucosa
   More complex than most other mucosae
    the typical digestive mucosa consists of
    three sublayers
    – A surface epithelium
    – A lamina propria
    – A deep muscularis mucosae
                 Histology: Mucosa
   The epithelium of
    the mucosa is a
    simple columnar
    epithelium that is
    rich in mucus
    secreting goblet cells
           Histology: Mucosa
   The slippery mucus it produces protects
    certain digestive organs from digesting
    themselves by enzymes working within
    their cavities and eases food passage
   In the stomach and small intestine the
    mucosa contain both enzyme-secreting
    and hormone-secreting cells
   Thus, in such sites, the mucosa is a
    diffuse kind of endocrine organ as well as
    part of the digestive organ
                Histology: Mucosa
   The lamina propria
    which underlies the
    epithelium is loose
    areolar connective
   Note lymph nodule
            Histology: Mucosa
   Its capillaries nourish the epithelium and
    absorb digested nutrients
   Its isolated lymph nodules are part of the
    mucosa associated lymphatic tissue (MALT)
    which collectively act as a defense against
    bacteria and other pathogens
   Large collections of lymph nodules occur at
    strategic locations such as within the
    pharynx (tonsils) and appendix
                Histology: Mucosa
   The muscularis
    mucosae is a scant
    layer of smooth
    muscle cells that
    produces local
    movements of the
    mucosa
            Histology: Mucosa
   The twitching of this muscle layer dislodges
    food particles that have adhered to the
    mucosa
   In the small intestine, it throws the mucosa
    into a series of small folds that immensely
    increase its surface area
              Histology: Submucosa
   The submocosa is a
    moderately dense
    connective tissue
    containing blood and
    lymphatic vessels,
    lymph nodules, and
    nerve fibers
   Its rich supply of
    elastic fibers enables
    the stomach to
    regain its normal
    shape after storing a
    large meal
              Histology: Submucosa
   The submocosa is a
    moderately dense
    connective tissue
    containing blood and
    lymphatic vessels,
    lymph nodules, and
    nerve fibers
   Its rich supply of
    elastic fibers enables
    the stomach to
    regain its normal
    shape after storing a
    large meal
       Histology: Muscularis Externa
   The muscularis
    externa is
    responsible for
    segmentation and
    peristalsis
   It mixes and propels
    foodstuffs along the
    digestive tract
   This thick muscular
    layer has an inner
    circular and an
    outer longitudinal
    layer
    Histology: Muscularis Externa
   In several places along the GI tract, the
    circular layer thickens to form sphincters
   Sphincters act as valves to prevent backflow
    and control food passage from one organ to
    the next
                 Histology: Serosa
   The serosa is the
    protective outermost
    layer of inter-
    peritoneal organ
   This visceral
    peritoneum is
    formed of areolar
    connective tissue
    covered with meso-
    thelium, a single
    layer of squamous
    epithelial cells
             Histology: Serosa
   In the esophagus, which is located in the
    thoracic instead of the abdominopelvic
    cavity, the serosa is replaced by an
    adventitia
   The adventitia is an ordinary fibrous
    connective tissue that binds the esophagus to
    surrounding structures
   Retroperitoneal organs have both a serosa
    (on the side facing the peritoneal cavity) and
    an adventitia (on the side abutting the dorsal
    body wall)
            Enteric Nervous System
   The alimentary
    canal has its own in-            Intrinsic
                                      Nerve
    house nerve supply                Plexes
   Enteric neurons
    communicate widely
    with each other to
    regulate digestive
    system activity
            Enteric Nervous System
   These enteric
    neurons constitute               Myenteric
                                      plexus
    the bulk of the two
                                     Submucosal
    major intrinsic                    plexus
    nerve plexuses
    found within the
    walls of the
    alimentary canal
    – Submucosal nerve
      plexus
    – Myenteric nerve
      plexus
            Enteric Nervous System
   A smaller third
    plexus is found                  Subserosa
                                       nerve
    within the serosa                 plexus
    layer
    – Subsersora nerve
      plexus
            Enteric Nervous System
   The submucosal
    nerve plexus chiefly             Myenteric
                                      plexus
    regulates the
                                     Submucosal
    activity of glands                 plexus
    and smooth muscle
    in the mucosa tunic
   The myenteric
    nerve plexus lies
    between the circular
    and longitudinal
    layers of smooth
    muscle of the
    muscularis externa
       Enteric Nervous System
   Via their communication with each other,
    with smooth muscle layers, and with
    submucosal plexus, the enteric neurons of
    the myenteric plexus provide the major
    nerve supply to the GI tract
   This plexus controls GI tract mobility by
    controlling the patterns of segmentation
    and peristalsis
   Control comes from local reflex arcs
    between enteric neurons in the same or
    different plexus or organs
       Enteric Nervous System
   The enteric nervous system is also linked
    to the CNS by afferent visceral fibers and
    sympathetic and parasympathetic
    branches of the ANS
   Digestive activity is subject to extrinsic
    control exerted by ANS which can speed
    up or slow secretory activity and mobility
Digestive System
Mouth, Pharynx, and Esophagus
   The mouth is the only part of the digestive
    system that is involved in the ingestion of
    food
   Most digestive function of the mouth
    reflect the activity of accessory organs
    chewing the food and mixing it with salvia
    to begin the process of chemical digestion
   The mouth also begin the propulsive
    process by which food is carried through
    the pharynx and esophagus to the
    stomach
                      The Mouth
   The oral cavity is a
    lined with mucosa
   It bounded by the
    lips anteriorly, and
    the tongue inferiorly
    and the cheeks
    laterally
   Its anterior opening
    is the oral orifice
   Posteriorly the oral
    cavity is continuous
    with the oropharynx
                     The Mouth
   The walls of the
    mouth are lined with
    stratified squamous
    epithelium
   The epithelium is
    highly ketatinized
    for extra protection
    against abrasion
    during eating
   The mucosa also
    produces defensins
    to fight microbes in
    the mouth
             The Lips and Cheeks
   The labia and the
    cheeks have a core
    of skeletal muscle
    covered by skin
   The orbicularis oris
    muscle forms the
    bulk of the lips
   The cheeks are
    formed largely by
    the buccinators
   The area between
    the teeth and gums
    is the vestibule
             The Lips and Cheeks
   The lips extend
    from the inferior
    margin of the nose
    to the superior
    boundary of the
    chin
   The reddened area
    is called red margin
   The labial frenulum
    is a median fold
    that joins the
    internal aspect of
    each lip to the gum
                    The Palate
   The palate which
    forms the roof of
    the mouth has two
    distinct parts
    – Hard palate
    – Soft palate
                       The Palate
   The hard palate is
    underlain by bone
    and is a rigid surface
    against which the
    tongue forces food
    during chewing
   There exists a
    centerline ridge
    called a raphe
   The mucosa is
    corrugated for
    friction
                       The Palate
   The soft palate is a
    mobile fold formed
    by skeletal muscle
   Projecting down
    from its free edge is
    the uvula
   The soft palate rises
    reflexively to close
    off the nasopharynx
    when swallowing
                      The Palate
   The soft palate is
    anchored to the
    tongue by the
    palantoglossal
    arches and to the
    wall of oropharynx
    by the
    palantopharyngeal
    arches
   These arches form
    the boundary of the
    facuces
               The Tongue
   The tongue occupies the floor of the
    mouth and fills most of the oral cavity
    when closed
   The tongue is composed of interlacing
    masses of skeletal muscle fibers
   The tongue grips the food and constantly
    repositions it between the teeth
   The tongue also mixes the food with
    salvia and form it into a mass called a
    bolus and then initiates swallowing by
    moving the mass into the pharynx
                      The Tongue
   The tongue has both
    intrinsic and extrinsic
    skeletal muscles
   The intrinsic muscles
    are confined within
    the tongue and are
    not attached bone
   The fibers allow the
    tongue to change its
    shape for speech and
    swallowing but not its
    position
                     The Tongue
   The extrinsic muscles
    extend the tongue
    from their points of
    origin
   The extrinsic muscles
    allow the tongue to be
    protruded, retracted
    and moved side to
    side
   The tongue is divided
    by a median septum
    of connective tissue
                      The Tongue
   A fold of mucosa
    called the lingual
    frenulum secures the
    tongue to the floor of
    the mouth
   This frenulum limits
    the posterior move-
    ment of the tongue
   You cannot swallow
    your tongue
                     The Tongue
   The conical filaform
    papillae give the
    tongue surface a
    roughness that aids
    in manipulating
    foods in the mouth
   They align in
    parallel rows on the
    dorsum
   They contain keratin
    which stiffens them
   House taste buds
                      The Tongue
   The mushroom
    shaped fungiform
    palillae are scattered
    over the surface
   Each has a vascular
    core that gives it a
    reddish hue
   Houses taste buds
                     The Tongue
   The circumvallate
    are located in a V-
    shaped row at the
    back of the tongue
   Appear similar to
    the fungiform
    papillae but with an
    additional
    surrounding furrow
         The Salivary Glands
   A number of glands both inside and
    outside the oral cavity produce and
    secrete saliva
   Saliva functions to
    – Cleanses the mouth
    – Dissolves food chemical so that they can be
      tasted
    – Moistens food and aids in compacting it into
      a bolus
    – Contains enzymes that begin the chemical
      breakdown of starches
               The Salivary Glands
   Most saliva is
    produced by three
    pairs of extrinsic
    salivary glands
    – Parotid
    – Submandibular
    – Sublingual
   These glands lie
    outside the oral
    cavity and empty
    their secretions into
    it
              The Salivary Glands
   The intrinsic
    salivary glands are
    small and are
    scattered throughout
    the oral cavity
         The Salivary Glands
   The salivary glands are composed of two
    types of secretory cells; mucus and serous
   The serous cells produce a watery
    secretion containing enzymes and the
    ions of saliva
   The mucus cells produce mucus a stringy
    viscous solution
                The Teeth
   The teeth lie in sockets in the gum
    covered margins of the mandible and
    maxilla
   Teeth function to tear and grind food and
    begin the mechanical process of digestion
                 Dentition
   Ordinarily we have two sets of teeth the
    primary and permanent dentitions
   The primary dentition consists of
    deciduous teeth
   The first teeth appear at six months and
    additional teeth continue to erupt until
    about 24 months when all 20 teeth have
    emerged
                 Dentition
   As the deeper permanent teeth enlarge
    and develop, the root of the milk teeth
    are resorbed from below causing them to
    loosen and fall out between the ages of 6
    and 12 years
   Generally, all the teeth of the permanent
    dentition have erupted by adolescence
                             The Teeth
   Teeth are classified
    according to their
    shape and function
    –   Incisors / cutting
    –   Canines / tear
    –   Premolars / grind
    –   Molars / crush
   There are 20 milk
    teeth and 32
    permanent teeth
                   Tooth Structure
   Each tooth has two
    major regions; the
    crown and the root
   The crown represents
    the visible portion of the
    tooth exposed above the
    gum
   The root is the portion
    of the tooth that is
    imbedded in the
    jawbone
                   The Pharynx
   From the mouth, the
    food passes
    posteriorly into the
    oropharnyx
   The mucosa consists
    of stratified
    squamous epithelium
   The epithelium is
    supplied with mucus
    producing glands for
    lubrication
                     The Pharynx
   The external muscle
    layer consists of two
    skeletal muscle layers
   The cells of the inner
    layer run
    longitudinally
   The outer layer of
    muscles pharyngeal
    constrictor muscles,
    encircle the wall
   Sequential
    contractions propel
    food into esophagus
                 The Esophagus




   The esophagus takes a fairly straight course through
    the mediastinum of the thorax, pierces the diaphragm
    at the esophageal hiatus to enter the abdomen
                  The Esophagus
   The esophagus joins
    the stomach at the
    cardiac orifice
   The cardica orifice is
    surrounded by the
    cardiac esophogeal
    sphincter
               The Pharynx
   The esophageal mucosa contains a non-
    ketatinized stratified squamous epithelium
    which changes abruptly simple columnar
    epithelium upon reaching the stomach
   When empty the esophagus is empty with its
    mucosa drawn into folds which flatten out
    when food is in passage
   The mucosa contains mucus secreting
    esophageal glands which are compressed by
    a passing bolus of food resulting in the
    glands secreting a lubricant
               The Pharynx
   The muscularis externa changes from
    skeletal muscle to a mix of skeletal and
    smooth to finally all smooth as it
    approaches the stomach
   Instead of a serosa, the esophagus has a
    fibrous adventitia composed entirely of
    connective tissue, which blends with
    surrounding structures along its route
             Digestive Processes
   The mouth and its accessory digestive
    organs are involved in most digestive
    processes
    –   The mouth ingests food
    –   Begins mechanical digestion by chewing
    –   Initiates propulsion by swallowing
    –   Starts the process of chemical digestion
            – The pharynx and the esophagus serve as conduits to
              pass food from the mouth to the stomach
Digestive Processes: Mastication
   Mastication is the mechanical process of
    breaking down food
   The cheeks and closed lips hold the food
    between the teeth
   The tongue mixes the food with saliva to
    soften it
   The teeth cut and grind food into smaller
    pieces
     Digestive Processes: Deglutition
   In deglutition, food is
    first compacted by the
    tongue into a bolus and
    swallowed
   Swallowing is a process
    that requires the
    coordination of tongue
    soft palate, pharynx,
    esophagus and 22
    separate muscles
     Digestive Processes: Deglutition
   In deglutition, food is
    first compacted by the
    tongue into a bolus and
    swallowed
   Swallowing is a process
    that requires the
    coordination of tongue
    soft palate, pharynx,
    esophagus and 22
    separate muscles
     Digestive Processes: Deglutition
   Food passage into
    respiratory
    passageways by rising
    of the uvula and larynx
   Relaxation of the upper
    esophageal sphincter
    allows food entry into
    the esophagus
     Digestive Processes: Deglutition
   The constrictor muscles
    of the pharynx
    contract, forcing food
    into the esophagus
    inferiorly
   The upper esophageal
    sphincter contracts
    after entry
     Digestive Processes: Deglutition
   Food is conducted
    along the length of the
    esophagus to the
    stomach by peristaltic
    waves
              Digestive Processes
   The gastroesophageal
    sphincter enters opens
    and food enters the
    stomach
               The Stomach
   The stomach functions as a temporary
    storage tank where the chemical
    breakdown of protein begins and food is
    converted to a creamy paste called chyme
   The stomach lies in the upper left
    quadrant of the abdominal cavity
   Though relatively fixed at both ends, it is
    free to move in between
      The Stomach: Gross Anatomy
   The stomach varies
    from 6 to 10 inches in
    length, but its
    diameter and volume
    depend on how much
    food it contains
   Empty it may
    contain on 50 ml but
    can expand to hold
    about 4 liters of food
      The Stomach: Gross Anatomy




   When empty, the stomach collapses inward, throwing
    its mucosa into large, longitudinal folds called rugae
      The Stomach: Gross Anatomy




   The major region of the stomach are the cardia region,
    the fundus, body, pyloric region, and the greater and
    lesser curvatures
      The Stomach: Gross Anatomy




   The lesser omentum runs from the liver to the lesser
    curvature where it becomes continuous with the
    visceral peritoneum of the stomach
      The Stomach: Gross Anatomy




   The greater omentum drapes inferior from the greater
    curvature of the stomach to cover the coils of the small
    intestine
Stomach: Microscopic Anatomy
   The stomach wall exhibits the four tunics
    of most of the alimentary canal but its
    muscularis and mucosa are modified for
    the special roles of stomach
   The muscularis externa has an extra
    oblique layer of muscle that enables it to
    mix, churn and pummel food
   The epithelium lining the stomach
    mucosa is simple columnar epithelium
    composed entirely of goblet cells, which
    produce a protective coating of mucus
               Microscopic Anatomy
   The four tunics
    typical of the
    alimentary canal
    –   Mucosa
    –   Submucosa
    –   Muscularis Externia
    –   Serosa
             Microscopic Anatomy
   The otherwise smooth
    lining is dotted with
    millions of gastric pits
    which lead to gastric
    glands that produce
    gastric juice
   The glands of the
    stomach body are
    substantially larger
    and produce the
    majority of the
    stomach secretions
             Microscopic Anatomy
   Mucus neck cells
    produce a different
    type of mucus from
    that secreted by the
    mucus secreting cells
    of the surface
    epithelium
   The special function of
    this unique mucus is
    not yet understood
              Microscopic Anatomy
   Parietal cells scattered
    among the chief cells
    secrete hydrochloric
    acid (HCl) and intrinsic
    factor
   The parietal cells have
    a large surface area
    adapted for secreting
    HCl in the stomach
   Intrinsic factor is
    required for absorption
    of B12 in the small
    intestine
             Microscopic Anatomy
   Chief cells produce
    pepsinogen, the inactive
    form of the protein-
    digesting enzyme
    pepsin
   The cells occur mainly
    in the basal regions of
    the gastric glands
   Pepsinogen is activated
    by HCl
              Microscopic Anatomy
   Parietal cells scattered
    among the chief cells
    secrete hydrochloric
    acid (HCL) and
    intrinsic factor
   The parietal cells have
    a large surface area
    adapted for secreting
    HCL in the stomach
   Intrinsic factor is
    required for absorption
    of B12 in the small
    intestine
              Microscopic Anatomy
   Enteroendocrine
    release a variety of
    hormones directly into
    the lamina propria
   These products diffuse
    into capillaries and
    ultimately influence
    several digestive system
    target organs which
    regulate stomach
    secretion and mobility
                 Mucosal Barrier
   Gastric juice is 100,000 more concentrated
    than that found in the blood
   Under such harsh conditions the stomach
    must protect itself from self digestion with a
    mucosal barrier
    –   Bicarbonate rich mucus is on the stomach wall
    –   Epithelial cells are joined by tight junctions
    –   Glandular cells are impermeable to HCl
    –   Surface epithelium is replace every 3 to 6 days
    Digestive Processes: Stomach
   The stomach is involved in the whole
    range of digestive activities
    – It serves as a holding area for ingested food
    – Breaks down food further chemically and
      mechanically
    – It delivers chyme to the small intestine at a
      controlled rate
    Digestive Processes: Stomach
   Protein digestion is initiated in the
    stomach and is essentially the only type of
    enyzmatic digestion that occurs there
   The most important protein digesting
    enzyme produced by the gastric mucosa
    is pepsin
   In children, the stomach glands also
    secrete rennin, an enzyme that acts on
    milk protein converting it to a curdy
    substance appearing like sour milk
    Digestive Processes: Stomach
   Despite its many functions in the
    digestive system the only one that is
    essential for life is secretion of intrinsic
    factor
   Intrinsic factor is required for intestinal
    absorption of vitamin B12, needed to
    produce mature erythrocytes
   Without B12 the individual will develop
    prenicious anemia unless administered
    by injection
Regulation of Gastric Secretion
   Gastric secretion is controlled by both
    neural and hormonal mechanisms
   Under normal conditions the gastric
    mucosa creates as much as 3 liters of
    gastric juice every day
   Gastric juice is an acid solution that has
    the potential to dissolve nails
Regulation of Gastric Secretion
   Nervous control is regulated by long
    (vagus nerve mediated) and short (local
    enteric) nerve reflexes
   When the vagus nerves actively stimulate
    the stomach, secretory activity of
    virtually all of its glands increase
   The sympathetic nerves depress secretory
    activity
Regulation of Gastric Secretion
   Hormonal control of gastric secretion is
    largely from the presence of gastrin
   Gastrin stimulates the secretion of both
    enzymes and HCL in the stomach
   Hormones produced by the small
    intestine are largely gastrin antagonists
Regulation of Gastric Secretion
   Stimuli acting at three distinct sites, the
    head, stomach, and small intestine,
    provoke or inhibit gastric secretory
    activity
   Accordingly the three phases are called
    cephalic, gastric, and intestinal phases
   However, the effector site is the stomach
    in all cases and once initiated, one or all
    threephases may be occurring at the
    same time
       Phase 1: Cephalic reflex
   The cephalic reflex phase of gastric
    secretion occurs before food enters the
    stomach
   It is triggered by the aroma, taste, sight,
    or though of food
   During this phase the brain gets the
    stomach ready for food
       Phase 1: Cephalic reflex
   Inputs from activated olfactory receptors
    and taste buds are relayed to the
    hypothalamus which in turn stimulates
    the vagal nuclei of the medulla oblongata,
    causing motor impulses to be transmitted
    via the vagus nerves to the
    parasympathetic nerve ganglia
   Eneteric ganglionic neurons in turn
    stimulate the stomach glands
       Phase 1: Cephalic reflex
   The enhanced secretory activity that
    results when we see or think of food is a
    conditioned reflex and occurs only when
    we like or want the food
   If we are depressed or have no appetite,
    this part of the cephalic reflex is
    suppressed
        Phase 2: Gastric reflex
   Once food reaches the stomach, local
    neural and hormonal mechanisms initiate
    the gastric phase
   This phase provides about two-thirds of
    the gastric juice released
   The most important stimuli are distension,
    peptids, and low acidity
        Phase 2: Gastric reflex
   Stomach distension activates stretch
    receptors and initiates both local
    (myentertic) reflexes and the long
    vagovagal reflexes
   In vagovagal reflex, impulses travel to the
    medulla and then back to the stomach via
    vagal fibers
   Both types of reflexes lead to acetylcholine
    (ACH) release, which in turn stimulates the
    output of more gastric juice by cells
        Phase 2: Gastric reflex
   Though neural influences initiated by
    stomach distension are important, the
    hormone gastrin probably plays a greater
    role in stimulating stomach gland
    secretion during the gastric phase
   Chemical stimuli provided by partially
    digested proteins (peptids)caffine (colas,
    coffee) and rising pH directly active
    gastrin secreting entoendocrine cells
    called G cells
        Phase 2: Gastric reflex
   Although gastrin also stimulates the
    release of enzymes, its main target is the
    HCL secreting parietal cells, which it
    prods to spew out even more HCL
   Highly acidic (pH below 2) gastric
    contents inhibit gastrin secretion
        Phase 2: Gastric reflex
   When protein foods are in the stomach,
    the pH of the gastric contents generally
    rises because proteins act as buffers to tie
    up H+
   The rise in pH stimulates gastrin and
    subsequently HCL release, which in turn
    provides the acidic conditions needed for
    protein digestion
        Phase 2: Gastric reflex
   The more protein in the meal, the greater
    the amount of gastrin and HCL released
   As proteins are digested, the gastric
    contents gradually become more acidic,
    which again inhibits the gastrin secreting
    cells
   This negative feedback mechanism helps
    maintain optimal pH and working
    conditions for the gastric enzymes
        Phase 2: Gastric reflex
   G cells are also activated by the neural
    reflexes already described
   Emotional upsets, fear, anxiety, or
    anything that triggers the fight-or-flight
    response inhibits gastric secretion
    because (during such times) the
    sympathetic division overrides
    parasympathetic controls of digestion
        Phase 2: Gastric reflex
   The control of the HCL secreting parietal
    cells is unique and multifaceted
   Basically, HCL secretion is stimulated by
    three chemicals, all of which work
    through second-messenger systems Ach
    released by parasympathetic nerve fibers
    and gastrin secreted by G cells
         Phase 2: Gastric reflex
   Ach released by
    para-
    sympathetic
    nerve fibers and
    gastrin secreted
    by G cells bring
    about their
    effects by
    increasing
    intercellular
    Ca++ levels
         Phase 2: Gastric reflex
   Histamine
    released by
    mucosal cells
    called
    histaminocytes
    acts through
    cyclic AMP
    (cAMP)
        Phase 2: Gastric reflex
   When only one of the three chemicals
    binds to the parietal cells, HCL secretions
    are minimul
   When all three of the chemicals bind to
    the parietal cells volumes of HCL pour
    forth as if pushed out under pressure
        Phase 2: Gastric reflex
   The process of HCL formation within the
    parietal cells is complicated and poorly
    understood
   The consensus is that H+ is actively
    pumped into the stomach lumen against a
    tremendous concentration gradient
        Phase 2: Gastric reflex
   As hydrogen ions are secreted, chloride
    ions (Cl-) are also pumped into the lumen
    to maintain an electrical balance in the
    stomach
   The Cl- is obtained from blood plasma,
    while the H+ appears to come from a
    breakdown of carbonic acid formed by
    the combination of carbon dioxide and
    water and within the parietal cells
         Phase 2: Gastric reflex
   CO2 + H2O 
    H2CO3  H+ +
    HCO3-
   As H+ is
    pumped from
    the cell and
    HCO3- is ejected
    through the
    basal cell
    membrane into
    the capillary
    blood
        Phase 2: Gastric reflex
   The result of ejection of the bicarbonate
    ion into the capillary blood is that blood
    draining from the stomach is more
    alkaline than the blood serving it
   The phenomenon is called the alkaline
    tide
           Phase 3: Intestinal
   The intestinal phase of gastric secretion
    has two components
    – One excitatory
    – One inhibitory
           Phase 3: Intestinal
   The excitatory aspect is set into motion as
    partially digested food begins to fill the
    initial part (duodenum) of the small
    intestine
   This stimulates intestinal mucosal cells to
    release a hormone that encourages the
    gastric glands to continue their secretory
    activity
           Phase 3: Intestinal
   The effects of this hormone imitate those
    of gastrin, so it has been named intestinal
    (enteric) gastrin
   However, intestinal mechanisms stimulate
    gastrin secretion only briefly
   As the intestine distends with chyme
    containing large amounts of H+, fats,
    partially digested proteins, and irritating
    substances, the inhibitatory component is
    triggered in the form of the enterogastric
    reflex
           Phase 3: Intestinal
   The enterogastric reflex is actually a trio
    of reflexes that
    – Inhibit the vagal nuclei in the medulla
    – Inhibit local reflexes
    – Activate sympathetic fibers that cause the
      pyloric sphincter to tighten and prevent
      further food entry into the small intestine
   As a result, gastric secretory activity
    declines
           Phase 3: Intestinal
   These inhibitions on gastric activity
    product the small intestine to harm due
    to excessive acidity and match the small
    intestine’s processing abilities to the
    amount of chyme entering it at a given
    time
             Phase 3: Intestinal
   In addition, the factors just named trigger
    the release of several intestinal hormones
    collectively called enterogastrones which
    include
    –   Secretin
    –   Cholecystokinin (CCK)
    –   Vasoactive intestinal peptide (VIP)
    –   Gastric inhibitory peptide (GIP)
   All of these hormones inhibit gastric
    secretion when the stomach is very active
Gastric Motility and Emptying
   Stomach contractions, accomplished by the
    tri-layered muscularis, not only cause its
    emptying but also compress, knead, twist,
    and continually mix the food with gastric
    juice to produce chyme
   Because the mixing movements are
    accomplished by a unique type of peristalis
    (bidirectional) the process of mechanical
    digestion and propulsion are inseparable in
    the stomach
Gastric Motility: Stomach Filling
   Although the stomach stretches to
    accommodate incoming food, internal
    stomach pressure remains constant until
    about 1 liter of food has been ingested
   The relatively unchanging pressure in the
    filling stomach is due to 1) reflex mediated
    relaxation of the stomach muscle and 2)
    plasticity of visceral smooth muscle
Gastric Motility: Stomach Filling
   Reflexive relaxation of stomach muscle in
    the fundus and body occurs both in
    anticipation of and in response to food
    entry into the stomach
   As food travels through the esophagus,
    the stomach muscles relax
   This receptive relaxation is coordinated
    by the swallowing center in the brain
    stem and mediated by the vagus nerves
Gastric Motility:Stomach Filling
   The stomach also actively dilates in
    response to gastric filling, which activates
    stretch receptors in the wall
   The phenomenon called adaptive
    relaxation appears to depend on local
    reflexes involving nitric oxide (NO)
    releasing hormones
Gastric Motility: Stomach Filling
    Plasticity is the intrinsic ability of visceral
     smooth muscle to exhibit the stress-
     relaxation response, that is, to be
     stretched without greatly increasing its
     tension and contractile strength
Gastric Motility and Emptying




   After a meal peristalsis begins near the
    cardiac sphincter, where it produces only
    gentle rippling movements of the stomach
    wall
Gastric Motility and Emptying




   As contractions approach the pylorus,
    where the stomach musculature is
    thicker, the contractions become more
    powerful
Gastric Motility and Emptying




   Consequently, the contents of the fundus
    remain relatively undisturbed, while the
    foodstuffs close to the pylorus receive a
    very active mixing
Gastric Motility and Emptying




   The pyloric region of the stomach, which
    holds about 30 ml of chyme, acts as a
    “dynamic filter” that allows only liquids
    and small particles of food to pass
Gastric Motility and Emptying




   Normally, each peristaltic wave reaching
    the pyloric muscle squirts 3 ml or less of
    chyme into the small intestine
Gastric Motility and Emptying




   While the stomach delivers small amounts
    of chyme into the doudenum it also
    simultaneously forces most of the
    contained material backward into the
    stomach for further mixing
    Gastric Motility and Emptying
   Although the intensity of the stomach’s
    peristaltic waves can be modified, the rate
    is always constant at around 3 per minute
   The contractile rhythm is set by the
    spontaneous activity of pacemaker cells
    located at the margins of the longitudinal
    smooth muscle layer
Gastric Motility and Emptying
   The pacemaker cells, are believed to be
    muscle-like noncontractile cells called
    interstitial cells of Cajal which depolarize
    the repolarize spontaneously three times
    each minute
   This depolarization and repolarization
    establish the so-called cyclic slow waves
    of the stomach or its basic electrical
    rhythm (BER)
    Gastric Motility and Emptying
   Since the pacemakers are electrically
    coupled to the rest of the smooth muscle
    sheet by gap junctions, their “beat” is
    transmitted efficiently and quickly to the
    entire muscularis
   The pacemakers set the maximum rate of
    contraction, but they do not initiate the
    contractions or regulate their force
   They generate subthreshold depolarization
    waves, which are then enhance by neural
    and hormonal factors
Gastric Motility and Emptying
   Factors that increase the strength of
    stomach contractions are the same
    factors that enhance gastric secetory
    activity
   Distension of the stomach wall by food
    activates stretch receptors and gastric
    secreting cells, which both ultimately
    gastric smooth muscle and so increase
    gastric motility
Gastric Motility and Emptying
   Thus, the more food there is in the
    stomach, the more vigorous the stomach
    mixing and emptying movements will be
    evident
   The stomach usually empties completely
    within four hours after a meal
   However, the larger the meal (greater
    distension) and the more liquid the meal,
    the faster the stomach empties
Gastric Motility and Emptying
   Fluids pass quickly through the stomach
   Solids linger, remaining until they are
    well mixed with gastric juice and
    converted to a liquid state
     Gastric Motility and Emptying
   The rate of emptying depends as much on the
    contents of the duodenum as on whats
    happening in the stomach
   The stomach and duodenum act in tandem
   As chyme enters the duodenum, receptors in
    its wall respond to chemical signals and to
    stretch, initiating the enterogastric reflex and
    hormonal mechanisms described earlier
   These factors inhibit gastric secretory activity
    and prevent further duodenal filling by
    reducing the force of pyloric contractions
Gastric Motility and Emptying
   A carbohydrate-rich meal moves through
    the duodenum rapidly, but fats form an
    oily layer at the top of the chyme and are
    digested more slowly by enzymes acting
    in the intestines
   Thus, when chyme entering the
    duodenum is fatty, food may remain in
    the stomach six hours or more
       The Small Intestine and
        Associated Structures
   In the small intestine, usable food is
    finally prepared for its journey into the
    cells of the body
   However, this vital function cannot be
    accomplished without the aid of
    secretions from the liver (bile) and
    pancreas (digestive enzymes)
   Thus the accessory organ are discussed in
    this section
                 Small Intestine
   The small
    intestine is a
    convoluted tube
    extending from
    the pyloric
    sphincter in the
    epigastric
    region to the
    iliocecal valve
    where it joins
    the large
    intestine
               Small Intestine
   It is the longest part of the alimentary
    tube, but its diameter is only about 2.5 cm
   In the cadaver, the small intestine is 6 - 7
    meters long because of loss of muscle tone,
    while it is only 2 - 4 meters long in the
    living individual
   The small intestine has three subdivisions
    – Duodenum
    – Jejunum
    – Ileum
             Gross Anatomy




   The relatively immovable duodenum which
    curves about the head of the pancreas
               Small Intestine
   The duodenum is about 10 inches long
   Although it is the shortest subdivision,
    the duodenum has the most features of
    interest
    –   The bile duct
    –   Main pancreatic duct
    –   Hepatopancreatic ampulla
    –   Major duodenal papilla
                  Gross Anatomy




   The bile duct, delivering bile from the liver
   The main pancreatic duct, carries pancreatic juice from
    the pancreas
                  Gross Anatomy




   The hepatopancreatic ampulla is where these two ducts
    unite in the wall of the duodenum
   The papilla is where this sphincter enters the duodenum
                 Small Intestine
   The jejunum is
    about 8 ft long
    and extends
    from the
    duodenum to
    the ileum
   This central
    section twists
    back and forth
    within the
    abdominal
    cavity
                  Small Intestine
   The ileum is
    approximately
    12 ft. in length
   It joins the
    large intestine
    at the ileocecal
    valve
                     Small Intestine
   The jejunum
    and ileum
    hang in coils
    in the central
    and lower
    part of the
    abdominal
    cavity
                Small Intestine
   The jejunum
    and ileum are
    suspended
    from the
    posterior
    abdominal
    wall by the
    fan shaped
    mesentery
             Small Intestine
   Nerve fibers serving the small intestine
    include the parasympathetics from the
    vagus nerves and sympathetics from the
    long splanchic nerves
   These are relayed through the superior
    mesenteric and celiac plexus
             Small Intestine
   The arterial supply is primarily from the
    superior and mesenteric artery
   The veins run parallel to the arteries and
    typically drain into the superior
    mesenteric vein
   From the mesenteric vein, the nutrient
    rich venous blood from the small
    intestine drains into the hepatic portal
    vein which carries it to the liver
        Microscopic Anatomy
   The small intestine is highly adapted for
    nutrient absorption
   Its length provides a huge surface area
    for absorption
   There are three structural modifications
    which increase the surface area for
    absorption
    – Plicae circulares
    – Villi
    – Microvilli
Microscopic Anatomy
          Digestive System Organs




   In this view you can see the plicae circulares
    and the villi of the small intestine
         Microscopic Anatomy
   Structural modifications increase the
    intestinal surface area tremendously
   It is estimated that the surface area of the
    small intestine is equal to 200 square
    meters or roughly equivalent to the floor
    space of a two story house
   Most absorption occurs in the proximal
    part of the small intestine, with these
    structural modifications decreasing
    toward the distal end
           Microscopic Anatomy
   The circular
    folds or plicae
    circularis are
    deep permanent
    folds of the
    mucosa and
    submucosa
   These folds are
    nearly 1 cm tall
           Microscopic Anatomy
   The folds force
    chyme to spiral
    through the
    lumen, slowing
    its movement
    and allowing
    time for full
    nutrient
    absorption
           Microscopic Anatomy
   Villi are finger
    like projections
    of the mucosa
   Over 1 mm tall
    they give a
    velvety texture
    to the mucosa
            Microscopic Anatomy
   The epithelial
    cells of the villi
    are chiefly
    absorptive
    columnar cells
    called
    enterocytes
           Microscopic Anatomy
   In each villus is a capillary     Enterocyte
    bed and a wide lymphatic
    capillary called a lacteal
   Digested food is absorbed
    through the epithelial cells
    into both the capillary blood
    and the lacteal
   Villi become gradually
    narrower and shorter along
    the length of the sm. intestine
           Microscopic Anatomy
   Microvilli are tiny
    projections of the
    plasma membrane of
    the absorptive cells of
    the mucosa
   It gives the mucosal
    surface a fuzzy
    appearance
    sometimes called a
    brush border
        Microscopic Anatomy
   Beside increasing the absorptive surface,
    the plasma membrane of the microvilli
    bear enzymes referred to as the brush
    border enzymes
   These enzymes complete the final stages
    of digestion of carbohydrates and
    proteins in the small intestine
             Histology of the Wall
   The four tunics of
    the digestive tract
    are modified in the
    small intestine by
    variations in
    mucosa and sub-
    mucosa
         Histology of the Wall
   The epithelium of the mucosa is largely
    simple columnar epithelium serving as
    absorptive cells
   The cells are bound by tight junctions
    and richly endowed with microvilli
   Also present are many mucus-secreting
    goblet cells
         Histology of the Wall
   Scattered among the epithelial cells of the
    wall are T cells called intraepithelial
    lymphocytes
   These T cells provide an immunological
    component
   Finally, there scattered enteroendocrine
    cells which are the source of secretin and
    cholecystokinin
           Histology of the Wall
   Between villi the
    mucosa is
    studded with pits
    that lead into
    tubular intestinal
    glands called
    intestinal crypts
    or crypts of
    Lieberkuhn
          Histology of the Wall
   The epithelial cells that line these crypts
    secrete intestinal juice
   Intestinal juice is a watery mixture
    containing mucus that serves as a carrier
    fluid for absorption of nutrients from chyme
         Histology of the Wall
   Located deep on the crypts are
    specialized secretory cells called Paneth
    cells
   Paneth cells fortify the small intestine by
    releasing lysozyme an antibacterial
    enzyme
   The number of crypts decreases along the
    length of the wall of the small intestine,
    but the number of goblet cells becomes
    more abundant
         Histology of the Wall
   The various epithelial cells arise from
    rapidly dividing stem cells at the base of
    the crypts
   The daughter cells gradually migrate up
    the villi where they are shed from the
    villis tips
   In this way the villus of the epithelium is
    renewed every three to six days
         Histology of the Wall
   The rapid replacement of the intestinal
    (and gastric) epithelial cells has clinical
    as well as physiological implications
   Treatments for cancer, such as radiation
    therapy and chemotherapy preferentially
    target the cells in the body that divide
    most quickly
   This kills cancer cells, but it also nearly
    obliterates the GI epithelium causing
    nausea, vomiting, and diarrhea after each
    treatment
         Histology of the Wall
   The submucosa is typical areolar
    connective tissue, and it contains both
    individual and aggregated lymphoid
    follicles (Peyer’s patches)
   Peyer’s patches increase in abundance
    toward the end of the small intestine,
    reflecting the fact that the large intestine
    contains huge numbers of bacteria that
    must be prevented from entering the
    bloodstream
          Histology of the Wall
   A set of
    elaborated
    mucus-secreting
    duodenal glands
    (Brunner’s) is
    found in the
    submucosa of the
    duodenum only
         Histology of the Wall
   These glands produce an alkaline
    (bicarbonate-rich) mucus that helps
    neutralize the acidic chyme moving in
    from the stomach
   When this protective mucus barrier is
    inadequate, the intestinal wall is eroded
    and duodenal ulcers results
         Histology of the Wall
   The muscularis is typical and bilayered
   Except for the bulk of the duodenum,
    which is retroperitoneal and has an
    adventitia, the external intestinal surface
    is covered by visceral peritoneum
    (serosa)
             Intestinal Juice
   The intestinal glands normally secrete
    between 1 and 2 liters of intestional juice
    daily
   The major stimulus for its production is
    distension or irritation of the intestinal
    mucosa by hypertonic or acidic chyme
              Intestinal Juice
   Normally, the pH range of intestinal juice
    is slightly alkaline (7.4-7.8), and it is
    isotonic with blood plasma
   Intestinal juice is largely water but it also
    contains some mucus, which is secreted
    both by the duodenal glands and by
    goblet cells of the mucosa
   Intestinal juice is relatively enzyme poor
    because intestinal enzymes are largely
    limited to the bound enzymes of the
    brush border
     The Liver and Gallbladder
   The liver and gallbladder are accessory
    organs associated with the small intestine
   The liver has many metabolic and
    regulatory roles
   Its digestive function is to produce bile
    for export to the duodenum
   Bile is a fat emulsifier which breaks up
    fat into tiny particles so that they are
    more accessible to digestive enzymes
   The gallbladder is a storage site for bile
                  The Liver




   The ruddy, blood rich liver is the largest
    gland in the body weighing about 1.4 kg in the
    average adult
                  The Liver
   Shaped like a wedge, it
    occupies most of the right
    hypochondriac and
    epigastric regions
    extending farther to the
    right of the body midline
    than the left
                 The Liver
   Located under the diaphragm, the liver
    lies almost entirely within the rib cage
   The location of the liver within the rib
    cage offers this organ some degree of
    protection
                   The Liver




   The liver has four primary lobes; right, left,
    caudate and quadrate
                   The Liver




   A mesentery, the falciform ligament,
    separates the right and left lobes anteriorly
    and suspends the liver from the diaphragm
                  The Liver




   Running along the free inferior edge of the
    falciform ligament is the ligamentum teres a
    remnant of the fetal umbilical vein
                   The Liver




   Except for the superiormost liver area, which
    is fused to the diaphragm, the entire liver is
    enclosed by a serosa (visceral peritoneum)
                  The Liver
   A dorsal
    mesentery, the
    lesser omentum,
    anchors the liver
    to the lesser
    curvature of the
    stomach
                  The Liver




   The hepatic artery and hepatic portal vein,
    enter the liver at the porta hepatis
                  The Liver




   The common bile duct, which runs inferiorly
    from the liver, travels through the lesser
    omentum
                    The Liver




   The gallbladder rests in a recess of the
    inferior surface of the right lobe of the liver
                   The Liver




   Bile leaves the liver through several bile ducts
    that ultimately fuse to form the large common
    hepatic duct which travels to the duodenum
                The Liver




   The common hepatic duct and the cystic
    duct fuse to form the bile duct
    Microscopic Anatomy of Liver
   The liver is
    composed of
    seed sized
    structural &
    functional
    units called
    liver lobules
   Each lobule
    is roughly
    hexagonal
     Microscopic Anatomy of Liver
   Hepatocytes
    or live cells
    are organized
    to radiate out
    from a central
    vein running
    the length of
    the
    longitudinal
    axis of the
    lobule
Microscopic Anatomy of Liver
   To make a rough “model” of a liver
    lobule, open a paperback book until the
    two covers meet
   The pages represent the plates of
    hepatocytes and the hollow cylinder
    formed by the rolled spine represents the
    central vein
Microscopic Anatomy of Liver
   The liver’s main function is to filter and
    process the nutrient rich blood delivered
    to it
   At each of the six corners of a lobule is a
    portal triad so named because three basic
    structures are always present there:
    – A branch of the hepatic artery
    – A branch of the hepatic portal vein
    – A bile duct
Microscopic Anatomy of Liver




   The hepatic artery supplies oxygen rich
    arterial blood to the liver
Microscopic Anatomy of Liver




   The hepatic vein carries blood laden with
    nutrients from the digestive viscera
Microscopic Anatomy of Liver




   A bile duct to carry secreted bile toward
    the common bile duct and ultimately to
    the duodenum
    Microscopic Anatomy of Liver
   Between the
    hepatocyte plates
    are enlarged,
    very leaky
    capillaries, the
    liver sinusoids
    Microscopic Anatomy of Liver
   Blood from both
    the hepatic portal
    vein and the
    hepatic artery
    percolates from
    the triad regions
    through these
    sinusoids and
    empties into the
    central vein
    Microscopic Anatomy of Liver
   From the central
    vein blood
    eventually enters
    the hepatic veins,
    which drain the
    liver, and empty
    into the inferior
    vena cava
    Microscopic Anatomy of Liver
   Inside the
    sinusoids are star
    shaped hepatic
    macrophages,
    also called
    Kupffer cells,
    which remove
    debris such as
    bacteria and
    worn-out blood
    cells
Microscopic Anatomy of Liver
   The hepatocytes (liver cells) are virtual
    organelle storehouses with large amounts
    of both rough and smooth endoplasmic
    reticulum, Golgi apparatuses, peroxisomes,
    and mitochondria
   Thus equipped, the hepatocytes produce
    not only bile but also
    – Process blood borne nutrients
    – Store Fat-soluble vitamins
    – Detoxify the blood
Microscopic Anatomy of Liver
   In processing nutrients the hepatocytes
    store glycogen and make plasma proteins
   Fat soluble vitamins are stored until such
    time as they are needed for metabolism
   Detoxification occurs are the hepatocytes
    rid the blood of ammonia by converting it
    to urea
   The net result is that the blood leaving the
    liver contains fewer nutrients and waste
    materials than the blood that entered
    Microscopic Anatomy of Liver
   Secreted bile
    flows through
    tiny canals, called
    bile canaliculi
    that run between
    adjacent hepato
    cytes toward the
    bile branch ducts
    in the portal triad
    Microscopic Anatomy of Liver
   Note that the bile
    and the blood
    flow in opposite
    directions in the
    liver lobule
   Bile entering the
    bile ducts
    eventually leaves
    the liver via the
    common hepatic
    duct
Microscopic Anatomy of Liver
   Bile is a yellow-green, alkaline solution
    containing
    –   Bile salts
    –   Bile pigments
    –   Cholesterol
    –   Neutral fats
    –   Phospholipids (lecithin and others)
    –   Electrolytes
   Only bile salts and phospholipids aid the
    digestive process
Microscopic Anatomy of Liver
   Bile salts, primarily cholic acid and
    chenodeoxycholic acids are cholesterol
    derivatives
   Their role is to emulsify fats which
    distributes them throughout the watery
    intestinal contents
   As a result, large fat globules entering the
    small intestine are physically separated
    into millions of small fatty droplets
Microscopic Anatomy of Liver
   Millions of tiny fat droplets vastly
    increase the surface area for the fat
    digesting enzymes to work on
   Bile salts also facilitate fat and
    cholesterol absorption and help solubilize
    cholesterol, both that contained in bile
    and that entering the small intestine for
    food
    Microscopic Anatomy of Liver
   Although many substances secreted in bile
    leave the body in feces, bile salts are not
    among them
   Bile salts are conserved by a means of a
    recycling mechanism called enterohepatic
    circulation
   In this process bile salts are
    – Reabsorbed into the small intestine
    – Returned to the liver via the hepatic portal vein
    – Resecreted in newly formed bile
Microscopic Anatomy of Liver
   The chief bile pigment is bilirubin, a
    waste product of hemoglobin (heme)
    during the breakdown of worn-out
    erythrocytes
   The globin and iron parts of hemoglobin
    are saved and recycled, but bilirubin is
    absorbed from the blood by the liver cells
    and actively excreted into the bile
Microscopic Anatomy of Liver
   Most of the bilirubin in bile is metabolized
    in the small intestine by resident bacteria
   A breakdown by-product is urobilirubin
    which give feces its brown color
   In the absence of bile, feces are grey-white
    in color and have fatty streaks because
    essentially no fats are digested or
    absorbed
Microscopic Anatomy of Liver
   The liver produces 500 to 1000 ml of bile
    daily, and bile production is stepped up
    when the GI tract contains fatty chyme
   Bile salts themselves are a major stimulus
    for enhance bile secretion
     Microscopic Anatomy of Liver
   The single most
    important
    stimulus of bile
    secretion is an
    increased level of
    bile salts in the
    enterohepatic
    circulation
              The Gallbladder
   The gallbladder is
    a thin-walled,
    green muscular
    sac, rouhgly the
    size of a kiwi fruit
   It snuggles in a
    shallow fossa on
    the ventral
    surface of the
    liver
            The Gallbladder




   The gallbladder stores bile that is not
    immediately needed for digestions
            The Gallbladder
   Bile that is not needed is concentrated by
    absorbing some of its water and ions
   When empty, its mucosa adopts the ridge
    like folds or rugae of the stomach
   Its muscular walls can contract to expell
    its contents into the cystic duct which
    then flows into the bile duct
   Like most of the liver it is covered by
    visceral peritoneum
              The Gallbladder




   When digestion is not occurring, the
    hepatopancreatic sphincter is tightly closed
              The Gallbladder




   Bile then backs up the cystic duct into the
    gallbladder where it is stored until needed
            The Gallbladder
   Although the liver makes bile continuously
    bile does not usually enter the small
    intestine until the gallbladder contract
   The major stimulus for gallbladder
    contraction is the intestinal hormone
    cholecystokinin (CCK)
   CCK is released to the blood when acidic,
    fatty chyme enters the duodenum
             The Gallbladder
   Besides causing the gallbladder to
    contract, CCk has two other important
    effects
    – It stimulates secreation of pancreatic juice
    – It relaxes the hepatppancreatic sphincter so
      that bile and pancreatic juice can enter the
      duodenum
   Parasympathetic impulses delivered by
    the vagus nerves have a minor impact on
    stimulating gallbladder contraction
                The Pancreas




   The pancreas is a soft, tadpole-shaped
    gland that extends across the abdomen
                The Pancreas




   Most the pancreas is retroperitoneal and lies
    deep to the greater curvature of stomach
              The Pancreas
   An accessory organ, the pancreas is
    important to the digestive process
    because it produces a broad spectrum of
    enzymes
   These enzymes break down all categories
    of foodstuffs, which the pancreas then
    delivers to the duodenum
   This exocrine product is called pancreatic
    juice
                The Pancreas




   Pancreatic juice drains from the pancreas via
    the centrally located main pancreatic duct
                The Pancreas




   The pancreatic duct generally fuses with the
    bile duct just as it enters the duodenum
               The Pancreas




   A smaller accessory pancreatic duct empties
    directly into the main duct
                 The Pancreas
   Within the
    pancreas are
    the acini,
    clusters of
    secretory cells
    surrounding
    ducts
                 The Pancreas
   The acini cells
    are full of
    rough
    endoplasmic
    reticulum and
    exhibit deeply
    staining
    zymogen
    granules
    containing
    digestive
    enzymes
                The Pancreas
   The pancreas
    also has an
    endocrine
    function
   Scattered
    amidst the
    acini are the
    more lightly
    staining
    pancreatic
    islets
                 The Pancreas
   These Islets of
    Langerhans
    release insulin
    and glucagon,
    hormones that
    regulate
    carbohydrate
    metabolism
            Pancreatic Juice
   Approximately 1200 to 1500 ml of clear
    pancreatic juice is produced daily
   It consists mainly of water and contains
    enzymes and electrolytes
   The acinar cells produce the enzyme rich
    pancreatic juice
   The epithelial cells lining the smallest
    pancreatic ducts release the bicarbonate
    ions that make it alkaline (pH 8)
              The Pancreas
   The high pH enables pancreatic fluid to
    neutralize the acid chyme entering the
    duodenum
   It also provides the optimal environment
    for activity of intestinal and pancreatic
    enzymes
   Like pepsin of the stomach, pancreatic
    protein digesting enzymes are produced
    and released in active forms, which are
    then activated in the duodenum
              The Pancreas
   Within the duodenum trypsinogen is
    activated to trypsin by enterokinase an
    intestinal brush border enzyme
   Trypsin in turn activates two other
    pancreatic enzymes
    – Procarboxypeptidase > carboxypeptidase
    – Chymotrypsinogen > chymotrypsin
   Other pancreatic enzymes (amylase, lipase,
    and nucleases) are secreted in active form
    but require ions in the bile for activity
Regulation of Pancreatic Secretion
    Secretion of pancreatic juice is regulated
     both by local hormones and by the
     parasympathetic nervous system
    Regulation of Pancreatic Secretion
   Secretin is
    released in
    response to the
    presence of HCL
    in the intestine
   Cholecystokinin
    is released in
    response to the
    entry of proteins
    and fats
Regulation of Pancreas Secretion
   Both hormones act on the pancreas, but
    secretin targets the duct cells, prompting
    their release of watery bicarbonate-rich
    pancreatic juice, Whereas CCK stimulates
    the acini to release enzyme-rich pancreatic
    juice
   Vagal stimulation causes release of
    pancreatic juice primarily during the
    cephalic and gastric phases of gastric
    secretion
Regulation of Pancreatic Secretion
    Normally, the amount of HCL produced
     in the stomach is exactly balanced by the
     amount of bicarbonate (HCO3) actively
     secreted by the pancreas
    HCO3 is secreted into the pancreatic
     juice, and H+ enters the blood
Regulation of Pancreatic Secretion
    Consequently, the pH of venous blood
     returning to the heart remains relatively
     unchanged because alkaline blood
     draining from the stomach is neutralized
     by the acidic blood draining the pancreas
      Digestion: Small Intestine
   Although food reaching the small intestine
    is unrecognizable, it is far from being
    digested chemically
   Carbohydrates and proteins are partially
    degraded, but virtually no fat digestion has
    occurred to this point
   The process of food digestion is accelerated
    during the chyme’s journey of 3 to 6 hours
    through the small intestine, it is here that
    virtually all nutrient absorption occurs
     Optimal Intestinal Activity
   Although the primary functions of the small
    intestine are digestion and absorption,
    intestinal juice provides little of what is
    needed to perform these functions
   Most substances required for chemical
    digestion - bile, digestive enzymes (except
    for brush border enzymes) and bicarbonate
    ions (to provide the proper pH for
    enzymatic catalysis) are imported from the
    liver and pancreas
     Optimal Intestinal Activity
   Anything that impairs liver or pancreatic
    function or delivery of their juices to the
    small intestine severely hinders the
    individual’s ability to digest food and
    absorb nutrients
     Optimal Intestinal Activity
   Optimal digestive activity in the small
    intestine also depends on a slow, measured
    delivery of chyme from the stomach
   The small intestine can process only small
    amounts of chyme at one time
   Chyme enter the small intestine is usually
    hypertonic
     Optimal Intestinal Activity
   If large amounts of chyme were rushed
    into the small intestine, the osmotic water
    loss from the blood into the intestinal
    lumen would result in dangerously low
    blood volume
   Additionally, the low pH of entering
    chyme must be adjusted upward and the
    chyme must be well mixed with bile and
    pancreatic juice for digestion to continue
   These adjustments take time
     Optimal Intestinal Activity




   Food movement into the small intestine is
    carefully controlled by the pumping
    action of the stomach pylorus which
    prevents the duodenum from being
    overwhelmed
    Motility of the Small Intestine
   Intestinal smooth muscle mixes chyme
    thoroughly with bile and pancreatic and
    intestinal juices and moves food residues
    through the ileocecal valve and into the
    large intestine
   In contrast to the peristaltic waves of the
    stomach, which both mix and propel
    food, segmentation is the most common
    motion of the small intestine
     Motility of the Small Intestine
   In segmentation,
    chyme is moved
    backward and
    forward a few
    centimeters at a
    time by alternating
    contraction and
    relaxation of rings
    of smooth muscles
      Motility of the Small Intestine
   These segmenting
    movements of the
    intestine are
    initiated by
    intrinsic
    pacemaker cells
    (interstitial cells of
    Cajal) in the
    longitudinal
    smooth muscle
    layer
    Motility of the Small Intestine
   Unlike the somach pacemakers, which have
    only one rhythm, the pacemakers in the
    duodenum depolarizes more frequently
    (12-14 contractions per minute) than those
    of the ileum (8-9 contractions per minute)
   As a result, segmentation moves intestinal
    contents slowly and steadily toward the
    ileocecal valve at a rate that allows ample
    time to complete digestion and absorption
    Motility of the Small Intestine
   The intensity of the segmentation is
    altered by hormones and long and short
    reflexes
     – Parasympathetic enhances segmentation
     – Sympathetic decreases segmentation
   The more intense the contractions, the
    greater the mixing effect, however the
    basic contractile rhythms of the various
    intestinal regions remain unchanged
     Motility of the Small Intestine
   True peristalsis
    occurs only
    after most
    nutrients have
    been absorbed
   Segmentation
    movements
    wane, and
    peristaltic
    waves begin
    Motility of the Small Intestine
   Peristaltic waves initiated in the duodenum
    begin to sweep slowly along the intestine,
    moving 10 - 70 cm before dying out
   Each successive wave is initiated a bit more
    distally, and this pattern of peristaltic
    activity is called the migrating mobility
    complex
   A complete migration from the duodenum
    to the ileum takes about two hours and
    then repeats itself
    Motility of the Small Intestine
   Peristalsis serves to sweep out the last
    remnants of the meal plus bacteria,
    sloughed-off mucosal cells, and other
    debris into the large intestine
   This “housekeeping” function is critical
    for preventing the overgrowth of bacteria
    that migrate from the large intestine to
    the small intestine
   As food enters the stomach with the next
    meal segmentation replaces peristalsis
    Motility of the Small Intestine
   The local enteric neurons of the GI tract
    wall coordinate intestinal mobility patterns
   The physiological diversity of the enteric
    neurons allows a variety of effects to occur
    depending on which neurons are activated
    or inhibited
    Motility of the Small Intestine
   A given ACh-releasing (cholinergic)
    sensory neuron in the small intestine, once
    activated, may simultaneously send
    messages to several different interneurons
    in the myenteric plexus that regulate
    peristalsis:
     – Impulses sent proximally by cholingeric
       neurons cause contraction and shortening of
       the circular muscular layer
    Motility of the Small Intestine
   …interneurons in the myenteric plexus
    that regulate peristalsis:
     – Impulses sent distally to certain interneurons
       cause shortening of the longitudinal muscle
       layers and distension of the intestine, in
       response to Ach-releasing neurons
     – Other impulses sent distally by activated VIP
       or NO-releasing enteric neurons cause
       relaxation of the circular muscle
    Motility of the Small Intestine
   As a result, as the proximal area constricts
    and forces chyme along the tract, the
    lumen of the distal part of the intestine
    enlarges to receive it
    Motility of the Small Intestine
   Most of the time, the ileocecal sphincter
    is constricted and closed
   Two mechanisms, one neural and one
    hormonal , cause it to relax when ileal
    mobility increases and allow food
    residues to entry the cecum
   Enhance activity of the stomach initiates
    the gastroileal reflex, a long reflex than
    enhances the force of segmentation in the
    ileum
    Motility of the Small Intestine
   In addition, gastrin released by the
    stomach increases the motility of the
    ileum and relaxes the ileocecal sphincter
   Once the chyme has passes through, it
    exerts backward pressure that closes the
    valve’s flaps, preventing regurgitation
    into the ileum
             Large Intestine




   The large intestine frames the small
    intestine on three sides and extends from
    the ileocecal valve to the anus
             Large Intestine
   Its diameter is greater than that of the
    small intestine, but is less than half as
    long 1.5 meters
   Its major function is to absorb water
    from indigestible food residues (delivered
    to it in fluid state) and eliminate them
    from the body as semisolid feces
             Large Intestine




   Over most of its length, the large intestine
    exhibits three features not seen elsewhere;
    teniae coli, haustra, epiploic appendages
             Large Intestine




   Teniae coli are three bands of smooth
    muscle which are the remnants of the
    smooth muscle layer
             Large Intestine




   The muscle tone of the teniae coli cause
    the wall of the large intestine to form
    pocketlike sacs called haustra
             Large Intestine




   Epiplocic appendages are small fat-filled
    pouches of visceral peritoneum that hang
    from its surface. Significance is not known
             Large Intestine




   The large intestine has the following
    subdivisions; cecum, appendix, colon,
    rectum, and anal canal
              Large Intestine




   The saclike cecum, or blind pouch, lies
    below the ileocecal valve is the first part of
    the large intestine
             Large Intestine




   Attached to the cecum is the blind,
    wormlike, vermiform appendix
             Large Intestine
   The appendix contains masses of lymphoid
    tissue, and as part of the MALT it plays an
    important role in body immunity
   It has a significant structural problem in
    that its twisted tissue provides an ideal
    location for enteric bacteria to accumulate
    and multiply
              Large Intestine




   The colon has several distinct regions;
    ascending, transverse, and descending colon
    segments connected by flexures
             Large Intestine




   The ascending colon travels up the right
    side of the abdominal cavity to the level of
    the right kidney
             Large Intestine




   At the level of the kidney the colon makes a
    right-angle turn, the right colic, or hepatic
    flexure
             Large Intestine




   The transverse colon travels across the top
    of the abdominal cavity
             Large Intestine




   Directly anterior to the spleen, it bends
    downward to form the left colic or splenic
    flexure
            Large Intestine




   The descending colon descends down the
    left side of the abdominal cavity
             Large Intestine




   As the descending colon enters the pelvis it
    forms the S-shaped sigmoid colon
               Large Intestine
   The transverse
    and sigmoid
    portions of the
    colon are
    anchored to the
    posterior
    abdominal wall
    by mesentary
    sheets called
    mesocolons
               Large Intestine




   In the pelvis, at the level of the third sacral
    vertebra, the sigmoid colon joins the rectum,
    which is positioned anterior to the sacrum
               Large Intestine
   The natural
    orientation of
    the rectum
    allows for a
    number of
    pelvic organs to
    be examined
    digitally during
    a rectal exam
              Large Intestine




   The rectum has three lateral curves or bends
    represented internally are transverse folds
    called rectal valves
               Large Intestine
   Rectal valves
    separate feces
    from flatus, thus
    allowing gas to
    passed
              Large Intestine
   The anal canal
    lies entirely
    external to the
    abdominopelvic
    cavity
   About 3 cm long
    the canal begins
    where the rectum
    penetrates the
    muscles of the
    pelvic floor
                Large Intestine
   The anal canal
    has two
    sphincters
    – External anal
      sphincter
    – Internal anal
      sphincter
             Large Intestine
   The involuntary internal anal sphincter is
    composed of smooth muscle
   The voluntary external anal sphincter is
    composed of voluntary muscle
   These sphincters which act rather like
    purse strings to open and close the anus,
    are ordinarily closed excepts during
    defecation
    Large Intestine: Microscopic
   The wall of the large intestine differs in
    several ways from that of the small
    intestine
   The colon mucosa is simple columnar
    epithelium except in the anal canal
   Because most food is absorbed before
    reaching the large intestine, there are no
    circular folds, no villi, and no cells that
    secrete digestive enzymes
    Large Intestine: Microscopic
   Its mucosa is thicker, its abundant crypts
    are deeper, and there are tremendous
    numbers of goblet cells in the crypts
   Lubricating mucus produced by goblet
    cells eases the passage of feces and
    protects the intestinal wall from irritating
    acids and gases released by resident
    bacteria in the colon
      Large Intestine: Microscopic
   The mucosa of
    the anal canal is
    different from
    the rest of the
    colon, reflecting
    the greater
    abrasion that
    this region
    receives
      Large Intestine: Microscopic
   The mucosa
    hangs in long
    ridges or folds
    called anal
    columns and
    contains
    stratified
    squamous
    epithelium
               Large Intestine
   The anal sinuses
    are recesses
    between the anal
    columns which
    exude mucus
    when
    compressed by
    feces
   This aids in the
    emptying of the
    canal
                Large Intestine
   The horizontal
    lines that
    parallels the
    inferior margin
    of the anal
    sinuses is called
    the pectinate line
   The line
    separates areas
    of visceral and
    somatic sensory               Pectinate line
    innervation
    Large Intestine: Microscopic
   The mucosa superior to the line is
    innervated by visceral sensory fibers and
    so are relatively insensitive to pain
   The are inferior to the pectinate line is
    innervated by somatic sensory fibers and
    is very sensitive to pain
    Large Intestine: Microscopic
   Two superficial venous plexuses are
    associated with the anal canal, one with
    the anal columns and the other with the
    anus itself
   Where these veins (hemorhoidal) are
    inflamed, itchy varicosities called
    hemorrhoids result
    Large Intestine: Microscopic
   In contrast to the more proximal regions
    of the large intestine, teniae coli and
    haustra are absent in the rectum and
    anal canal
   Consistent with its need to generate
    strong contractions to perform its
    expulsive role, the rectum’s muscularis
    muscle layers are complete and well
    developed
              Bacterial Flora
   Although most bacteria entering the
    cecum from the small intestine are dead
    having been killed by the action of
    lysozyme, defensins, HCL, and protein
    digesting enzymes
   The bacteria that survive, together with
    the bacteria that enter the GI tract via
    the anus, constitute the bacterial flora of
    the large intestine
             Bacterial Flora
   The bacterial flora colonize the colon and
    ferment some of the indigestible carbo-
    hydrates (cellulose and others) releasing
    irritating acids and a mixture of gases
    – Dimethyl sulfide, H2, N2, CH4, and CO2
   About 500 ml of gas is produced each day
    with much more when certain
    carbohydrate rich foods are eaten
   The bacterial flora also synthesize B
    complex vitamins and most of vitamin K
      Processes: Large Intestine
   What is finally delivered to the large
    intestine contains few nutrients, but still
    has 12 to 24 hours more digestive system
   Except for the small amount of digestion
    of residue by the enteric bacteria, no
    further food breakdown takes place in
    the large intestine
     Processes: Large Intestine
   Although the large intestine harvests
    vitamins made by the bacterial flora and
    reclaims most of the remaining water and
    some of the electrolytes (particularly
    sodium and chloride) absorption is not a
    major function of this organ
   The primary concern of the large
    intestine are propulsive activities that
    force the fecal material toward the anus
    and then eliminate it from the body
      Processes: Large Intestine
   While the large intestine is undeniably
    essential for our comfort, it is not
    essential for life
   Several different surgical procedures
    remove a part or all of the large intestine
    in order to save life
      Motility: Large Intestine
   The large intestine musculature is
    inactive much of the time, and when it is
    mobile, its contractions are sluggish and
    of short duration
   The most frequent movements seen in the
    colon are haustral contractions, which
    are slow segmenting movements that
    occurs every 30 minutes or so
       Motility: Large Intestine
   Haustral contractions reflect local controls
    of smooth muscle within the walls of
    individual haustra
   As a haustrum fills with food residue, the
    distension stimulates its muscle to contract,
    which propels the luminal contents into the
    next haustrum
   These movements also mix the residue
    which aids in water absorption
        Motility: Large Intestine
   Mass movements (mass peristalsis) are long,
    slow-moving, but powerful contractile
    waves that move over large areas of the
    colon three or four times daily and force the
    contents toward the rectum
   Typically these movements occur during or
    just after eating when the presence of food
    in the stomach activates the gastroileal
    reflex in the small intestine and the
    propulsive gastrocolic reflex in the colon
      Motility: Large Intestine
   Bulk, or fiber, in the diet increases the
    strength of colon contractions and softens
    the stool, allowing the colon to act more
    efficiently
                   Defecation
   The rectum is
    usually empty,
    but when feces
    are forced into it
    by mass
    movements,
    stretching of the
    rectal walls
    initiates the
    defecation reflex
                  Defecation
   This is a spinal
    cord mediated
    reflex that causes
    the walls of the
    sigmoid colon
    and the rectum to
    contract and the
    anal sphincters to
    relax
                  Defecation
   Distension or
    stretch of the
    rectal walls
    triggers a
    depolarization of
    sensory (afferent)
    fibers which
    synapse with the
    spinal cord
                  Defecation
   Parasympathetic
    motor (efferent)
    fibers, in turn,
    stimulate
    contraction of the
    rectal walls and
    relaxation of the
    internal anal
    sphincter
                   Defecation
   If it is convenient
    to defecate,
    voluntary signals
    stimulate the
    relaxation of the
    external anal
    sphincter
                  Defecation
   As feces are forced into the anal canal,
    impulses reach the brain allowing us to
    decide whether the external(voluntary) anal
    sphincter should remain open or closed
   If defection is delayed, the reflex
    contractions end within a few seconds and
    the walls relax
   With the next mass movement, the reflex is
    initiated again and again until one chooses to
    defecate

				
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