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.
3
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.
4
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
8
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
9
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
10
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
12
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
14
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
15
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”
16
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
17
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.
18
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
19
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
20
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
21
22. Motility in Small intestine
A. Segmentation
2. Peristalsis (propulsive) motility
22
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
23
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
24
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:
27
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…
28
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)
29
30. Esophageal secretion
The esophagus
wall contains
mucus cells that
produce mucous.
Mucous is used for
lubrication and
thus creates easy
swallowing
30
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)
31
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…
32
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
33
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)
34
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
35
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
36
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
37
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
38
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)
39