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The Digestive System


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Title: The Digestive System

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

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

  • Ingestion is simply the process of taking food
    into the digestive tract via the mouth

  • Propulsion is the process that moves food through
    the alimentary canal
  • It includes swallowing (voluntary process) and
    peristalsis (involuntary process)

  • 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 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

  • 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 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
  • 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

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

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
  • 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
  • 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

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

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
    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
  • The hepatic, splenic and left gastric branches of
    the celiac trunk serve the spleen, liver, and
  • 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
  • Major functions are
  • Secretion of mucus, digestive enzymes and
  • Absorption
  • Protection

Histology Mucosa
  • The mucosa is the moist epithelial membrane that
    lines the length of the lumen of the alimentary
  • Major functions are
  • Secretion of mucus, digestive enzymes and
  • 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

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

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
  • Retroperitoneal organs have both a serosa (on the
    side facing the peritoneal cavity) and an
    adventitia (on the side abutting the dorsal body

Enteric Nervous System
  • The alimentary canal has its own in-house nerve
  • Enteric neurons communicate widely with each
    other to regulate digestive system activity

Intrinsic Nerve Plexes
Enteric Nervous System
  • These enteric neurons constitute the bulk of the
    two major intrinsic nerve plexuses found within
    the walls of the alimentary canal
  • Submucosal nerve plexus
  • Myenteric nerve plexus

Myenteric plexus
Submucosal plexus
Enteric Nervous System
  • A smaller third plexus is found within the serosa
  • Subsersora nerve plexus

Subserosa nerve plexus
Enteric Nervous System
  • The submucosal nerve plexus chiefly regulates the
    activity of glands 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

Myenteric plexus
Submucosal plexus
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
  • 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
  • The cheeks are formed largely by the buccinators
  • The area between the teeth and gums is the

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

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
  • 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
  • Moistens food and aids in compacting it into a
  • 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

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

  • 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

  • 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
  • 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
  • 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
  • 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
  • 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

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
  • 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
  • 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

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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

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
  • 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
  • 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
  • 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
  • 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
  • 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

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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

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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
  • 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

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
  • One excitatory
  • One inhibitory

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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 intestines 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
  • 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 MotilityStomach 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

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 stomachs
    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

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
  • 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

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
  • 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

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

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

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

Microscopic Anatomy
  • The small intestine is highly adapted for
    nutrient absorption
  • Its length provides a huge surface area for
  • 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

Microscopic Anatomy
  • The epithelial cells of the villi are chiefly
    absorptive columnar cells called enterocytes

Microscopic Anatomy
  • In each villus is a capillary bed and a wide
    lymphatic capillary called a lacteal
  • Digested food is absorbed through the epithelial
    cells into both the capillary blood and the
  • 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

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 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 (Peyers patches)
  • Peyers 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 elaborated mucus-secreting duodenal
    glands (Brunners) 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

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
  • 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

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 th
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