The document provides an overview of gastrointestinal physiology. It discusses the major functions of the digestive system including ingestion, digestion, absorption, and excretion. It describes the anatomy and layers of the gastrointestinal tract from the mouth to anus. Key sections include descriptions of motility in different parts of the GI tract, the secretions of saliva, gastric juices, bile, pancreatic juices and intestinal secretions. It also briefly discusses common GI disorders.

1. GIT Physiolagy
Dr.Mowlid A(MD)
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2. GI PHYSIOLOGY
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3. Introduction to GIT
Food is required by the body for the:
o Production of energy.
o Growth & repair of tissues.
 Each day an average adult man consumes
o 1 kg of solid food &
o 1- 2 liters of fluid.
 Majority of this material is in a form that cannot be
used immediately by the body for cellular metabolism.
• It must be broken down into simple
molecules which can be absorbed into the
bloodstream for distribution to the tissues.
:.The GIS performs this task.
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4. The major functions of the digestive system
The ingestion & mastication (chewing) of food;
 Transport of food through the GI tract
• at a rate that allow optimal digestion & absorption;
 The secretion of fluid, salts, and digestive enzymes;
• ( Exocrine + Endocrine secretions)
 Digestion (mechanical & chemical);
Supplies energy
 Absorption of end products of digestion;
 Excretion: Removal of indigestible remains, hydrophobic
substances from the body.
 Host defense.
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5. Introduction…
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6. Introduction…
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7. GIT and fluid balance (homeostatic function)
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8. Microbial defense by the GIT
Mouth
 Saliva contains lysozymes and
IgA that attacks microbes in the
mouth
Stomach
 HCl secreted in the stomach has
bactericidal actions
Small intestine:
 Payer's patches lymph tissue
attack microbes in the SI
Macrophages:
 In SI, developed from Monocytes
and destroy microbes by
Phagocytosis
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9. Anatomically & functionally, the digestive system can be divided into:
Tubular GIT
(Alimentary Canal)
1. Mouth
2. Pharynx
3. Esophagus
4. Stomach
5. Small intestine
6. Large intestine
7. Rectum
8. Anus
Accessories
Organs
1. Teeth,
tongue
2. Salivary
glands
3. Pancreas
4. Liver &
5. Gallbladder
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10. Functional layers of the GIT (Esophagus-Anus)
- From the lumen outward they are:
1. The mucosa- Protection, secretion & absorption from the luminal
surfaces
2. The sub mucosa,
 Contains blood vessels, lymph nodes and some nerve fibers
3. The muscularis externa-
 Contraction of the circular muscle decreases the diameter of the lumen
 Contraction of the longitudinal muscle, decreases the length of the GIT
4. The serosa or fibrosa
 Outer most protective layer, consists of connective tissues
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12. Neuronal plexuses in the wall of GIT
A. Submucosal (Messiner’s) plexus
 Between submucosa and circular muscle of muscularies externa layer
 Involved in GIT secretions & blood flow
B. Myenteric (Aurebach’s) plexus
 Between the circular & longitudinal smooth layers of muscularies externa
 It is involved in GIT motility
These two plexuses together with other neurons of the GIT constitute the Enteric
Nervous System (ENS)
 The ENS
 Integrate the motor & secretory activities of the GI system
 Control most of the GI functions even in the absence of ANS innervations
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13. Enteric nervous system (ENS)…
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14. Autonomic (PSN and SNS) connection to the GIT
A. Parasympathetic
 Is cholinergic (Ach)
 Cranial:
 All are in vagus nerve
 Innervate Esophagus, stomach, SI,
and the proximal large intestine
 Sacral:
 Innervates distal LI, sigmoid, rectum,
and anus
 PNS is excitatory to the GIT &
causes increased motility
 However, at the sphincters, PNS has
a dilatatory action
B. Sympathetic fibers
 Are adrenergic (NE & Epinephrine)
 The sympathetics is inhibitory to the
GIT- decreases motility
 At sphincters, it causes constriction
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15. Blood flow to the GIT
 GIT blood flow is classified under
splanchnic circulation
 Which includes: Gut, Spleen, pancreas
etc…
 Venous blood collected from these regions
pass through the portal circulation and end in
the liver
 liver sinusoids > hepatic vein > inferior
vena cava
 Advantage of portal circulation:
 Potentially harmful agents that come
with the blood are destroyed by
macrophages of the liver (Reticulo-
endothelial cells)
 blood flow to the GIT is controlled by
Metabolic, Decreased O2-tension, Neural
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16. 1. Motility in the Esophagus
 Upper 1/3 of esophagus is skeletal muscle
 Lower 2/3 of esophagus is smooth muscles
Swallowing reflex (deglutition)
It is the movement of food from the
mouth into the stomach
1. Oral /Voluntary phase
2. Pharyngeal phase
Involuntary from pharynx into the
esophagus
 Cause of primary peristalsis
3. Esophageal phase
 Involuntary
 Distention of the esophageal walls
causes “secondary peristalsis”
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17. Motility in the Esophagus…
Swallowing reflex…
• Secondary peristalsis is caused by
stimulation of solid food on the
walls of the esophagus
• It is powerful enough to push the
bolus towards the stomach
• Turning “upside-down” can not
cause the food to move towards the
mouth unless pathological
• Gravity plays a role in swallowing,
but not very potent
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18. Stomach
Structure
 Cardiac sphincter (LES)
 Lesser curvature
 Greater curvature
 Cardia
 Fundus
 Corpus (body)
 Pyloric antrum
 Pyloric sphincter
 Oblique muscle
The four regions of the stomach are cardia , fundus , body and pylorus.
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19. Stomach…
 Muscular region is
made up of
 Circular muscle
 Longitudinal muscle
 Oblique muscle
 Having oblique
muscle is responsible
for its movement in
every directions
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20. Stomach…
 Stomach has pacemaker region
on its body part made up of
cells called cells of Cajal
 The pacemaker controls the
basic electrical rhythm (BER)
of the stomach
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21. Motility in the stomach
 Motility of the stomach contents are
accomplished by:
 A. Mixing processes
- Are beneficial to mix the bolus
with gastric juice (chyme)
Pacemaker cells (cells of Cajal)
causing slow waves (BER)
- Slow waves result in tonic, weak
contractions of the stomach walls
(3 slow waves/min)
B. Peristaltic waves
 Orad contraction
 Caudad relaxation
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22. Motility in Small intestine
A. Segmentation
 2. Peristalsis (propulsive) motility
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23. Migrating Motility Complex (MMC) in small intestine
Migrating Motility Complex (MMC)
 Is a strong peristaltic wave that sweeps over the intestinal wall
after most meals are absorbed
 Important in completely pushing and sweeping the contents of
the small intestine into the large intestine
 Therefore, given the name ‘the house keeper’
 It also prevents the intestine from colonic bacteria
 It can sweep heavy and indigestible substances (e.g. coin) to
colon
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24. Motility in Large intestine
 Two types
A. Haustral movements
 large circular constrictions (haustrations) that are
powerful enough to close the diameter to a
narrow loop
B. Mass (propulsive) mov’t
 Mass movements occur seldom (1-3/day
or once after a breakfast)
 Local reflexes such as Gastro-colic or
duodeno-colic reflex initiate mass
movement
 Reflex contraction of the rectum
 Relaxation of the internal anal sphincter
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25. GI Secretions
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26. Mouth(oral cavity)
 Oral cavity
 Lips
 Muscles/cheeks
 Tongue
 Teeth
 Responsible for mechanical
(physical) digestion
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27. Secretion in Mouth
Types of Salivary glands and their secretions
Parotid (25%)
 the largest, just below and in front
of the ears
 Secrete mainly serous watery fluid
rich in ptyaline (salivary amylase)
Submandibular (70%)
 Produce both serous and mucous
fluid
Sublingual (~5%)
 Secrete mainly thick mucous with
little serous fluid
The three principal pair glands of salivation are:
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28. Salivary secretion…
Composition and function of saliva
Composition : Total secretion = about 1-1.5 L/day
 H2O (99.5%): facilitates taste and dissolution
 Electrolytes ( 0.5%): Na+, Cl-, K+, HCO3
-, etc
 Mucin (proteoglycan) for lubrication
 Enzymes: salivary amylase (ptyalin), lingual lipase
 Lysozymes (an enzyme that has a weak antibacterial action)
 IgA (prevent bacterial infection) etc… defense
Functions
 Lubrication
 Speech
 Mixing
 Bacterial attack, etc…
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29. Three phases and Reflex control of salivary
secretion
 Cephalic (brain) phase
 Triggered by thought, smell, or sight of food
 Oral phase
 Triggered by food that stimulate touch & test receptors
in the mouth
 Gastric phase
 Triggered by substances which stimulate the gastric
mucosa (acids or sour tastes) in the stomach
 Salivation can also be controlled by higher centers like
hypothalamus which has nerve connections with salivatory
nuclei in the Medulla oblongata (MO)
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30. Esophageal secretion
 The esophagus
wall contains
mucus cells that
produce mucous.
 Mucous is used for
lubrication and
thus creates easy
swallowing
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31. Glands that secrete different hormones in the stomach
1. Mucus neck cells: mucus
2. Parietal cells: HCl+IF
3. Chief cells:
Pepsinogen+gastric lipase
4. ECL(H) cells: histamine
secreting cells
5. G-cells: gastrin
 Enterocytes: migrate
from the crypt to the
surface to replace worn-
out cells
The Functions of the stomach
 Storage
 Mixing
 emptying
 Antibacterial effect (HCl)
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32. Function of HCl in the stomach
1. Kills most of the bacteria in food
2. Stimulates hunger – important in appetite
regulation
3. Activates the pro-enzyme Pepsinogen into its
active pepsin that denatures peptide bonds to
produce amino acids
4. Enhances motility of the stomach etc…
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33. Emptying and delaying of stomach contents
Thus, liquids empty faster than solids
Moreover, the following steps take
place for organic chyme when emptying
stomach
CHO > protein > fat
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34. Absorption in the stomach
 An insignificant
absorptive functions takes
place in the stomach,
these are:
 Alcohol
 Certain drugs (aspirin,
morphine etc.)
 Small quantities of H2O
Organic nutrients
(glucose, amino acids
and FFA etc… are not
absorbed)
• The following effects
protect the stomach wall
from acidic attacks (i.e.
pH<3)
a. Production of thick
mucous
b. HCO3
- secretion
(buffers the acid)
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35. Pancreas
 The pancreatic
secretion contains:
1. Bicarbonate (HCO3
-)
2. Electrolytes (Na+, K+,
Cl- etc.)
3. Digestive enzymes:
(inactive pro-enzymes)
a. Pancreatic Amylase
b. Pancreatic lipase
c. Proteases
d. Nucleases
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36. Bile Production in the liver
Bile
 Is an alkaline fluid (pH 8)
 Secreted in the liver
 Functions in emulsification
and absorption of fat in the
duodenum
 Emulsification means changing
greater fat globules into smaller
fat-droplets called micelles
 Excretion of several toxic end
products from the blood
These include: Bilirubin and Excess
cholesterol
Gall stone formation
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37. The Small intestine (SI)
 SI is specialized for
completion of digestion
and absorption of nutrients
a. Duodenum: ~ 25 cm,
mainly secretory, mucous,
hormones, enzymes
b. Jejunum : ~ 1.5 m mainly
absorptive
c. Ileum : ~1.7 m mainly
absorptive
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38. Small intestine: secretions and villi
Brush border (microvilli)
• Increase the absorptive surface
area to a great extent in the small
intestine
• Site of digestion and absorption
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39. Function of the large intestine
1. Water absorption
2. Electrolyte mainly NaCl
absorption
2. Mucous & HCO3
- Secretion
3. Storage, transport, and
evacuation of feces
4. Absorption of some drugs
5. Microbial fermentation
• The microbes in the LI
– Produce enzymes capable of
digesting cellulose
– Stimulates synthesis of some
vitamins (Vit. K, and B-groups)
The large intestine (colon)
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41. Common disorders of the GIT
1. Gastro-esophageal reflux (heart
burn)
2. Achalasia
3. Appendicitis
4. Constipation
5. Diarrhea
6. Peptic ulcers
7. Vomition
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