Download the digestive system

THE DIGESTIVE SYSTEM
• Digestive system organs fall into two main
groups: the alimentary canal and the
accessory organs:
– Alimentary canal, or the gastrointestinal (GI) tract,
is the continuous muscular digestive tube that winds
through the body digesting and absorbing foodstuff;
its organs include: the mouth, pharynx, esophagus,
stomach, small intestine, and large intestine
– Accessory digestive organs aid digestion physically
and produce secretions that break down foodstuff in
the GI tract; the organs involved are the teeth, tongue,
gallbladder, salivary glands, liver, and pancreas
ORGANS OF THE ALIMENTARY CANAL
THE DIGESTIVE SYSTEM
•
Digestive Processes:
– Ingestion is the simple act of putting food into the mouth
– Propulsion moves food through the alimentary canal and includes both
swallowing and peristalsis (alternate waves of contraction and relaxation of
muscles in the organ walls)
• Its main effect is to squeeze food along the tract
– Mechanical digestion is the physical process of preparing the food for chemical
digestion by enzymes and involves chewing, mixing, churning, and segmentation
(rhythmic local constrictions of the intestine)
• Segmentation mixes food with digestive juices and increases the efficiency of
absorption by repeatedly moving different parts of the food mass over the intestinal wall
– Chemical digestion is a series of catabolic steps in which complex food
molecules are broken down to their chemical building blocks by enzymes
secreted into the lumen of the alimentary canal
– Absorption is the passage of digested end products (plus vitamins, minerals,
and water) from the lumen of the GI tract through the mucosal cells by active and
passive transport into the blood or lymph
• The small intestine is the major absorptive site
– Defecation eliminates indigestible substances from the body via the anus as
feces
GASTROINTESTINAL TRACT
ACTIVITIES
PERISTALSIS
PERISTALSIS
• (a): Peristalsis: adjacent
segments of the intestine (or
other alimentary tract organs)
alternately contract and relax,
which moves food along the
tract distally
• (b): Segmentation:
nonadjacent segments of the
intestine alternately contract
and relax, moving the food
now forward and then
backward
– Results in food mixing rather
than food propulsion
Basic Functional Concepts
• The lumen (cavity) of the GI tract is an
area that is actually outside the body, and
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
Sensors (mechanoreceptors and
chemoreceptors) involved in controls
of GI tract activity are located in the
walls of the tract organs
These sensors respond to several
stimuli; the most important are
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
When stimulated, these receptors
initiate reflexes that:
–
–
1.Activate or inhibit glands that secrete
digestive juices into the lumen or
hormones into the blood
2.Mix lumen contents and move them
along the tract by stimulating smooth
muscle of the GI tract walls
Basic Functional Concepts
•
•
•
•
Controls of digestive activity are
both extrinsic and intrinsic
Many of the controlling systems of the
digestive tract are intrinsic—a product
of local nerve plexuses (short reflexes)
or local hormone-producing cells
Long reflexes are initiated by stimuli
arising inside or outside the GI tract
and involve CNS centers and extrinsic
autonomic nerves
The stomach and small intestine also
contain hormone-producing cells that,
when appropriately stimulated, release
their products to the extracellular
space
–
These hormones are distributed via
blood and interstitial fluid to their target
cells in the same or different tract
organs, which they prod to secrete or
contract
NEURAL REFLEX PATHWAYS
THE DIGESTIVE SYSTEM
• The digestive system creates an optimal
internal environment for its functioning in
the lumen of the GI tract, an area that is
technically outside of the body
– Digestive activities within the GI tract are
triggered by mechanical and chemical stimuli
– Controls of the digestive activity are both
extrinsic and intrinsic (nervous and hormonal)
Digestive System Organs:
Relationship and Structural Plan
• Relationship of Digestive Organs to the Peritoneum
– Most digestive system organs reside in the abdominopelvic
cavity
– All ventral body cavities are lined by serous membranes
• The peritoneum of the abdominopelvic cavity is lined by a slippery
serous membrane
– The visceral peritoneum covers the external surfaces of most of the
digestive organs, and the parietal peritoneum lines the body wall of
the abdominopelvic cavity
– Peritoneal cavity is located between the visceral and parietal
peritoneums and is filled with serous fluid
– The serous fluid lubricates the mobile digestive organs, allowing them
to glide easily across one another and along the body wall as they carry
out their digestive activities
Digestive System Organs:
Relationship and Structural Plan
• (a): Mesentery is a double
layer of peritoneum (a sheet
of two serous membranes
fused back to back) that
extends to the digestive organs
from the body wall
(intraperitoneal or peritoneal
organs)
– It allows blood vessels,
lymphatics, and nerves
to reach the digestive
organs, and holds the
organs in place as well
as stores fat
Digestive System Organs:
Relationship and Structural Plan
• (b): not all alimentary
canal organs are
suspended by a
mesentery
(retroperitoneal organs)
– Some parts of the
small intestine adhere
to the dorsal
abdominal wall
(pancreas and parts of
the large intestine)
PERITONEUM
HOMEOSTATIC IMBALANCE
• Peritonitis: inflammation of the
peritoneum
– The peritoneal coverings tend to stick
together around the infection site
• Localizes the infection, providing time for
macrophages to attack to prevent the inflammation
from spreading
• If widespread, it is dangerous and often lethal
The Splanchnic Circulation
• The splanchnic circulation serves the
digestive system and includes those arteries that
branch off the abdominal aorta to serve the
digestive organs and the hepatic portal
circulation
– Normally receives ¼ of the cardiac output
• Increases after a meal
– 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
Mucosa
•
The innermost, moist, epithelial
membrane that line the entire digestive
tract from mouth to anus
•
•
•
•
It secretes mucus, digestive enzymes, and
hormones
Absorbs digestive end products into the blood
Protects against infectious disease
Digestive mucosa consists of three
sublayers:
–
1. Lining epithelium
•
•
•
–
2. Lamina propria
•
•
•
–
Rich in mucus-secreting goblet cells
Slippery mucus protects certain digestive
organs from being digested themselves by
enzymes working within their cavities and eases
food passage along the tract
Stomach and small intestine secretes both
enzymes and hormones (kind of endocrine
gland)
Connective tissue
Capillaries nourish the epithelium and absorb
digested nutrients
Lymph nodes: mucosa-associated lymphatic
tissue (MALT) helps defend us against bacteria
and other pathogens
3. Muscularis mucosae
•
Scant layer of smooth muscle cells that
produces local movements of the mucosa
Histology of the Alimentary Canal
Submucosa
• Moderately dense
connective tissue layer
containing blood and
lymphatic vessels,
lymphoid follicles, and
nerve fibers
• Its extensive vascular
network supplies
surrounding tissues of the
GI tract wall
Histology of the Alimentary Canal
Muscularis Externa
• Typically consists of smooth
muscle and is responsible
for peristalsis and
segmentation
• Typically has an inner circular
layer and an outer longitudinal
layer of smooth muscle
• In several places, the circular
layer thickens, forming
sphincters, that act as valves
to prevent backflow and control
food passage from one organ
to the next
Histology of the Alimentary Canal
Serosa
• Protective outer
layer of the
intraperitoneal
organs
• It is the visceral
peritoneum
FOUR BASIC LAYERS OF THE
ALIMENTARY WALL
Enteric Nervous System
of the
Alimentary Canal
• The alimentary canal has its own nerve
supply made up of enteric neurons that
communicate widely with each other to
regulate digestive activity
– Intrinsic nerve plexuses found in the walls of the
alimentary canal
• Submucosal nerve plexus occupies the submucosa and
chiefly regulates the activity of glands and smooth muscle in
the mucosa
– Linked to the CNS by afferent visceral fibers and
by sympathetic and parasympathetic branches
• In general, parasympathetic inputs enhance secretory activity
and motility, whereas sympathetic impulses inhibit digestive
activities
Mouth
• Mouth is the only part of
the alimentary canal
involved in ingestion
• Mouth ( oral cavity,
buccal cavity) is a
stratified squamous
epithelial mucosa-lined
cavity with boundaries of
the lips, cheeks, palate,
and tongue
Mouth
• The lips (labia) and cheeks
have a core of skeletal muscle
covered externally by skin that
helps to keep food between
the teeth when we chew and
plays a small role in speech
• The palate forms the roof of
the mouth and has two
parts: the hard palate
anteriorly and the soft palate
posteriorly
– Projecting posteriorly from the
soft palate is the uvula
• Rises reflexively to close off
the nasopharynx when we
swallow
Tongue
• The tongue is made of
interlacing bundles of
skeletal muscle and is
used to reposition food
(bolus) when chewing,
mix food with salvia,
initiate swallowing, and
help form consonants for
speech
– Lingual frenulum: secures
the tongue to the floor of
the mouth and limits
posterior movements of the
tongue
ORAL CAVITY
Tongue
• Filiform papillae give the
tongue surface a
roughness that provides
friction for manipulating
food
– They contain keratin, which
stiffens them and gives the
tongue its whitish
appearance
• Fungiform and
circumvallate papillae
contain taste buds
TONGUE
Salivary Glands
• Salivary glands
produce saliva,
which cleanses the
mouth, dissolves food
chemicals so that
they can be tasted,
moistens food, and
contains chemicals
that begin the
breakdown of
starches
Salivary Glands
• Most saliva is produced
by extrinsic salivary
glands that lie outside the
oral cavity and empty
their secretions into it
• Their output is
augmented slightly by
small intrinsic salivary
glands, also called buccal
glands, scattered
throughout the oral cavity
mucosa
Salivary Glands
• Salivary glands are composed of two
types of secretory cells:
– Serous cells produce a watery secretion
containing enzymes, ions, and a tiny bit of
mucin (glycoprotein that forms a slimy
solution in water)
– Mucous cells produce mucus, a stringy,
viscous solution
Salivary Glands
• Parotid glands:
– Paired
– Lie anterior to the ear
between the masseter
muscle and the skin
– Opens into the
vestibule next to the
second upper molar
– Only serous cells
Parotid Duct
HOMEOSTATIC IMBALANCE
• Mumps:
– Inflammation of the parotid glands caused by
the mumps virus (myxovirus)
– Spreads from person to person in saliva
– In adult males, there is a 25% risk that the
testes may become infected leading to sterility
Salivary Glands
• Submandibular gland:
– Paired
– Lies along the medial
aspect of the mandibular
body on each side
– Duct runs beneath the
mucosa of the oral cavity
floor and opens at the base
of the lingual frenulum
– Equal numbers of serous
and mucous cells
Salivary Glands
• Sublingual gland:
– Paired
– Lies anterior to the
submandibular gland under
the tongue
– Opens via 10-12 ducts into
the floor of the mouth
– Contains mostly mucous
cells
• Numerous minor glands
are located in the oral
cavity
SALIVARY GLANDS
SALIVARY GLANDS
Saliva
•
•
•
•
97-99.5% water
Hypo-osmotic
Slightly acidic (pH 6.75-7.00)
Contains:
– Solutes include electrolytes: Na+, K+, Cl-, PO4-, and HCO3– Digestive enzyme: salivary amylase
– Protein mucin: forms thick mucus when dissolved in water
• Lubricates the oral cavity and hydrates food
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Lysozyme: bacteriostatic enzyme capable of destroying certain bacteria
IgA: immunoglobulin G; antibodies
Metabolic waste: urea and uric acid
Defensins: produced by WBC to destroy bacteria
• Friendly bacteria that live on the back of the tongue convert
food-derived nitrates into nitrites which are converted into
nitric acid (bactericidal)
Control of Salivation
• Controlled primarily by the parasympathetic
division of the autonomic nervous system
• Chemoreceptors and pressoreceptors in the
mouth send signals to the salivatory nuclei in the
brain stem (pons and medulla)
• Parasympathetic nervous system activity
increases and impulses sent via motor fibers in
the facial (VII) and glossopharyngeal (IX) nerves
trigger a increase output of watery (serous),
enzyme-rich saliva
• Sympathetic nervous system causes release
of a thick mucin-rich saliva
Teeth
• Teeth lie in sockets (alveoli) in the gumcovered margins of the mandible and maxilla
• We masticate, or chew, by opening and closing
our jaws and moving them from side to side
while continually using our tongue to move the
food between our teeth
• The teeth tear and grind food, breaking it into
smaller pieces
Dentition
• Generally, all the teeth of the permanent
dentition but the third molars have erupted
by the end of adolescence
– The third molars (wisdom teeth), emerge between the
ages of 17 and 25 years
• There are usually 32 permanent teeth in a full
set, but sometimes the wisdom teeth never
erupt or are completely absent
HOMEOSTATIC IMBALANCE
• When a tooth remains embedded in the
jawbone, it is said to be impacted
• Impacted teeth can cause a good deal of
pressure and pain and must be removed
surgically
• Wisdom teeth are most commonly
impacted
Dentition
•
Teeth are classified according
to their shape and function:
– Incisors:
• Chisel-shaped
• Adapted for cutting or nipping off
pieces of food
– Canines: cuspids or eyeteeth
• Conical or fanglike
• Tear and pierce
– Premolars: bicuspids
• Broad crowns with rounded cusps
(tips)
• Best suited for grinding or
crushing
– Molars: four or five cusps
• Broad crowns with rounded cusps
(4-5)
• Best grinders
TEETH
Dental Formula
• Shorthand way of indicating the numbers and
relative positions of the different types of teeth in
the mouth
• Written as a ratio, uppers over lowers, for ½ of
the mouth
– Since the other side is a mirror image, the total
dentition is obtained by multiplying the dental formula
by 2
• The permanent dentition (two incisors, one
canine, two premolars, and three molars)
– 2I,1C,2PM,3M X (32 teeth)
– 2I,1C,2PM,3M
Tooth Structure
•
•
Five major regions:
1. Crown:
– Exposed part of the tooth above
the gingiva (gum), which
surrounds the tooth like a tight
collar
• Where the gingiva borders on a
tooth, It dips downward to form a
shallow groove called the gingival
sulcus
– In youth, the gingiva adheres
tenaciously to the enamel
covering the crown
– As the gums begin to recede with
age, the gingiva adheres to the
more sensitive cementum
covering the superior region of
the root
– As a result, the teeth appear to
get longer in old age
– Outer surface is enamel
Tooth Structure
•
Enamel: an acellular, brittle material
that directly bears the force of chewing
–
–
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Hardest substance in the body (96%
calcium salts)
Heavily mineralized with calcium
salts (calcium carbonate)
Densely packed hydroxyapatite
(mineral) crystals (form of calcium
phosphate) are oriented in forceresisting columns perpendicular to
the tooth’s surface
•
•
–
The cells that produce enamel
degenerate when the tooth erupts
•
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Soluble in the acids of soft drinks
Becomes decay-resistant fluoroapatite
after combing with fluoride ions present
in fluoridated water or fluoride
toothpastes
Any decayed or cracked areas of the
enamel will not heal and must be
artificially filled
Proteins amelogenins and enamelins
(4%): role not fully understood but
believed to aid in framework and
support
Tooth Structure
•
2. Root:
– Portion of the tooth embedded in
the jawbone
– Canine, incisors, and premolars
have one root
• First two upper premolars have
two roots (upper jaw)
– Molars:
• First two upper have three roots
• Lower have two roots
• Third molars vary: usually one
root
– Cementum: outer surface of the
root
• Calcified connective tissue
• Attaches the tooth to the thin
periodontal ligament
– Anchors the tooth in the bony
alveolus of the jaw, forming a
fibrous joint called a gomphosis
Tooth Structure
•
3. Neck:
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•
Connects the crown and root
4. Dentin:
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Bonelike material
Underlines the enamel cap and forms the bulk
of a tooth
Dentinal tubules: each tubule contains cell
types that secretes and maintains the dentin
(odontoblast)
•
•
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Formed throughout adult life
New dentin can be formed rapidly to
compensate for tooth damage or decay
Softer (less brittle) than enamel since less
mineralized (75% of enamel)
Surrounds a central pulp cavity
•
Contains a number of soft tissue structures
(pulp)
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–
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Connective tissue, blood vessels, nerve
fibers
Supplies nutrients to the tooth tissues and
provides for tooth sensation
5. Root canal:
•
At the proximal end of each root canal is an
apical foramen that provides a route for
blood vessels, nerves, and other structures
to enter the pulp cavity of the tooth
–
Trigeminal nerve and maxillary artery
HOMEOSTATIC IMBALANCE
• Death of a tooth’s nerve and consequent
darkening of the tooth is commonly caused by a
blow to the jaw
• Swelling in the local area pinches off the blood
supply to the tooth and the nerve dies
• Typically the pulp becomes infected by bacteria
some time later and must be removed by root
canal therapy
• After the cavity is sterilized and filled with an
inert material, the tooth is capped
Tooth Structure
• Although enamel, dentin, and cementum
are all calcified and resemble bone, they
differ from bone in that they are
avascular
• Enamel also differs from cementum
and dentin because it lacks collagen as
its main organic component
Tooth and Gum Disease
•
Dental caries (cavities):
– Result from gradual demineralization of enamel and underlying dentin by
bacterial action
– Begins when dental plaque (film of sugar, bacteria, and other mouth debris)
adheres to the teeth
• Bacterial metabolism of the trapped sugars produces acids, which can dissolve the
calcium salts of the teeth
• Once the salts are leached out, the remaining organic matrix of the tooth is readily
digested by protein-digesting enzymes released by the bacteria
•
Calculus: tartar
– Plaque on gums
– Disrupts the seals between the gingivae and the teeth, putting the gums at risk
for infection
– Gingivitis: early stages of infection
• Gums red, swollen, bleeding
• If neglected: periodontal disease (periodontitis)
– The bacteria invade the bone around the teeth forming pockets of infection (dissolving bone)
– Accounts for 80-90% of tooth loss in adults
– Concerns about its relationship to heart disease
» Bacteria in the blood stimulating clot formations that enter the heart coronary circulation
CANINE TOOTH
Pharynx
• From the mouth, food passes
posteriorly into the oropharynx,
and then the laryngopharynx
• The pharynx (oropharynx
and laryngopharynx)
provides a common
passageway for food, fluids,
and air (nasopharynx has no
digestive role)
• Constriction of the pharyngeal
muscles propels food into the
esophagus
Esophagus
• (a): Muscular tube collapses
when not involved in food
propulsion
• (b): Arrow shows the point of
abrupt transition from the
stratified squamous epithelium
of the esophagus (top) to the
simple columnar epithelium of
the stomach (bottom)
• The submucosa contains
mucus-secreting esophageal
glands
– As a bolus moves through the
esophagus, it compresses
these glands, causing them to
secrete mucus that lubricates
the esophageal walls and aids
food passage
ESOPHAGUS
Esophagus
• Takes a fairly straight course
through the mediastinum of the
thorax pierces the diaphragm
at the esophageal hiatus to
enter the abdomen
• After food moves through
the laryngopharynx, it is
routed into the esophagus
posteriorly as the epiglottis
closes off the larynx to food
entry
• The esophagus provides a
passageway for food and fluids
from the laryngopharynx to the
stomach where it joins at the
cardiac orifices
Esophagus
• Joins the stomach at
the cardiac
(gastroesophageal)
sphincter which is a
physiological sphincter
– Acts as a valve
– A slightly thicken circular
muscle
• The muscular diaphragm,
which surrounds this
sphincter, helps keep it
closed when food is not
being swallowed
HOMEOSTATIC IMBALANCE
• Heartburn:
– First symptom of gastroesophageal reflux disease
(GERD)
– Burning, radiating substernal pain that occurs when
the acidic gastric juice regurgitates into the
esophagus
• Hiatal hernia:
– Structural abnormality in which the superior part of
the stomach protrudes slightly above the diaphragm
• Since the diaphragm no longer reinforces the cardiac
sphincter, gastric juice may flow into the esophagus
Digestive Processes Occurring in the Mouth
• Ingestion of food
• Begins mechanical digestion by chewing
initiates propulsion by swallowing
• Initiates the chemical digestion of
carbohydrates (polysaccharides: starch and
glycogen) by enzyme salivary amylase in
saliva
– Hydrolysis into smaller fragments of linked glucose
molecules
– Except for a few drugs that are absorbed through the
oral mucosa (nitroglycerine), essentially no absorption
occurs in the mouth
Digestive Processes Occurring in the
Pharynx and Esophagus
• Merely serve as conduits to pass food
from the mouth to the stomach
• Their single digestive function is food
propulsion, accomplished by the role they
play in swallowing
Mastication (Chewing)
• Mastication, or chewing, begins the
mechanical breakdown of food and
mixes the food with saliva
• Partly voluntary:
– We put food in our mouth
– We contract muscles
• Partly reflexive:
– Response to stretch and pressure receptors
in cheeks, gums, and tongue
Deglutition (Swallowing)
•
•
Involves coordination of over 22
separate muscle groups
Complicated process that involves two
major phases:
–
–
–
1. The buccal phase (a) is voluntary
and occurs in the mouth where the
bolus is forced into the oropharynx
Food enters the pharynx and stimulates
tactile receptors passing out of our
control and into the realm of involuntary
reflex activity
2. The pharyngeal-esophageal phase is
involuntary and occurs when food is
squeezed through the pharynx and into
the esophagus
•
Controlled by the swallowing center
located in the medulla and lower pons
–
Motor impulses are transmitted via
various cranial nerves, most importantly
the vagus nerve, to the muscles of the
pharynx and esophagus
DEGLUTITION
Deglutition (Swallowing)
• (b): Once food enters the
pharynx, all routes except
the desired one—into the
digestive tract—are blocked
off:
– Tongue blocks the mouth and
presses on the epiglottis
– Soft palate rises to close off
the nasopharynx (uvula)
– Larynx rises so that the
epiglottis covers its opening
into the respiratory
passageways, and the upper
esophageal sphincter relaxes
(b)
Deglutition (Swallowing)
• Food is squeezed through
the pharynx and into the
esophagus by wavelike
muscular peristaltic
contractions (c-e)
• Upper esophageal sphincter
contracts (c)
• (d): food is moved through
the esophagus by peristalsis
• (e):Just before the peristaltic
wave (and food) reaches the
end of the esophagus, the
gastroesophageal sphincter
relaxes reflexively to allow
food to enter the stomach
DEGLUTITION
Stomach
• Temporary storage
• Initiation of chemical
breakdown of proteins
• Food is converted to a
creamy paste called
chyme
• Lies in the left
hypochondriac,
epigastric, and umbilical
regions of the abdomen
ABDOMINAL REGIONS
Stomach
• The stomach is a temporary
storage tank where the
chemical breakdown of
proteins is initiated and food
is converted to chyme
• The adult stomach varies from
15-25 cm long, but its diameter
and volume vary depending on
the amount of food it contains
– The major regions of the
stomach include the cardiac
region, fundus, body, and the
pyloric region
• Pyloric sphincter: controls
stomach emptying
Stomach
• The convex lateral
surface of the
stomach is its greater
curvature, and its
concave medial
surface is its lesser
curvature
– Extending from these
curvatures are two
mesenteries (omenta)
Stomach
•
•
Lesser omentum: runs from the
liver to the lesser curvature of the
stomach where it becomes
continuous with the visceral
peritoneum covering the stomach
Greater omentum:
– Drapes inferiorly from the greater
curvature of the stomach to cover
the coils of the small intestine
– Dorsally and ventrally wraps the
spleen, transverse portion of large
intestine, blending with the
mesocolon (secures colon to
parietal peritoneum of the
posterior abdominal wall)
– Riddled with fat deposits giving it
an appearance of a lacy apron
– Contains large deposits of lymph
nodes (protecting the peritoneal
cavity and intraperitoneal organs)
Stomach
• Served by the
autonomic nervous
system:
– Sympathetic fibers
from the thoracic
splanchnic nerve
relayed through the
celiac plexus
– Parasympathetic
fibers are supplied by
the vagus nerve
SYMPATHETIC NERVOUS
SYSTEM
PARASYMPATHETIC NERVOUS
SYSTEM
Stomach
• Arterial supply
provided by branches
(gastric and splenic)
of the celiac trunk
• Veins are part of the
hepatic portal system
and ultimately drain
into the hepatic
portal vein
ARTERIES OF THE ABDOMEN
VEINS OF THE ABDOMEN
STOMACH
Stomach
Microscopic Anatomy
•
Wall contains the four tunics
typical of most of the alimentary
canal ( mucosa, submucosa,
muscularis externa, adventita), but
its muscularis and mucosa are
modified for the special roles of
the stomach
– Mucosa lining is folded (rugae)
•
Besides the usual circular and
longitudinal layers of smooth
muscle, the muscularis externa
has an innermost smooth
muscle layer that runs obliquely
– This arrangement allows the
stomach not only to move food
along the tract, but also to churn,
mix, and pummel the food,
physically breaking it down into
smaller fragments
Stomach
Microscopic Anatomy
• The surface epithelium of the
stomach mucosa is a simple
columnar epithelium
composed of goblet cells,
which produce a protective
two-layer coat of alkaline
mucus in which the surface
layer consists of viscous
mucus that traps a layer of
bicarbonate-rich fluid beneath
it
• Mucosa is also dotted with
millions of gastric pits which
lead to gastric glands that
produce the stomach secretion
called gastric juice
Stomach
Microscopic Anatomy
Gastric Glands
• Cardiac and pyloric
regions primarily secrete
mucus
• Pyloric antrum secrete
mucus and several
hormones (gastrin)
• Fundus and body
regions secrete:
– Most chemical digestion
– Majority of secretions
– Variety of Secretory
Cells: Mucous, Parietal,
Chief, Enteroendocrine
Gastric Glands
Secretory Cells
•
Mucous neck cells: upper neck
region of the gland
–
•
Produces a different type of mucus
(acidic) secreted by the goblet cells
of the epithelium
Parietal cells: middle region of the
gland
–
–
Scattered among the chief cells
Secrete HCl: makes stomach contents
extremely acidic (pH 1.5-3.5)
•
•
•
•
•
Condition necessary for activation
and optimal activity of pepsin
Harsh enough to kill many of the
bacteria ingested with foods
Denatures proteins
Breaks down cell walls of plants
Chief cells: basal region of the gland
–
Pepsinogen: the inactive form of the
protein-digesting enzyme pepsin
•
–
Activated by HCl
Lipases: small amount of fat-digesting
enzymes
Gastric Glands
Secretory Cells
•
Enteroendocrine cells ( G cells):
– Secrete a variety of hormone or
hormonelike products directly
into the blood and influence
several digestive system target
organs
• Gastrin: hormone
– Stimulates gastric acid secretion
– Causes the lower esophageal
sphincter to contract and the
ileocecal valve to relax
• Histamine: hormonelike
– Dilation of blood vessels
• Endorphins (natural opiates):
– Hormonelike
– Decreasing sensation of pain
• Serotonin: hormone
– Vasoconstrictor
• Cholecystokinin: hormone
– Stimulates contraction of gall
bladder and pancreas
• Somatostatin: hormonelike
– Inhibits gastric acid secretion
Stomach Mucosa
• Exposed to some of the harshest conditions in the
entire digestive tract
• Gastric juice is corrosively acidic and its proteindigesting enzymes can digest the stomach itself
• Produces a mucosal barrier to protect itself:
– 1. Thick coating of bicarbonate-rich mucus
– 2. Epithelial cells joined together by tight junctions that prevent
gastric juice from leaking into the underlying tissue layers
– 3. Deep in the gastric glands, where the protective alkaline
mucus is absent, the plasma membranes are impermeable to
HCl
– 4. Damaged epithelial cells are shed and replaced by
division of undifferentiated stem cells
MICROSCOPIC ANATOMY OF STOMACH
HOMEOSTATIC IMBALANCE
• Gastritis: anything that breaches the gel-like mucosal barrier
causes inflammation of the stomach wall
– Persistent damage to the underlying tissues can promote gastric ulcers,
erosions of the stomach wall
• Danger posed by ulcers is perforation of the stomach wall followed by
peritonitis (inflammation of the membranes that line the abdominal cavity)
and, perhaps, massive hemorrhage
• Causes:
–
–
–
–
–
–
–
Hypersecretion of HCl
Hyposecretion of mucus
Aspirin: lipid soluble (absorbed easily through mucosa)
Coffee
Alcohol: lipid soluble (absorbed easily through mucosa)
Stress
(90%): Acid-resistant bacteria: Helicobacter pylori
» Burrow beneath the mucus and destroy the protective mucosal layer leaving
denuded areas
» Release several chemical
» PROBLEM: also found in healthy people and might be linked to cancer
Digestive Processes Occurring in
the Stomach
• Protein digestion is initiated in the stomach and
is essentially the only type of enzymatic
digestion that occurs there
• Proteins are denatured by HCl produced by
stomach glands in preparation for enzymatic
digestion
• Only function essential for life is the secretion of
intrinsic factor:
– required for intestinal absorption of vitamin B12
• Needed to produce mature erythrocytes
– In its absence: pernicious anemia results
Regulation of Gastric Secretion
• Gastric secretion is controlled by both neural
and hormonal mechanisms
– Nervous control is provided by long (vagus nervemediated) and short (local enteric) nerve reflexes
• Parasympathetic nervous system stimulates secretion of the
glands via the cranial nerve (vagus)
• Sympathetic nervous system depresses secretory activity
– Hormonal control of gastric secretion is largely
controlled by gastrin which stimulates secretion of
enzymes and HCl, and of hormones produced by the
small intestine, which are mostly gastrin antagonists
Phases of Gastric Secretion
• Three distinct phases:
– Cephalic
– Gastric
– Intestinal
Neural and hormonal mechanism that
regulates release of gastric juice
Cephalic Phase
•
•
•
•
•
•
•
Reflex phase
Only a few minutes
Gastric secretion occurs before
food enters the stomach
Triggered by aroma, taste, sight,
or thought of food
Senses stimulate the
hypothalamus which in turn
stimulates the vagal nuclei of the
medulla oblongata, causing motor
impulses to be transmitted via the
vagus nerves to parasympathetic
enteric ganglia resulting in
stimulation of the stomach glands
Conditioned reflex
Depression or lack of appetite
suppresses the cephalic reflex
Gastric Phase
• Initiated by local neural
and hormonal
mechanisms once food
reaches the stomach
• 3-4 hours
• Provides 2/3 of gastric
juice
• Stretch receptors
stimulate additional
neural responses
Gastric Phase
•
•
•
Hormone gastrin (secreted by
enteroendocrine: G cells) plays
a greater role in stimulating
gastric juice secretion
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 gastrinsecreting cells
– This negative feedback
mechanism helps maintain
optimal pH and working
conditions for the gastric
enzymes
Gastric Phase
•
•
•
Binding of histamine, gastrin, and
acetylcholine (Ach) to parietal cell
membrane receptors initiates
intracellular events (mediated by
second-messenger systems) that lead
to HCl secretion into the stomach
lumen
H+ and HCO3- (bicarbonate ions) are
generated from the dissociation of
carbonic acid (H2CO3)
As H+ / K+ ATPase pumps H+ into the
lumen, K+ enters the cell, HCO3- is
pumped into the interstitial space in
exchange for chloride ions (Cl-)
–
•
Cl- is obtained from blood plasma
Cl- and K+ then diffuse into the lumen
through membrane channels
Regulation and mechanism of HCl secretion
Gastric Phase
• G cells (enteroendocrine
cells) that produce gastrin
are activated by both:
– Chemical (amounts of
proteins, HCl, gastrin etc.) in
negative feedback mechanism
– Neural reflexes stimulate or
inhibit gastrin/gastric juice
production
• Inhibitory:
– 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
Neural and hormonal mechanism that
regulates release of gastric juice
Regulation and mechanism of HCl secretion
Intestinal Phase
Excitatory
• Two components—one excitatory and the
other inhibitory
– Excitatory:
• Set into motion as partially digested food fills the initial part
(duodenum) of the small intestine
• Stimulates intestinal mucosal cells to release a hormone
(intestinal gastrin) that encourages the gastric glands to
continue to secrete gastric juice
• This stimulatory effect is brief because as the intestine
distends with chyme containing large amounts of H+, fats,
partially digested proteins, and various irritating substances,
the inhibitory component is triggered in the form of the
enterogastric reflex
Intestinal Phase
Inhibitory
• Enterogastric reflex is actually a trio of reflexes that:
– 1. Inhibit the vagal nuclei in the medulla
– 2. Inhibit local reflexes
– 3. Activate sympathetic fibers that cause the pyloric sphincter to
tighten and prevent further food entry into the small intestine
• Gastric secretory activity declines
– Protects the small intestine from excessive acidity
• Triggers the release of several intestinal hormones
(enterogastrones: inhibits gastric juice secretion and
stimulates intestinal secretions)
–
–
–
–
Secretin
Cholecystokinin (CCK)
Vasoactive intestinal peptide (VIP)
Gastric inhibitory peptide (GIP)
Neural and hormonal mechanism that
regulates release of gastric juice
Peristaltic Waves in the Stomach
•
•
•
(a): moves toward the pylorus
(b): most vigorous peristalsis and mixing action occurs close to the pylorus
(c): pyloric end of the stomach acts as a pump that delivers small amounts
of chyme into the duodenum, simultaneously forcing most of its contained
material backward into the stomach, where it undergoes further mixing (3ml
of chyme squirts through at any given moment and the valve closes
propelling the stomach contents backward for further mixing) (back and
forth pumping effectively breaks up solids in the gastric contents)
Peristaltic Rate
• Constant rate: 3 waves per minute
– Can be modified
• Set by the spontaneous activity of pacemaker cells
located in the longitudinal smooth muscle layer
(interstitial cells of Cajal)
– Establish the stomach’s basic electrical rhythm of peristaltic
waves
– Depolarize and repolarize spontaneously three times each
minute
• The rate at which the stomach empties is determined
by both the contents of the stomach and the
processing that is occurring in the small intestine
Regulation of Gastric Emptying
• Usually 4 hours after a meal
– Liquid faster
– Solids slower
• Fatty foods slower
• Depends on the contents of the duodenum
as on what is happening in the stomach
PERISTALTIC WAVES
Neural and hormonal factors
Inhibiting Gastric Emptying
• Controls ensure that
the food will be well
liquefied in the
stomach and
prevent the small
intestine from being
overwhelmed
Neural and hormonal factors
inhibiting gastric emptying
Small Intestine and Associated Structures
• The small intestine is the site of the completion of
digestion and absorption of nutrients:
– These vital functions cannot be accomplished without the
aid of secretions from the liver (bile) and pancreas
(digestive enzymes)
– It extends from the pyloric sphincter to the ileocecal valve where
it joins the large intestine
– It has three subdivisions: the duodenum, the jejunum, and
the ileum
– It is highly adapted for absorption with three microscopic
modifications: plicae circulares, villi, and microvilli
– The intestinal crypts, or the crypts of Lieberkuhn, secrete
intestinal juice that serves as a carrier fluid for absorbing
nutrients from chyme
DUODENUM
Small Intestine
• Body’s major digestive
organ
• Virtually all absorption
• A convoluted tube
extending from the pyloric
sphincter in the epigastric
region to the ileocecal
valve in the right iliac
region where it joins the
large intestine
ORGANS OF THE ALIMENTARY CANAL
Small Intestine
Small Intestine
• Longest part of the alimentary tube, but is
only about half the diameter of the large
intestine, ranging from 2.5 to 4cm (1-1.6 in)
– In a cadaver: 6-7 m long (approximately 20 feet)
– In a living human: 2-4 m (approximately 8-13 feet)
• Has three subdivisions:
– Duodenum: which is mostly retroperitoneal (space
behind the peritoneum
– Jejunum: intraperitoneal (within the peritoneal cavity)
– Ileum: intraperitoneal (within the peritoneal cavity)
Duodenum
• About 25 cm (10 inches)
long
• It is the shortest intestinal
subdivision
• Hepatopancreatic ampulla:
union of two ducts which
opens into the duodenum via
the major duodenal papilla
(secretions controlled by the
hepatopancreatic sphincter)
– Bile duct delivers bile from the
liver
– Pancreatic duct delivers
pancreatic juice from the
pancreas
Jejunum
• 2.5 m (8 feet) long
• Extends from the
duodenum to the
ileum
Ileum
• Approximately 3.6 m
(12 feet) in length
• Joins the large
intestine at the
ileocecal valve
• Both the jejunum and
ileum suspend from
the posterior
abdominal wall by the
fan-shaped
mesentery
ORGANS OF THE ALIMENTARY CANAL