OVERVIEW OF GIT FUNCTIONS
& REGULATION
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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 (GIT)
 Digestive Tract:







Mouth
Pharynx
Esophagus
Stomach
Small intestine
- Duodenum
- Jejunum
- Ileum
Large intestine
- Cecum
- Appendix
- Colon
- Rectum
Anus
 Accessory Digestive Organs:
 Salivary glands
 Exocrine pancreas
 Biliary system
- Liver
- Gall bladder
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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.
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DIGESTIVE SYSTEM
Primary Function:
 Transfer nutrients, water, and electrolytes from
ingested food into body’s internal environment.
 Food is ingested – digested – absorbed – distributed
and used.
The Digestive System Performs Four Functions:
1- Motility
2- Secretion
3- Digestion
4- Absorption
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FUNCTIONS OF THE DIGESTIVE SYSTEM
1- Motility:
 Muscular contractions that mix and move
forward the contents of the digestive tract.
 Two Types of Digestive Motility:


Propulsive (peristaltic) movements
Mixing (segmenting) movements
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Types of Digestive Motility
1- Propulsive (= Peristaltic) 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. For example:
* Rapid movements in esophagus.
* Slow movements in Small intestine.
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GIT Motility

Movements of contents through most of digestive
tract is accomplished by contraction of smooth
muscles (involuntary component) except:
1- Mouth (chewing).
2- Early part of esophagus (swallowing).
3- External anal sphincter (defecation).

In these regions, motility involves skeletal muscle
(voluntary component).
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Types of GIT Motility
2- Mixing (=segmenting) Movements:

Serve Two Functions:
1- Mixing food with digestive juices & hence promotes
digestion of foods.
2- Facilitates absorption by exposing all parts of intestinal
contents to absorbing surfaces of digestive tract.
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Segmentation
Gut law: Distension of gut leads to
peristaltic wave starting at the point
of distension & proceeds anal wards.
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FUNCTIONS OF THE DIGESTIVE SYSTEM
2- Secretions:
 Digestive juices are secreted in to GIT lumen
by exocrine glands (through ducts).
 Digestive secretions consists of:
1- Water.
2- Electrolytes.
3- Specific organic constituents (enzymes, bile
salts, or mucus) important in digestive
process.
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GIT SECRETIONS
 Secretions are released into the digestive tract lumen on
appropriate neural or hormonal stimulation.
 Normally absorbed in one form or another into blood
after their participation in digestion.
 Total quantity of fluid that is secreted from digestive
glands into GIT lumen equals 7 liters / day.
 Failure of absorption of digestive juices, as in diarrhea &
vomiting results in loss of fluid (dehydration).
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FUNCTIONS OF THE DIGESTIVE SYSTEM
3- Digestion:
 Biochemical
breakdown of structurally complex
foodstuffs into smaller, absorbable units by enzymes
produced within GIT.
 Complex food stuffs:
i). Carbohydrate
ii). Proteins
iii). Fats
These are large molecules, therefore, they are digested
and then absorbed into the blood or lymph.
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i). CARBOHYDRATES
 Carbohydrates are absorbed as monosaccharides e.g.
glucose, fructose, and galactose.
 Cellulose is a polysaccharide found in plants. It can not be
digested, therefore, works as bulk or ingestible fibers.
ii). PROTEIN
 Proteins are absorbed as
- Amino acids.
- Small polypeptides.
iii). FAT
 Fat are absorbed as
- Monoglyceride [glycerol with one fatty acid].
- Free fatty acid.
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FUNCTIONS OF THE DIGESTIVE SYSTEM
4- Absorption:
 In the small intestine, digestion is completed & most absorption
occurs. The small intestine, with its epithelial folds, villi, and
microvilli, has an internal surface area of 200 m2.
 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.
 Total quantity of fluid that must be absorbed: 2 L (ingested)
+ 7 L (secreted) = 9 liters / day.
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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.
 Musculosa (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.
 Of Three layers:
1- Mucous membrane
2- Lamina propria
3- Muscularis mucosa
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MUCOSA
1- Mucous Membrane:


Inner epithelial layer serves as a 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
2- Lamina Propria:

Middle layer of connective tissue on which epithelium rests.

Houses gut-associated lymphoid tissue (GALT)

Important in defense against disease-causing intestinal
bacteria.
3- 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 Submucosal nerve plexus or
Meissners
Secretion.
plexus
that
regulates
GIT
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MUSCULOSA (= MUSCULARIS EXTERNA)
 Major smooth muscle coat of digestive tube. In most
areas, it consists of two layers:
1- Circular layer (Inner layer):
 Contraction decreases diameter of lumen.
2- Longitudinal layer (Outer layer):
 Contraction shortens the tube.
 Together, contractile activity of these layers produces
propulsive and mixing movements.
 Myenteric nerve plexus lies between the two muscle
layers that controls Movements of GIT.
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SEROSA
 Outer connective tissue covering of GIT.
 Secretes serous fluid (watery & slippery fluid) that
Lubricates and prevents friction between digestive
organs and surrounding viscera.
 Continuous with mesentery throughout much of the
tract. This Attachment provides relative fixation and
supports digestive organs in proper place while still
allowing them freedom for mixing and propulsive
movements.
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REGULATION OF GIT FUNCTION
 Digestive motility and secretion are carefully
regulated to optimize the digestion.
 Four factors are involved in regulating digestive
system function:
1- Autonomous smooth muscle function.
2- Intrinsic local nerve plexuses.
3- Extrinsic autonomic nerves.
4- 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 in between circular & longitudinal
layer of smooth muscles.
 These are self-excitable cells that display
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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Slow Wave Potential
 If
slow
wave
potential
(slow
depolarization)
reaches threshold, action potentials are triggered
resulting in rhythmic cycles of contraction.
 Reaching threshold depends on mechanical, neural
and hormonal factors that influence starting point of
slow wave potential (e.g. presence of food bolus in
GIT).
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Slow Wave Potential
 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, the higher the
cytosolic calcium, the stronger the contraction.
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2- INTRINSIC NERVE PLEXUSES
 Submucosal plexus and myentric plexus, together are
often termed as Enteric
nervous system.
 Primarily coordinate local activity in GIT.
 Intrinsic nerve plexuses can affect all functions of
digestive tract, i.e. motility, secretion of digestive
juices and gastrointestinal hormones.
 Intrinsic nerve plexuses activity can be influenced by
endocrine, paracrine and nerve signals.
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 Submucosal
nerve
plexus
or
Meissners plexus that regulates GIT
Secretion.
 Myenteric nerve plexus lies between
the two muscle layers that controls
Movements of GIT.
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3- EXTRINSIC NERVES (AUTONOMIC)
 Both branches of ANS influence GIT motility
& secretion either by:
 Modifying activity of intrinsic nerve plexuses.
 Altering level of GIT hormones secretion.
 Directly acting on smooth muscle and glands.
 Sympathetic inhibits the motility and secretion
and parasympathetic increases both.
 Extrinsic nervous system coordinates activity
between different regions of GIT.
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Parasympathetic
Sympathetic
(Dominant)
* Vagus supplies from the * Greater splanchnic nerve:
esophagus till 1st half of large
from LHCs of lower 8 Thoracic
intestine.
segments.
* Lesser splanchnic nerve:
* Sacral (S2, 3, 4) segments supply
from LHCs of upper 2 Lumbar
the rest of GIT till anal region.
segments.
1. Contraction of GIT wall
Relaxation of sphincters.
& 1. Relaxation
of GIT wall
Contraction of sphincters.
2. Increase GIT secretions.
2. Decrease GIT secretions.
3. VD.
3. VC.
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&
GASTROINTESTINAL HORMONES
 Endocrine gland cells are tucked within mucosa of
certain regions of GIT that release hormones 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.
 E.g. CCK, Secretin, Gastrin……………etc
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Pathways Controlling GIT activities
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GIT RECEPTORS & REFLEXES
(i)
Chemoreceptors: sensitive to chemical changes in lumen.
(ii) Mechanoreceptors: sensitive to stretch on the wall.
(iii) Osmoreceptors: sensitive to osmolarity of luminal
contents.
 Stimulation of these receptors causes neural reflexes or
secretion of hormones which effect motility and secretion of
digestive juices.
 In GIT, two types of reflexes occur:
1. Short reflexes: local enteric reflex in wall of digestive tract.
2. Long reflexes: between CNS and Digestive system.
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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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Thank you
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