The Digestive System

1
The Digestive System

• Chapter 22

2
The Digestive System

• The digestive system
• Takes in food
• Breaks it down into nutrient molecules
• Absorbs the nutrient molecules into the
  bloodstream
• Rids the body of indigestible remains

3
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

4
The Digestive System

• The alimentary canal or gastrointestinal (GI)
  tract is the continuous muscular digestive tube
  that winds through the body

5
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

6
Digestive Processes

• The digestive tract can be viewed as a process by
  which food becomes less complex at each step of
  processing and nutrients become available to the
  body

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Ingestion

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

8
Propulsion

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

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Propulsion

• Peristalsis involves alternate waves of
  contraction and relaxation of muscles in the
  organ walls
• Its main effect is to squeeze food from one organ
  to the next
• Some mixing occurs as well

10
Mechanical Digestion

• Mechanical digestion physically prepares food for
  chemical digestion by enzymes

11
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

12
Chemical Digestion

• Chemical digestion is a series of catabolic steps
  in which complex food molecules are broken down
  to their chemical building blocks

13
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

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Absorption

• Absorption is the passage of digested end
  products (plus vitamins, mineral and water) from
  the lumen of the GI tract into the blood or lymph
  capillaries located in the wall of the canal

15
Absorption

• For absorption to occur these substances must
  first enter the mucosal cells by active or
  passive transport processes
• The small intestine is the main absorption site

16
Defecation

• Defecation is the elimination of indigestible
  substances from the body as feces

17
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

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Basic Functional Concepts

• Digestive activity is provoked by a range of
  mechanical and chemical stimuli
• Receptors are located in the walls of the tract
  organs
• These receptors respond to several stimuli
• The most important being the stretching of the
  organ by food in the lumen, osmolarity (solute
  concentration) and pH of the contents and the
  presence of substrates and end products of
  digestion

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Basic Functional Concepts

• When appropriately stimulated, these receptors
  initiate reflexes that
• Activate or inhibit glands that secrete digestive
  juices into the lumen or hormones into the blood
• Mix lumen contents along the length of the tract
  by stimulating the smooth muscle of the GI tract
  walls

20
Basic Functional Concepts

• Controls of digestive activity are both extrinsic
  and intrinsic
• Another novel trait of the digestive tract is
  that many of its controlling systems are
  intrinsic - a product of in-house nerve plexuses
  or local hormone-producing cells
• The walls of the alimentary canal contain nerve
  plexuses
• These plexuses extend essentially the entire
  length of the GI tract and influence each other
  both in the same and in different organs

21
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

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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

23
Digestive System Organs

• Most of the digestive organs reside in the
  abdominal-pelvic cavity
• All ventral body cavities contain serous
  membranes
• The peritoneum of the abdominal cavity is the
  most extensive serous membrane of the body

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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

25
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

26
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

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Digestive System Organs

• Mesenteries provide routes for blood vessels,
  lymphatics and nerves to reach the digestive
  viscera

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Digestive System Organs

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

29
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

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Digestive Processes

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

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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

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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

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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

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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

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Histology of the Alimentary Canal

• From internal to external the four layers of the
  alimentary canal are
• Mucosa
• Submucosa
• Muscularis Externa
• Serosa

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Histology Mucosa

• The mucosa is the moist epithelial membrane that
  lines the length of the lumen of the alimentary
  canal
• Major functions are
• Secretion of mucus, digestive enzymes and
  hormones
• Absorption
• Protection

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Histology Mucosa

• The mucosa is the moist epithelial membrane that
  lines the length of the lumen of the alimentary
  canal
• Major functions are
• Secretion of mucus, digestive enzymes and
  hormones
• Absorption
• Protection

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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

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Histology Mucosa

• The epithelium of the mucosa is a simple columnar
  epithelium that is rich in mucus secreting goblet
  cells

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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

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Histology Mucosa

• The lamina propria which underlies the epithelium
  is loose areolar connective
• Note lymph nodule

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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

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Histology Mucosa

• The muscularis mucosae is a scant layer of smooth
  muscle cells that produces local movements of the
  mucosa

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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

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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

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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

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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

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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

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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

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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)

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Enteric Nervous System

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

Intrinsic Nerve Plexes
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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
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Enteric Nervous System

• A smaller third plexus is found within the serosa
  layer
• Subsersora nerve plexus

Subserosa nerve plexus
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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
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Enteric Nervous System

• Via their communication with each other, with
  smooth muscle layers, and with submucosal plexus,
  the enteric neurons of the myenteric plexus
  provide the major nerve supply to the GI tract
• This plexus controls GI tract mobility by
  controlling the patterns of segmentation and
  peristalsis
• Control comes from local reflex arcs between
  enteric neurons in the same or different plexus
  or organs

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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

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Digestive System
58
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

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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

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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

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The Lips and Cheeks

• The labia and the cheeks have a core of skeletal
  muscle covered by skin
• The orbicularis oris muscle forms the bulk of the
  lips
• The cheeks are formed largely by the buccinators
• The area between the teeth and gums is the
  vestibule

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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

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The Palate

• The palate which forms the roof of the mouth has
  two distinct parts
• Hard palate
• Soft palate

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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

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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

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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

67
The Tongue

• The tongue occupies the floor of the mouth and
  fills most of the oral cavity when closed
• The tongue is composed of interlacing masses of
  skeletal muscle fibers
• The tongue grips the food and constantly
  repositions it between the teeth
• The tongue also mixes the food with salvia and
  form it into a mass called a bolus and then
  initiates swallowing by moving the mass into the
  pharynx

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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

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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

70
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

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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

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The Tongue

• The mushroom shaped fungiform palillae are
  scattered over the surface
• Each has a vascular core that gives it a reddish
  hue
• Houses taste buds

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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

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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

75
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

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The Salivary Glands

• The intrinsic salivary glands are small and are
  scattered throughout the oral cavity

77
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

78
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

79
Dentition

• Ordinarily we have two sets of teeth the primary
  and permanent dentitions
• The primary dentition consists of deciduous teeth
• The first teeth appear at six months and
  additional teeth continue to erupt until about 24
  months when all 20 teeth have emerged

80
Dentition

• As the deeper permanent teeth enlarge and
  develop, the root of the milk teeth are resorbed
  from below causing them to loosen and fall out
  between the ages of 6 and 12 years
• Generally, all the teeth of the permanent
  dentition have erupted by adolescence

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The Teeth

• Teeth are classified according to their shape and
  function
• Incisors / cutting
• Canines / tear
• Premolars / grind
• Molars / crush
• There are 20 milk teeth and 32 permanent teeth

82
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

83
The Pharynx

• From the mouth, the food passes posteriorly into
  the oropharnyx
• The mucosa consists of stratified squamous
  epithelium
• The epithelium is supplied with mucus producing
  glands for lubrication

84
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

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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

86
The Esophagus

• The esophagus joins the stomach at the cardiac
  orifice
• The cardica orifice is surrounded by the cardiac
  esophogeal sphincter

87
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

88
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

89
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

90
Digestive Processes Mastication

• Mastication is the mechanical process of breaking
  down food
• The cheeks and closed lips hold the food between
  the teeth
• The tongue mixes the food with saliva to soften
  it
• The teeth cut and grind food into smaller pieces

91
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

92
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

93
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

94
Digestive Processes Deglutition

• The constrictor muscles of the pharynx contract,
  forcing food into the esophagus inferiorly
• The upper esophageal sphincter contracts after
  entry

95
Digestive Processes Deglutition

• Food is conducted along the length of the
  esophagus to the stomach by peristaltic waves

96
Digestive Processes

• The gastroesophageal sphincter enters opens and
  food enters the stomach

97
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

98
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

99
The Stomach Gross Anatomy

• When empty, the stomach collapses inward,
  throwing its mucosa into large, longitudinal
  folds called rugae

100
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

101
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

102
The Stomach Gross Anatomy

• The greater omentum drapes inferior from the
  greater curvature of the stomach to cover the
  coils of the small intestine

103
Stomach Microscopic Anatomy

• The stomach wall exhibits the four tunics of most
  of the alimentary canal but its muscularis and
  mucosa are modified for the special roles of
  stomach
• The muscularis externa has an extra oblique layer
  of muscle that enables it to mix, churn and
  pummel food
• The epithelium lining the stomach mucosa is
  simple columnar epithelium composed entirely of
  goblet cells, which produce a protective coating
  of mucus

104
Microscopic Anatomy

• The four tunics typical of the alimentary canal
• Mucosa
• Submucosa
• Muscularis Externia
• Serosa

105
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106
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

107
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

108
Microscopic Anatomy

• Parietal cells scattered among the chief cells
  secrete hydrochloric acid (HCl) and intrinsic
  factor
• The parietal cells have a large surface area
  adapted for secreting HCl in the stomach
• Intrinsic factor is required for absorption of
  B12 in the small intestine

109
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

110
Microscopic Anatomy

• Parietal cells scattered among the chief cells
  secrete hydrochloric acid (HCL) and intrinsic
  factor
• The parietal cells have a large surface area
  adapted for secreting HCL in the stomach
• Intrinsic factor is required for absorption of
  B12 in the small intestine

111
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

112
Mucosal Barrier

• Gastric juice is 100,000 more concentrated than
  that found in the blood
• Under such harsh conditions the stomach must
  protect itself from self digestion with a mucosal
  barrier
• Bicarbonate rich mucus is on the stomach wall
• Epithelial cells are joined by tight junctions
• Glandular cells are impermeable to HCl
• Surface epithelium is replace every 3 to 6 days

113
Digestive Processes Stomach

• The stomach is involved in the whole range of
  digestive activities
• It serves as a holding area for ingested food
• Breaks down food further chemically and
  mechanically
• It delivers chyme to the small intestine at a
  controlled rate

114
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

115
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

116
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

117
Regulation of Gastric Secretion

• Nervous control is regulated by long (vagus nerve
  mediated) and short (local enteric) nerve
  reflexes
• When the vagus nerves actively stimulate the
  stomach, secretory activity of virtually all of
  its glands increase
• The sympathetic nerves depress secretory activity

118
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

119
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

122
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

123
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

126
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

127
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

128
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

129
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

130
Phase 2 Gastric reflex

• The more protein in the meal, the greater the
  amount of gastrin and HCL released
• As proteins are digested, the gastric contents
  gradually become more acidic, which again
  inhibits the gastrin secreting cells
• This negative feedback mechanism helps maintain
  optimal pH and working conditions for the gastric
  enzymes

131
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

132
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

133
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

134
Phase 2 Gastric reflex

• Histamine released by mucosal cells called
  histaminocytes acts through cyclic AMP (cAMP)

135
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

136
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

137
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

138
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

139
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

140
Phase 3 Intestinal

• The intestinal phase of gastric secretion has two
  components
• 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

143
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

144
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

145
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

146
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

147
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

148
Gastric Motility Stomach Filling

• Although the stomach stretches to accommodate
  incoming food, internal stomach pressure remains
  constant until about 1 liter of food has been
  ingested
• The relatively unchanging pressure in the filling
  stomach is due to 1) reflex mediated relaxation
  of the stomach muscle and 2) plasticity of
  visceral smooth muscle

149
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

150
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

151
Gastric Motility Stomach Filling

• Plasticity is the intrinsic ability of visceral
  smooth muscle to exhibit the stress- relaxation
  response, that is, to be stretched without
  greatly increasing its tension and contractile
  strength

152
Gastric Motility and Emptying

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

153
Gastric Motility and Emptying

• As contractions approach the pylorus, where the
  stomach musculature is thicker, the contractions
  become more powerful

154
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

155
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

156
Gastric Motility and Emptying

• Normally, each peristaltic wave reaching the
  pyloric muscle squirts 3 ml or less of chyme into
  the small intestine

157
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

158
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

159
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)

160
Gastric Motility and Emptying

• Since the pacemakers are electrically coupled to
  the rest of the smooth muscle sheet by gap
  junctions, their beat is transmitted
  efficiently and quickly to the entire muscularis
• The pacemakers set the maximum rate of
  contraction, but they do not initiate the
  contractions or regulate their force
• They generate subthreshold depolarization waves,
  which are then enhance by neural and hormonal
  factors

161
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

162
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

163
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

164
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

165
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

166
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

167
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

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Small Intestine

• It is the longest part of the alimentary tube,
  but its diameter is only about 2.5 cm
• In the cadaver, the small intestine is 6 - 7
  meters long because of loss of muscle tone, while
  it is only 2 - 4 meters long in the living
  individual
• The small intestine has three subdivisions
• Duodenum
• Jejunum
• Ileum

169
Gross Anatomy

• The relatively immovable duodenum which curves
  about the head of the pancreas

170
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

171
Gross Anatomy

• The bile duct, delivering bile from the liver
• The main pancreatic duct, carries pancreatic
  juice from the pancreas

172
Gross Anatomy

• The hepatopancreatic ampulla is where these two
  ducts unite in the wall of the duodenum
• The papilla is where this sphincter enters the
  duodenum

173
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

174
Small Intestine

• The ileum is approximately 12 ft. in length
• It joins the large intestine at the ileocecal
  valve

175
Small Intestine

• The jejunum and ileum hang in coils in the
  central and lower part of the abdominal cavity

176
Small Intestine

• The jejunum and ileum are suspended from the
  posterior abdominal wall by the fan shaped
  mesentery

177
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

178
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

179
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

180
Microscopic Anatomy
181
Digestive System Organs

• In this view you can see the plicae circulares
  and the villi of the small intestine

182
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

183
Microscopic Anatomy

• The circular folds or plicae circularis are deep
  permanent folds of the mucosa and submucosa
• These folds are nearly 1 cm tall

184
Microscopic Anatomy

• The folds force chyme to spiral through the
  lumen, slowing its movement and allowing time for
  full nutrient absorption

185
Microscopic Anatomy

• Villi are finger like projections of the mucosa
• Over 1 mm tall they give a velvety texture to the
  mucosa

186
Microscopic Anatomy

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

187
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
  lacteal
• Villi become gradually narrower and shorter along
  the length of the sm. intestine

Enterocyte
188
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

189
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

190
Histology of the Wall

• The four tunics of the digestive tract are
  modified in the small intestine by variations in
  mucosa and sub- mucosa

191
Histology of the Wall

• The epithelium of the mucosa is largely simple
  columnar epithelium serving as absorptive cells
• The cells are bound by tight junctions and richly
  endowed with microvilli
• Also present are many mucus-secreting goblet
  cells

192
Histology of the Wall

• Scattered among the epithelial cells of the wall
  are T cells called intraepithelial lymphocytes
• These T cells provide an immunological component
• Finally, there scattered enteroendocrine cells
  which are the source of secretin and
  cholecystokinin

193
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

194
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

195
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

196
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

197
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

198
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
  bloodstream

199
Histology of the Wall

• A set of elaborated mucus-secreting duodenal
  glands (Brunners) is found in the submucosa of
  the duodenum only

200
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

201
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)

202
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

203
Intestinal Juice

• Normally, the pH range of intestinal juice is
  slightly alkaline (7.4-7.8), and it is isotonic
  with blood plasma
• Intestinal juice is largely water but it also
  contains some mucus, which is secreted both by
  the duodenal glands and by goblet cells of the
  mucosa
• Intestinal juice is relatively enzyme poor
  because intestinal enzymes are largely limited to
  the bound enzymes of the brush border

204
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

205
The Liver

• The ruddy, blood rich liver is the largest gland
  in the body weighing about 1.4 kg in the average
  adult

206
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

207
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

208
The Liver

• The liver has four primary lobes right, left,
  caudate and quadrate

209
The Liver

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

210
The Liver

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

211
The Liver

• Except for the superiormost liver area, which is
  fused to the diaphragm, the entire liver is
  enclosed by a serosa (visceral peritoneum)

212
The Liver

• A dorsal mesentery, the lesser omentum, anchors
  the liver to the lesser curvature of the stomach

213
The Liver

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

214
The Liver

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

215
The Liver

• The gallbladder rests in a recess of th