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									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
    – 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
   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
   Ingestion is
    simply the
    process of
    taking food into
    the digestive
    tract via the
   Propulsion is the
    process that
    moves food
    through the
    alimentary canal
   It includes
    process) and
   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
       Mechanical Digestion
   Mechanical
    prepares food for
    digestion by
           Mechanical Digestion
   Mechanical
    processes include
    chewing, mixing of
    food with saliva by
    the tongue,
    churning of food by
    the stomach, and
   Segmentation
    mixes food with
    digestive juices
    and increases the
    rate of absorption
        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
   Absorption is
    the passage of
    digested end
    products (plus
    mineral and
    water) from the
    lumen of the GI
    tract into the
    blood or lymph
    located in the
    wall of the canal
   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 is the
    elimination of
    substances from
    the body as
    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
    Basic Functional Concepts
   Digestive activity is provoked by a
    range of mechanical and chemical
    – Receptors are located in the walls of the
      tract organs
    – These receptors respond to several
    – 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
       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
    – Mix lumen contents
      along the length of
      the tract by
      stimulating the
    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
            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
       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
   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
       Digestive System Organs
   Most of the
    digestive organs
    reside in the
   All ventral body
    cavities contain
    serous membranes
   The peritoneum of
    the abdominal
    cavity is the most
    extensive serous
    membrane of the
       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
       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
       Digestive System Organs

   Mesenteries provide routes for blood vessels,
    lymphatics and nerves to reach the digestive
       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
   These organs, which also include most of the
    pancreas and parts of the large intestine are
           Digestive Processes

   Digestive organs like the stomach that keep
    their mesentery and remain in the peritoneal
    cavity are called interperitoneal or peritoneal
   It is not known why some digestive organs end
            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
   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
   From internal
    to external the
    four layers of
    the alimentary
    canal are
    – Mucosa
    – Submucosa
    – Muscularis
    – Serosa
              Histology: Mucosa
   The mucosa is the
    moist epithelial
    membrane that
    lines the length of
    the lumen of the
    alimentary canal
   Major functions
    – Secretion of
      mucus, digestive
      enzymes and
    – Absorption
              Histology: Mucosa
   The mucosa is the
    moist epithelial
    membrane that
    lines the length of
    the lumen of the
    alimentary canal
   Major functions
    – Secretion of
      mucus, digestive
      enzymes and
    – Absorption
        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
         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
   Thus, in such sites, the mucosa is a
    diffuse kind of endocrine organ as
             Histology: Mucosa
   The lamina propria
    which underlies
    the epithelium is
    loose areolar
   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
   Large collections of lymph nodules
    occur at strategic locations such as
    within the pharynx (tonsils) and
            Histology: Mucosa
   The muscularis
    mucosae is a
    scant layer of
    smooth muscle
    cells that
    produces local
    movements of the
          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
          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
    Histology: Muscularis Externa
   The muscularis
    externa is
    responsible for
    segmentation and
   It mixes and
    propels foodstuffs
    along the
    digestive tract
   This thick
    muscular layer
    has an inner
    circular and an
Histology: Muscularis Externa
   In several places along the GI tract, the
    circular layer thickens to form
   Sphincters act as valves to prevent
    backflow and control food passage
    from one organ to the next
              Histology: Serosa
   The serosa is the
    outermost layer of
    inter- peritoneal
   This visceral
    peritoneum is
    formed of areolar
    connective tissue
    covered with
    meso- thelium, a
    single layer of
           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             Intrinsic
    in-house nerve                 Plexes
   Enteric neurons
    widely with each
    other to regulate
    digestive system
         Enteric Nervous System
   These enteric
    neurons                       Myenteric
    constitute the
    bulk of the two             plexus
    major intrinsic
    nerve plexuses
    found within the
    walls of the
    alimentary canal
    – Submucosal nerve
    – Myenteric nerve
         Enteric Nervous System
   A smaller third
    plexus is found               Subserosa
    within the serosa              plexus
    – Subsersora nerve
         Enteric Nervous System
   The submucosal
    nerve plexus                  Myenteric
    chiefly regulates
    the activity of             plexus
    glands and
    smooth muscle in
    the mucosa tunic
   The myenteric
    nerve plexus lies
    between the
    circular and
    layers of smooth
     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
     Enteric Nervous System
   The enteric nervous system is also
    linked to the CNS by afferent visceral
    fibers and sympathetic and
    parasympathetic branches of the
   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
   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
                     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
   Posteriorly the
    oral cavity is
    continuous with
                   The Mouth
   The walls of the
    mouth are lined
    with stratified
   The epithelium is
    highly ketatinized
    for extra
    protection against
    abrasion during
   The mucosa also
          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 Lips and Cheeks
   The lips extend
    from the inferior
    margin of the
    nose to the
    boundary of the
   The reddened
    area is called red
   The labial
    frenulum is a
    median fold that
                    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
   There exists a
    centerline ridge
    called a raphe
   The mucosa is
    corrugated for
                     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
    when swallowing
                   The Palate
   The soft palate is
    anchored to the
    tongue by the
    arches and to the
    wall of oropharynx
    by the
   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
   The tongue also mixes the food with
    salvia and form it into a mass called
                   The Tongue
   The tongue has
    both intrinsic and
    extrinsic skeletal
   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
                 The Tongue
   The extrinsic
    muscles extend the
    tongue from their
    points of origin
   The extrinsic
    muscles allow the
    tongue to be
    retracted and
    moved side to side
   The tongue is
    divided by a
    median septum of
                   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
   You cannot
    swallow your
                  The Tongue
   The conical
    filaform papillae
    give the tongue
    surface a
    roughness that
    aids in
    foods in the mouth
   They align in
    parallel rows on
    the dorsum
   They contain
    keratin which
                  The Tongue
   The mushroom
    shaped fungiform
    palillae are
    scattered over the
   Each has a
    vascular core that
    gives it a reddish
   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
       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
    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
   Teeth function to tear and grind food
    and begin the mechanical process of
   Ordinarily we have two sets of teeth
    the primary and permanent
   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
   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
                        The Teeth
   Teeth are
    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
   The mucosa
    consists of
   The epithelium is
    supplied with
    mucus producing
    glands for
                  The Pharynx
   The external
    muscle layer
    consists of two
    skeletal muscle
   The cells of the
    inner layer run
   The outer layer of
    muscles, encircle
               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 Esophagus
   The esophagus
    joins the stomach
    at the cardiac
   The cardica orifice
    is surrounded by
    the cardiac
             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
             The Pharynx
   The muscularis externa changes
    from skeletal muscle to a mix of
    skeletal and smooth to finally all
    smooth as it approaches the
   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
    – 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
       Digestive Processes:
   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:
   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
           Digestive Processes:
   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
          Digestive Processes:
   Food passage into
    passageways by
    rising of the uvula
    and larynx
   Relaxation of the
    upper esophageal
    sphincter allows food
    entry into the
          Digestive Processes:
   The constrictor
    muscles of the
    pharynx contract,
    forcing food into the
    esophagus inferiorly
   The upper
    esophageal sphincter
    contracts after entry
          Digestive Processes:
   Food is conducted
    along the length of
    the esophagus to the
    stomach by
    peristaltic waves
          Digestive Processes
   The
    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
   The stomach
    varies from 6 to 10
    inches in length,
    but its diameter
    and volume
    depend on how
    much food it
   Empty it may
    contain on 50 ml
    but can expand to
    hold about 4 liters
    of food
           The Stomach: Gross

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

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

   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

   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
    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
             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
   The special function
    of this unique
    mucus is not yet
           Microscopic Anatomy
   Parietal cells
    scattered among the
    chief cells secrete
    hydrochloric acid
    (HCl) and intrinsic
   The parietal cells
    have a large surface
    area adapted for
    secreting HCl in the
   Intrinsic factor is
    required for
           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
   Pepsinogen is
    activated by HCl
           Microscopic Anatomy
   Parietal cells
    scattered among the
    chief cells secrete
    hydrochloric acid
    (HCL) and intrinsic
   The parietal cells
    have a large surface
    area adapted for
    secreting HCL in the
   Intrinsic factor is
    required for
          Microscopic Anatomy
   Enteroendocrine
    release a variety of
    hormones directly
    into the lamina
   These products
    diffuse into
    capillaries and
    ultimately influence
    several digestive
    system target organs
    which regulate
    stomach secretion
              Mucosal Barrier
   Gastric juice is 100,000 more
    concentrated than that found in the
   Under such harsh conditions the
    stomach must protect itself from self
    digestion with a mucosal barrier
    – Bicarbonate rich mucus is on the stomach
    – Epithelial cells are joined by tight junctions
    – Glandular cells are impermeable to HCl
    – Surface epithelium is replace every 3 to 6
       Digestive Processes:
   The stomach is involved in the whole
    range of digestive activities
    – It serves as a holding area for ingested
    – Breaks down food further chemically
      and mechanically
    – It delivers chyme to the small intestine
      at a controlled rate
       Digestive Processes:
   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
       Digestive Processes:
   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
   Without B12 the individual will
    develop prenicious anemia unless
    administered by injection
       Regulation of Gastric
   Gastric secretion is controlled by
    both neural and hormonal
   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
   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
   The sympathetic nerves depress
    secretory activity
       Regulation of Gastric
   Hormonal control of gastric secretion
    is largely from the presence of
   Gastrin stimulates the secretion of
    both enzymes and HCL in the
   Hormones produced by the small
    intestine are largely gastrin
       Regulation of Gastric
   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
   Chemical stimuli provided by
    partially digested proteins
    (peptids)caffine (colas, coffee) and
    rising pH directly active gastrin
    secreting entoendocrine cells called
      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
      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
   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-
    nerve fibers
    and gastrin
    secreted by G
    cells bring
    about their
    effects by
       Phase 2: Gastric reflex
   Histamine
    released by
    mucosal cells
    es acts
    cyclic AMP
      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
      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
      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+ +
   As H+ is
    pumped from
    the cell and
    HCO3- is
    through the
    basal cell
    membrane into
    the capillary
      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
   As a result, gastric secretory activity
         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
        Gastric Motility and
   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
   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
   Although the stomach stretches to
    accommodate incoming food, internal
    stomach pressure remains constant
    until about 1 liter of food has been
   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
   Reflexive relaxation of stomach
    muscle in the fundus and body
    occurs both in anticipation of and in
    response to food entry into the
   As food travels through the
    esophagus, the stomach muscles
   This receptive relaxation is
    coordinated by the swallowing center
    in the brain stem and mediated by
    Gastric Motility:Stomach
   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
   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
        Gastric Motility and

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

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

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

   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

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

   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
          Gastric Motility and
   Although the intensity of the stomach’s
    peristaltic waves can be modified, the
    rate is always constant at around 3 per
   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
   The pacemaker cells, are believed to
    be muscle-like noncontractile cells
    called interstitial cells of Cajal which
    depolarize the repolarize
    spontaneously three times each
   This depolarization and
    repolarization establish the so-called
    cyclic slow waves of the stomach or
    its basic electrical rhythm (BER)
          Gastric Motility and
   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
        Gastric Motility and
   Factors that increase the strength of
    stomach contractions are the same
    factors that enhance gastric secetory
   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
   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
   However, the larger the meal (greater
    distension) and the more liquid the
    meal, the faster the stomach empties
        Gastric Motility and
   Fluids pass quickly through the
   Solids linger, remaining until they are
    well mixed with gastric juice and
    converted to a liquid state
          Gastric Motility and
   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
   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
        Gastric Motility and
   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
   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
    from the
    sphincter in
    the epigastric
    region to the
    iliocecal valve
    where it joins
    the large
             Small Intestine
   It is the longest part of the alimentary
    tube, but its diameter is only about 2.5
   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
    – Duodenum
    – Jejunum
           Gross Anatomy

   The relatively immovable duodenum
    which curves about the head of the
           Small Intestine
   The duodenum is about 10 inches
   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
              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
                Small Intestine
   The ileum is
    12 ft. in length
   It joins the
    large intestine
    at the
                   Small Intestine
   The
    and ileum
    hang in
    coils in the
    central and
    lower part
    of the
             Small Intestine
   The jejunum
    and ileum
    from the
    wall by the
    fan shaped
           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
            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
      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
   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
    folds of the
    mucosa and
   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
         Microscopic Anatomy
   Villi are finger
    projections of
    the mucosa
   Over 1 mm
    tall they give
    a velvety
    texture to the
         Microscopic Anatomy
   The epithelial
    cells of the
    villi are
    cells called
         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.
         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-
       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
   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
         Histology of the Wall
   Between villi
    the mucosa is
    studded with
    pits that lead
    into tubular
    glands called
    intestinal crypts
    or crypts of
         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
       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
        Histology of the Wall
   A set of
    (Brunner’s) is
    found in the
    submucosa of
    the duodenum
       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
   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
   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
                 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
   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
                 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
                 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
   The liver is
    of seed
    structural &
    units called
    liver lobules
   Each lobule
    is roughly
       Microscopic Anatomy of
   Hepatocytes
    or live cells
    organized to
    radiate out
    from a
    central vein
    running the
    length of the
    axis of the
     Microscopic Anatomy of
   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
   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

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

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

   A bile duct to carry secreted bile
    toward the common bile duct and
    ultimately to the duodenum
      Microscopic Anatomy of
   Between the
    plates are
    enlarged, very
    capillaries, the
    liver sinusoids
      Microscopic Anatomy of
   Blood from
    both the hepatic
    portal vein and
    the hepatic
    percolates from
    the triad
    regions through
    these sinusoids
    and empties
    into the central
      Microscopic Anatomy of
   From the
    central vein
    enters the
    hepatic veins,
    which drain the
    liver, and empty
    into the inferior
    vena cava
      Microscopic Anatomy of
   Inside the
    sinusoids are
    star shaped
    also called
    Kupffer cells,
    which remove
    debris such as
    bacteria and
    worn-out blood
     Microscopic Anatomy of
   The hepatocytes (liver cells) are virtual
    organelle storehouses with large
    amounts of both rough and smooth
    endoplasmic reticulum, Golgi
    apparatuses, peroxisomes, and
   Thus equipped, the hepatocytes
    produce not only bile but also
    – Process blood borne nutrients
    – Store Fat-soluble vitamins
    – Detoxify the blood
     Microscopic Anatomy of
   In processing nutrients the
    hepatocytes store glycogen and make
    plasma proteins
   Fat soluble vitamins are stored until
    such time as they are needed for
   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
      Microscopic Anatomy of
   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
   Note that the
    bile and the
    blood flow in
    directions in
    the liver lobule
   Bile entering
    the bile ducts
    leaves the liver
    via the common
     Microscopic Anatomy of
   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
   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
   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
   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
    – Resecreted in newly formed bile
     Microscopic Anatomy of
   The chief bile pigment is bilirubin, a
    waste product of hemoglobin (heme)
    during the breakdown of worn-out
   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
   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
   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
   The single
    most important
    stimulus of bile
    secretion is an
    increased level
    of bile salts in
            The Gallbladder
   The gallbladder
    is a thin-walled,
    green muscular
    sac, rouhgly the
    size of a kiwi
   It snuggles in a
    shallow fossa
    on the ventral
    surface of the
           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
   Its muscular walls can contract to
    expell its contents into the cystic
    duct which then flows into the bile
   Like most of the liver it is covered by
            The Gallbladder

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

   Bile then backs up the cystic duct into
    the gallbladder where it is stored until
          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
           The Gallbladder
   Besides causing the gallbladder to
    contract, CCk has two other
    important effects
    – It stimulates secreation of pancreatic
    – 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
             The Pancreas

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

   Most the pancreas is retroperitoneal and
    lies deep to the greater curvature of
            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
   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
             The Pancreas

   A smaller accessory pancreatic duct
    empties directly into the main duct
             The Pancreas
   Within the
    pancreas are
    the acini,
    clusters of
              The Pancreas
   The acini
    cells are full
    of rough
    reticulum and
    exhibit deeply
               The Pancreas
   The
    also has an
   Scattered
    amidst the
    acini are the
    more lightly
              The Pancreas
   These Islets
    insulin and
    that regulate
          Pancreatic Juice
   Approximately 1200 to 1500 ml of
    clear pancreatic juice is produced
   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 of pancreatic juice is
    regulated both by local hormones
    and by the parasympathetic nervous
       Regulation of Pancreatic
   Secretin is
    released in
    response to the
    presence of
    HCL in the
   Cholecystokini
    n is released in
    response to the
    entry of
    proteins and
      Regulation of Pancreas
   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
     Regulation of Pancreatic
   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
   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
   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
    Optimal Intestinal Activity
   Optimal digestive activity in the small
    intestine also depends on a slow,
    measured delivery of chyme from the
   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
   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
   In contrast to the peristaltic waves of
    the stomach, which both mix and
    propel food, segmentation is the
    most common motion of the small
          Motility of the Small
   In segmentation,
    chyme is moved
    backward and
    forward a few
    centimeters at a
    time by
    contraction and
    relaxation of
    rings of smooth
           Motility of the Small
   These
    movements of
    the intestine are
    initiated by
    pacemaker cells
    (interstitial cells
    of Cajal) in the
    smooth muscle
        Motility of the Small
   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
   The intensity of the segmentation is
    altered by hormones and long and
    short reflexes
    – Parasympathetic enhances
    – 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
   True
    occurs only
    after most
    nutrients have
   Segmentation
    wane, and
        Motility of the Small
   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
   Peristalsis serves to sweep out the
    last remnants of the meal plus
    bacteria, sloughed-off mucosal cells,
    and other debris into the large
   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
        Motility of the Small
   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
   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
   …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
   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
   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
   In addition, gastrin released by the
    stomach increases the motility of the
    ileum and relaxes the ileocecal
   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,
           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
           Large Intestine

   Epiplocic appendages are small fat-
    filled pouches of visceral peritoneum
    that hang from its surface.
           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
             Large Intestine
   The
    and sigmoid
    portions of
    the colon are
    anchored to
    the posterior
    wall by
    sheets called
            Large Intestine

   In the pelvis, at the level of the third
    sacral vertebra, the sigmoid colon joins
    the rectum, which is positioned anterior
             Large Intestine
   The natural
    orientation of
    the rectum
    allows for a
    number of
    pelvic organs
    to be
    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
   About 3 cm
    long the canal
    begins where
    the rectum
    penetrates the
    muscles of the
    pelvic floor
              Large Intestine
   The anal canal
    has two
    – External anal
    – Internal anal
           Large Intestine
   The involuntary internal anal
    sphincter is composed of smooth
   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
   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
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
    Large Intestine: Microscopic
   The mucosa of
    the anal canal
    is different
    from the rest
    of the colon,
    reflecting the
    abrasion that
    this region
    Large Intestine: Microscopic
   The mucosa
    hangs in long
    ridges or folds
    called anal
    columns and
             Large Intestine
   The anal
    sinuses are
    between the
    anal columns
    which exude
    mucus when
    compressed by
   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
    areas of                   Pectinate line
    visceral and
Large Intestine: Microscopic
   The mucosa superior to the line is
    innervated by visceral sensory fibers
    and so are relatively insensitive to
   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
   The bacterial flora also synthesize B
    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
    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
   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
     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
   The rectum is
    usually empty,
    but when feces
    are forced into
    it by mass
    stretching of
    the rectal walls
    initiates the
   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
   Distension or
    stretch of the
    rectal walls
    triggers a
    of sensory
    (afferent) fibers
    which synapse
    with the spinal
   Parasympatheti
    c motor
    (efferent) fibers,
    in turn,
    contraction of
    the rectal walls
    and relaxation
    of the internal
    anal sphincter
   If it is
    convenient to
    stimulate the
    relaxation of
    the external
    anal sphincter
   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
   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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