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OVERVIEW OF DIGESTIVE
SYSTEM
Lecture 1
Dr. Zahoor Ali Shaikh
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DIGESTIVE SYSTEM
• The primary function of the Digestive System is
to transfer nutrients, water and electrolytes from
the food into the body.
• Ingested food is source of energy from which cell
generate ATP to carry out energy dependent
activities e.g. active transport, contraction,
secretion.
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DIGESTIVE SYSTEM
• Food we take is digested or broken down into
small molecules that can be absorbed from the
GIT [Gastro Intestinal Tract] into circulatory
system, for distribution to the cells.
• Normally 95% of ingested food is available for
use by the body.
• Food is ingested – digested – absorbed –
distributed and used.
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DIGESTIVE SYSTEM
• We will have overview of Digestive System, we
will discuss:
1. Basic Digestive Process
2. Components of Digestive System
3. Regulation of Digestive System
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1. Basic Digestive Processes
• Digestive System performs four basic processes:
1. Motility
2. Secretion
3. Digestion
4. Absorption
We will discuss one by one.
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Basic Digestive Processes
1. ‘Motility’
• Motility means movements, muscle contraction
that mix and move the food forward.
• Smooth muscle in GIT has
- Tone [constant level of contraction] – tone is
important to maintain study pressure on the
contents present in the GIT.
- Action Potential – which induces contraction
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1. ‘Motility’
• There are two types of muscle contraction
(i). Propulsive movements
(ii). Mixing movements
• (i). Propulsive movement
• They propel or push or move forward the
contents. Rate of propulsion varies in GIT.
• Why? Because it depends on the function of the
region. E.g. movement of food is fast in
esophagus, and slow in small intestine.
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1. ‘Motility’
• (ii). Mixing movement
• It has two functions:
- mixing the food with digestive juices to help in
digestion of food
- help in absorption of food due to exposing the
contents of food to the absorbing surface of GIT.
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1. ‘Motility’
• In GIT, where smooth muscle is present, it is
functioning by Involuntary mechanism.
• We have voluntary control in GIT only on
chewing, swallowing, and defecation.
Why? Because in these areas skeletal muscles
are involved which are under voluntary control.
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Basic Digestive Processes
2. ‘Secretion’
• By exocrine glands (through ducts).
• Digestive secretion consist of water, electrolytes,
enzymes, mucus, or bile salts.
• Secretion of digestive juices is active process
(require energy).
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Basic Digestive Processes
3. ‘Digestion’
• We take three type of food:
i). Carbohydrate
ii). Proteins
iii). Fats
• These are large molecules, therefore, they are
digested and then absorbed into the blood or lymph.
• Word ‘Digestion’ means breakdown of complex
food stuff into smaller, absorbable units by enzymes.
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Basic Digestive Processes
3. ‘Digestion’
i). CARBOHYDRATES
• Carbohydrates are absorbed as monosaccharide
e.g. glucose, fructose, and galactose.
Remember
• Cellulose [polysaccharide found in plants] can
not be digested, therefore, works as bulk or
ingestible fiber.
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Basic Digestive Processes
3. ‘Digestion’
ii). PROTEIN
• Protein 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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Basic Digestive Processes
4. ‘Absorption’
• In small intestine, digestion is completed and
most absorption occurs.
• Water, vitamins and electrolytes are also
absorbed.
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GIT
ANATOMY
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GIT
FUNCTIONS
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2. DIGESTIVE TRACT & ACCESSORY
DIGESTIVE ORGANS
 Digestive Tract
• It is a tube about 4.5 meter [15 feet] in length.
• It includes mouth, pharynx [throat], esophagus,
stomach, small intestine [duodenum, jejunum
and ileum],
large intestine [cecum, appendix, ascending,
transverse, descending colon, rectum] and anus.
 NOTE – they are continuous but there are
regional modifications which allow them to
specialize in digestive activity.
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2. DIGESTIVE TRACT & ACCESSORY
DIGESTIVE ORGANS
 Digestive Tract
• It is continuous from mouth to the anus, the
lumen of this tube is continuous with external
environment.
• Contents in the lumen are technically outside the
body.
• Only after the substance is absorbed from the
lumen then it becomes the part of body. E.g. pH
of stomach content is 2 due to presence of HCL,
but body fluid pH compatible with life is 6.8 – 8.
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DIGESTIVE TRACT & ACCESSORY
DIGESTIVE ORGANS
 ACCESSORY DIGESTIVE ORGANS
- Salivary glands
- Pancreas [Exocrine]
- Biliary system – liver, gallbladder
• Exocrine gland means organs lie outside the
digestive tract and empty their secretion through
ducts into the digestive tract lumen.
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DIGESTIVE TRACT WALL
• Digestive Tract Wall has four layers:
1. Mucosa
2. Sub-mucosa
3. Muscular layer
4. Serosa
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DIGESTIVE TRACT WALL
 1. Mucosa
• It lines the luminal surface.
• It is divided into three layers:
- Mucus membrane
- Lamina propria
- Muscularis mucosa
 Mucus membrane – Inner epithelial layer,
protective and specialized for absorption. It has
exocrine gland cells for secretion of digestive juices.
It has also endocrine gland cell for secretion of GIT
hormones.
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DIGESTIVE TRACT WALL
 1. Mucosa [cont]
 Lamina propria – Middle layer of connective
tissue. It has Gut associated lymphoid tissue
[GALT] important in defense against intestinal
bacteria.
 Muscularis mucosa – Smooth muscle.
• Mucosal surface is highly folded which increases
the surface area for absorption.
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DIGESTIVE TRACT WALL
 2. Sub-mucosa [under the mucosa]
• It is thick layer of connective tissue which
provides distensibility and elasticity.
• It has large blood vessels and lymph.
• It has nerves – known as SUB-MUCOSAL
PLEXUS or MEISSNERS PLEXUS.
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DIGESTIVE TRACT WALL
 3. Muscular layer
• Smooth muscle coat of digestive tube which
surrounds sub-mucosa.
• It has two layers:
- Inner circular muscle layer
- Outer longitudinal muscle layer
• Contraction of inner circular layer decreases the
diameter of lumen at the point of contraction.
• Contraction of outer longitudinal layer shortens
the tube.
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DIGESTIVE TRACT WALL
 3. Muscular layer [cont]
• Contraction of both layers [circular and
longitudinal] causes propulsive and mixing
movements.
• Another nerve network called ‘Myentric plexus’
lies between the circular and longitudinal muscle
layer.
IMPORTANT – Gut Local activity is regulated
by sub-mucosal [meissners plexus] and Myentric
plexus and also by hormones.
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DIGESTIVE TRACT WALL
 4. Serosa
• It is outer connective tissue covering of digestive
tract.
• It secretes watery, slippery fluid that lubricates
and prevents friction between digestive organs
and surrounding viscera.
• Serosa is continuous with the mesentery, which
is attached to inner wall of abdominal cavity.
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3. Regulation of Digestive System
• Four factors regulate the digestive functions:
• 1. Smooth muscle [autonomous] function
• 2. Intrinsic nerve plexus [sub-mucosal and
myentric]
• 3. Extrinsic nerves
• 4. GIT hormones
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3. Regulation of Digestive System
 1. Smooth muscle [autonomous] function
• Some smooth muscle cells are specialized and
work as pace-maker cells called interstitial cells
of CAJAL [like cardiac muscle cell].
• WHY? Because they have automatic changes in
membrane potential. They are non-contractile
cells. They lie between circular and longitudinal
smooth muscle layers.
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3. Regulation of Digestive System
• 1. Smooth muscle [autonomous] function
• Self-induced electrical activity in smooth muscle
is slow wave potential or Basic Electrical
Rhythm [BER].
• Slow wave are below threshold level, if they
reach threshold level, action potential will occur
and lead to muscle contraction.
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3. Regulation of Digestive System
 2. Intrinsic nerve plexus [sub-mucosal and
myentric]
• They run throughout the GIT and have about 100
million neurons and do the self-regulation of
intestine. They are called ‘ENTERIC NERVOUS
SYSTEM’.
• ENTERIC NERVOUS SYSTEM – it has sensory
neurons which respond to local stimuli. It has motor
neuron which supply smooth muscle, exocrine and
endocrine cells, therefore, affect the motility,
secretion, and GIT hormones.
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3. Regulation of Digestive System
 3. Extrinsic nerves
• These are ANS [Autonomic Nervous System].
• It originates outside the digestive tract and supplies
the digestive organs.
• ANS – sympathetic and para-sympathetic ANS
affects motility, secretion by acting directly on
smooth muscle or affecting the intrinsic plexus [submucosal and myentric].
• Para-sympathetic nerve supply to GIT arise via
VAGUS NERVE. It increases motility and secretion.
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3. Regulation of Digestive System
 4. GIT hormones
• In Mucosa of some regions of Digestive tract are
located endocrine gland cells.
• GIT hormones are carried by blood to other
areas of GIT and cause excitation or inhibition of
smooth muscle of GIT.
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RECEPTORS PRESENT IN GIT
• Three types of sensory receptors are present
which respond to local changes.
• (i). Chemoreceptors – sensitive to chemical
changes in lumen.
• (ii). Mechano receptors [pressure receptors] –
sensitive to stretch on the wall.
• (iii). Osmo receptors – sensitive to Osmolarity of
luminal contents.
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RECEPTORS PRESENT IN GIT
• Stimulation of these receptors causes
- Neural reflexes or Secretion of hormones which
effect motility, secretion of digestive juices.
• In GIT, two type of reflexes occur:
1. Short reflexes – local reflex in wall of digestive
tract
2. Long reflexes – reflex between CNS and
Digestive system
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Thank you