Transcript GIT 323

PHYSIOLOGY OF THE
DIGESTIVE SYSTEM
FUNCTIONAL ANATOMY OF
THE GIT
DR. AMEL EASSAWI
DR. Sharique Ahmed Quadri
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OBJECTIVES
To know the components of GIT and their
functional significance.
Emphasize the functional importance of four
layers of GIT.
Outline four basic digestive processes.
Recognize the importance of regulatory factors
that controls digestive functions.
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DIGESTIVE SYSTEM
• Digestive Tract
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•
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•
•
Mouth
Pharynx
Esophagus
Stomach
Small intestine
– Duodenum
– Jejunum
– Ileum
• Accessory Digestive
Organs
– Salivary glands
– Exocrine pancreas
– Biliary system
• Liver
• Gallbladder
• Large intestine
– Cecum
– Appendix
– Colon
– Rectum
• Anus
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DIGESTIVE SYSTEM
• Digestive tract is 4.5 m (15 feet) in normal
contractile state.
• Lumen is continuous from mouth to anus and
hence is continuous with external environment.
• Conditions that are essential for digestive
process can be tolerated in the digestive tract
lumen that could not be tolerated in body proper.
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DIGESTIVE SYSTEM
Primary Function:
– Transfer nutrients, water, and electrolytes from
ingested food into body’s internal environment
The Digestive System Performs Four Functions:
– Motility
– Secretion
– Digestion
– Absorption
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FUNCTIONS OF THE DIGESTIVE
SYSTEM
Motility:
– Muscular contractions that mix and
move forward the contents of the
digestive tract
Two Types of Digestive Motility:
• Propulsive (peristalsis) movements
• Mixing (segmentation) movements
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FUNCTIONS OF THE DIGESTIVE
SYSTEM
Propulsive Movements:
– Push contents forward through the digestive tract
– Velocity with which contents are moved forward
(rate of propulsion) varies in different regions of
GIT, depending on functions of that region.
» Rapid movement in esophagus
» Slow movement in small intestine
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FUNCTIONS OF THE DIGESTIVE
SYSTEM
– Movements of contents through most of digestive
tract is accomplished by contraction of smooth
muscle except:
» Mouth
» Early part of esophagus
» External anal sphincter
– In these regions motility involves skeletal muscle
(voluntary component)
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FUNCTIONS OF THE DIGESTIVE
SYSTEM
Mixing Movements:
– Serve Two Functions:
» Mixing food with digestive juices & hence
promotes digestion of foods.
» Facilitates absorption by exposing all parts of
intestinal contents to absorbing surfaces of
digestive tract.
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Segmentation
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FUNCTIONS OF THE DIGESTIVE
SYSTEM
Secretions:
– Digestive juices are secreted in to GIT lumen
by exocrine glands.
– Digestive secretion consist of:
• Water
• Electrolytes
• Specific organic constituents(enzymes, bile
salts, or mucus) important in digestive
process.
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FUNCTIONS OF THE DIGESTIVE
SYSTEM
– Secretions are released into the digestive tract
lumen on appropriate neural or hormonal
stimulation.
– Normally reabsorbed in one form or another back
into blood after their participation in digestion.
Failure of reabsorption of digestive juices , as
in diarrhea & vomiting results in loss of fluid
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FUNCTIONS OF THE DIGESTIVE
SYSTEM
Digestion:
– Biochemical breakdown of structurally complex
foodstuffs into smaller, absorbable units by
enzyme produce within GIT
– Complex foodstuffs and their absorbable units
• Carbohydrates
→ Monosaccharides
(poly &disaccharides) (Glucose,fructose,galactose)
• Proteins
→
Amino acids
• Fats
→
Glycerol and Fatty acids
(triglyceride)
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FUNCTIONS OF THE DIGESTIVE
SYSTEM
Absorption:
• In the small intestine, digestion is completed
most absorption occurs.
– Through process of digestion small
absorbable units resulting from digestion,
along with water, vitamins, and electrolytes
are transferred from digestive tract lumen into
blood or lymph.
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Anatomy and Functions of the Digestive System
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DIGESTIVE TRACT WALL
• GIT wall has same general structure throughout
length from esophagus to anus (with some local
characteristic variations)
• Four major tissue layers
– Mucosa
• Innermost layer
– Submucosa
– Muscularis externa
– Serosa
• Outer layer
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Layers of Digestive Tract Wall
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MUCOSA
• Lines luminal surface of digestive tract
• Highly folded surface greatly increases
absorptive area
• Three layers:
– Mucous membrane
– Lamina propria
– Muscularis mucosa
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MUCOSA
Mucous Membrane:
• Inner epithelial layer serves as protective surface
• Modified in particular areas for secretion and absorption.
Contains:
– Exocrine gland cells – secrete digestive juices
– Endocrine gland cells – secrete blood-borne
gastrointestinal hormones
– Epithelial cells – specialized for absorbing digestive
nutrients
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MUCOSA
Lamina Propria
• Middle layer of connective tissue on which
epithelium rest.
• Houses gut-associated lymphoid tissue (GALT)
– Important in defense against disease-causing intestinal
bacteria.
Muscularis Mucosa
• Sparse layer of smooth muscle, contraction
modifies the pattern of surface folding.
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SUBMUCOSA
• Thick layer of connective tissue.
• Provides digestive tract with distensibility and
elasticity.
• Contains larger blood and lymph vessels
• Contains nerve network known as submucosal
plexus.
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MUSCULARIS EXTERNA
• Major smooth muscle coat of digestive tube
• In most areas consists of two layers
– Circular layer
• Inner layer
• Contraction decreases diameter of lumen
– Longitudinal layer
• Outer layer
• Contraction shortens the tube
• Together contractile activity of these layers produces
propulsive and mixing movements
• Myenteric plexus
– Lies between the two muscle layers
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SEROSA
• Outer connective tissue covering of GIT
• Secretes serous fluid(watery, slippery fluid)
– Lubricates and prevents friction between
digestive organs and surrounding viscera.
• Continuous with mesentery throughout much of
the tract
– This Attachment provides relative fixation
– Supports digestive organs in proper place
while still allowing them freedom for mixing
and propulsive movements
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REGULATION OF DIGESTIVE SYSTEM
FUNCTION
• Digestive motility and secretion are carefully
regulated to optimize the digestion.
• Four factors are involved in regulating digestive
system function.
– Autonomous smooth muscle function
– Intrinsic nerve plexuses
– Extrinsic nerves
– Gastrointestinal hormones
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AUTONOMOUS SMOOTH MUSCLE
FUNCTION
• In the wall of GIT some specialized smooth
muscle cells are pacemakers cells –known as
interstitial cells of Cajal.
• These cells lie between circular & longitudinal
layer of smooth muscles.
• These are self excitable cell that displays
rhythmic spontaneous variations in membrane
potential-known as SLOW WAVE POTENTIAL
OR basic electrical rhythm (BER).
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Autonomous Smooth Muscle Function
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AUTONOMOUS SMOOTH
MUSCLE FUNCTION
• If slow wave reaches threshold, depolarization is
triggered at the peak resulting in rhythmic cycles
of contraction.
• Whether threshold will reach or not depends on
various mechanical, neural and hormonal factors
that influence starting point of slow wave (e.g.
presence of food bolus in GIT).
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AUTONOMOUS SMOOTH MUSCLE
FUNCTION
• The rate of self induced contractile activity
depends on inherent rate established by
involved pacemaker.
• The intensity of contractions depends on
number of action potentials occurring at peak of
slow wave.
• Greater the number of contraction--higher the
cytosolic calcium--stronger the contraction.
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INTRINSIC NERVE PLEXUSES
• Submucosal plexus and myentric plexus, together
often termed as enteric nervous system.
• Primarily coordinate local activity in GIT.
• Intrinsic plexus can affect all functions of digestive
tract, i.e. motility, secretion of digestive juices and
gastrointestinal hormones.
• Intrinsic nerve activity can be influence by
endocrine, paracrine and nerve signals
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EXTRINSIC NERVES
• Are through both branches of ANS
• Influence GIT motility secretion either by
– Modifying activity of intrinsic plexuses.
– Altering level of GI hormone secretion.
– Directly acting on smooth muscle and glands.
• Sympathetic inhibits the motility and secretion
and parasympathetic increases both.
• Extrinsic nervous system coordinate activity
between different regions of GIT.
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GASTROINTESTINAL
HORMONES
• Tucked within mucosa of certain regions of GIT
are endocrine gland cells that releases hormone
into blood on appropriate stimulation.
• These hormones acts on other areas of GIT and
exert either stimulatory or inhibitory influences
on smooth muscle and exocrine cells.
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Pathways Controlling GIT activities
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REFERENCES
 Human Physiology, Lauralee Sherwood, seventh
edition.
 Text book Physiology by Guyton &Hall,11th edition.
 Text book of Physiology by Linda S. Contanzo,
third edition.
 Physiology by Berne and Levy, sixth edition.
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