OVERVIEW OF DIGESTIVE
SYSTEM
Dr. Abdelrahman Mustafa
Department of Basic Medical Sciences
Division of Physiology
Faculty of Medicine
Almaarefa Colleges
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
• Mouth
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Digestive tract is 4.5m
(15 feet) in normal
contractile state.
– Duodenum
– Jejunum
– Ileum
• Large intestine
–
–
–
–
Cecum
Appendix
Colon
Rectum
• Anus
Accessory digestive organs
• Salivary glands
• Exocrine pancreas
Digestive tract is 4.5m (15 feet) in normal
• Biliary system
contractile state.
• Liver
• Gallbladder
3
Digestive System
• Primary functions
– Digestion
– Absorption
• The Digestive System has TWO processes to
Perform these functions
– Motility
– Secretion
4
Digestive Tract Layers
• 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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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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Layers of 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
Layers of 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
– Nerve 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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Processes of Digestive System
• Motility
Definition Muscular contractions that mix and move
forward the contents of the digestive tract
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Digestive Motility
Example1) :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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Types of Digestive Motility
Example 2 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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Processes of Digestive System
• Secretions:
– Digestive juices are secreted in to GIT lumen
by exocrine glands.
– Digestive secretion consist of
• Water,
• Electrolytes, and
• Specific organic constituents(enzymes,
bile salts, or mucus) important in
digestive process.
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Processes of Digestive System
• Secretions:
– Secretions are released into 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 Digestive System
• Digestion
– Biochemical breakdown of structurally complex
foodstuffs into simplified ,absorbable units by
enzyme produce within GIT
– Complex foodstuffs and their absorbable units
• Carbohydrates
→
(poly &disaccharides)
• Proteins
→
• Fats
→
(triglyceride)
Monosaccharides
(Glucose,fructose,galactose)
Amino acids
Glycerol and Fatty acids
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Functions of Digestive System
Absorption:
• In the small intestine, digestion is
completed & most absorption occurs.
– Through process of absorption 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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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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1)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 potentialknown 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(rate and
intensity )
• 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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2)Role of Intrinsic nerve plexuses
• Submucosal plexus &
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 & nerve signals
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3)Role of 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 &
glands.
• Sympathetic inhibits the motility &
secretion and parasympathetic
increases both.
• Extrinsic nervous system coordinate
activity between different regions of
GIT.
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4)Role of 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 & exert
either stimulatory or
inhibitory influences on
smooth muscle and exocrine
cells.
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Summary of Pathways Controlling GIT activities
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Q1
• The extension of Digestive tract is
A)4.5m in normal contractile state.
B) 15m in normal contractile state
C) 15m in normal relaxation state
D) 4.5m in normal relaxation state.
Q2
• Important layer in defense against diseasecausing intestinal bacteria is :
• A) Mucous membrane
• B) Lamina propria
• C) Muscularis mucosa
• D)Serousa
Q3
• Which of the following is true regarding to
propulsive movements:
A)Mix contents through the digestive tract
B)Rate of propulsion fixed in different regions of
GIT
C) Propulsive movements is rapid movement in
esophagus
D)Propulsive movements not found movement
in small intestine
Q4
•
•
•
•
•
The enteric nervous system consist of
A) Interstitial cells of Cajal.
B) Sympathetic nervous system
C) Parasympathetic nervous system
D) Submucosal plexus
References
• Human physiology by Lauralee Sherwood,
seventh edition
• Text book physiology by Guyton &Hall,11th
edition
• Text book of physiology by Linda .s
contanzo,third edition
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