Secretory functions of GIT
The glands of the GIT have 2 main
functions:
1- Secretion of digestive enzymes from the
mouth to the ileum.
2- Secretion of mucus by the mucus
secreting glands from the mouth to the
anus, for lubrication of the content &
protection of the mucosa.
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Anatomical types of glands
1- Goblet cells, they are single cells present in
all parts of GIT & secret mucus.
2- Invagination of the mucosa down into the
submucosa forming pits, in the small
intestine they are called Crypts of Lieberkühn
3- In the stomach & duodenum there are deep
tubular glands.
4- Associated glands like salivary glands,
pancreas & liver.
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Stimulation of glandular secretion
1- By local contact of food with the epithelium,
this is partly mediated by the enteric NS.
2- Autonomic stimulation; parasympathetic
stimulation increases the secretion of salivary,
esophageal, gastric, pancreatic & duodenal
glands & the glands in the distal portion of large
intestine.
But secretions of the jejunum, ileum & The
proximal 2/3rd of large intestine are controlled
by local neural & hormonal stimuli.
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Stimulation of the sympathetic system causes
mild increase in the glandular secretion but
because it reduces the blood supply to the
glands, so it reduces the secretions of already
stimulated gland.
3- Hormonal control.
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Mucus
It is thick secretion compose of water,
electrolytes & glycoproteins.
Advantages of Mucus:
1. Lubrication of food.
2. Protection of mucosa from irritation.
3. Easier passage of food.
4. Has buffering properties & can neutralize
weak acids or bases.
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Saliva
It is secreted by the 2 parotids,
2 submandibular, 2 sublingual & many small
glands in the buccal mucosa.
 Its daily secretion is about 800 – 1500 ml (about
0.5 ml / min.)
 The pH of saliva is 6.0 – 7.0
 It contains 2 types of secretions:
1- Serous secretion: contains ptylin or α- amylase
which digest starch.
2- Mucus secretion: containing mucin for lubrication &
surface protection.
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The parotid secretion is purely serous, while others are
mixed & the small buccal glands secret only mucus.
Importance of saliva for oral hygiene:
1. Keeps the oral tissue moisten all the time.
2. Washes bacteria & remnants of food.
3. Contains substances that kill bacteria like lysozymes.
4. Contains immunoglobulins.
5. Contains enzymes for digestion of food.
In the absence of saliva the mucosa of the mouth
becomes dry with ulceration, infection & dental carries.
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Neural control of salivary secretions
1- Parasympathetic stimulation increases
salivary secretion.
Sensory stimuli from the tactile & taste
sensations (specially sour taste) travel to
the superior & inferior salivatory nuclei at
the junction between the pons & medulla.
2- Salivation can be stimulated or inhibited by
nervous signals arriving the salivatory
nuclei from the higher center, e.g when the
person smells food.
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3- Salivation increases in response to irritant
foods in the stomach & intestine, especially
when the person is nauseated, the swallowed
saliva helps to dilute or neutralize the irritating
substance.
4- Sympathetic stimulation also increases
salivation moderately, but lesser than
parasympathetic stimulation.
The sympathetic nerves arise from superior
cervical ganglia to the salivary glands.
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Esophageal secretions
It is entirely mucoid in character, they
protect the mucosa & provides
lubrication for swallowing.
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Gastric secretions
The mucosa of the stomach contains 3 types of
glands:
1- The entire surface of the stomach is lined by
mucus secreting cells.
2- The oxyntic or gastric glands, they are present
at the fundus & body of the stomach, they
secret HCl, pepsinogen, intrinsic factor &
mucus.
3- The pyloric glands, they are present at the
antrum, they secret mucus, some pepsinogen &
gastrin.
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Oxyntic (gastric) glands
These are tubular glands containing 3 types of
cells:
1- Mucus neck cells secreting mucus & some
pepsinogen.
2- Peptic or chief cells secrete large quantities
of pepsinogen.
3- The parietal cells (oxyntic cells) secretes acid
& intrinsic factor.
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Pepsinogen: is a protein which has no
proteolytic activity, but when it becomes in
contact with acid it is converted to pepsin which
is an active proteolytic enzyme acts in acidic
media at pH of 1.8 – 3.5.
Intrinsic factor: is secreted with HCl & it is
important for the absorption of vit. B12.
In chronic gastritis the acid producing cells
are destroyed & the patient develops
megaloblastic anemia due to B12 deficiency.
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Pyloric glands
They secrete:
1- Mucus by neck mucus cells.
2- Little pepsinogen by some peptic cells.
3- Gastrin hormone by the G cells.
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Surface mucus cells
These cells secrete thick alkaline mucous
secretion which forms a gel like layer of
about 1 mm thickness to protect the
mucosal cells & lubricate the food.
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Regulation of gastric acid secretion
Ach, histamine & gastrin are the basic
stimulators of the glands of the stomach.
Ach increases the secretion of all the cells
& glands, i.e it increases HCl from the
parietal cells, pepsinogen by the peptic
cells & mucus by the mucus secreting
cells.
Gastrin & histamine stimulates mainly acid
secretion.
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Stimulation of acid secretion
1- Nervous stimulation:
2 types of reflexes are initiated;
A- The long vagovagal reflexs.
They are transmitted from the gastric mucosa to
the brain stem & then back to the stomach
through the vagus N.
B- Short reflexes.
They are originated locally & are transmitted
entirely through the local enteric NS.
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Types of stimuli that can initiate
these reflexes are:
1) Distention of the stomach.
2) Tactile stimuli on the surface gastric
mucosa.
3) Chemical stimuli like AA, peptides & acids
that are already secreted by the gastric
glands.
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2- Role of gastrin:
Both the nerve signals from the vagus & enteric
reflexes cause the antral mucosa to secrete
gastrin hormone, which is carried by the blood
to the parietal cells to secrete HCl, also they are
carried to the peptic cells to secrete pepsinogen
but to lesser extent.
3- Role of histamine: Histamine is a very
important co-factor for the action of Ach &
gastrin, it acts through H2 receptors, so drugs
that block these receptors (like Cimetidin)
causes great reduction in acid secretion even in
the presence of Ach & gastrin.
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Regulation of pepsinogen
1- Release of Ach from the nerve endings
stimulates the peptic cells to secrete
pepsinogen.
2- Acid in the stomach stimulates
peosinogen secretion.
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Phases of gastric secretion
1- The cephalic phase:
It results in secretion of about 20% of gastric
secretion.
It occurs in response to sighting, smelling or
tasting food even before it enters the stomach.
The stimuli originates from the cerebral cortex
or from the appetite center in the hypothalamus,
then transmitted through the vagus to the
stomach.
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2- The gastric phase:
It accounts for 70% of gastric secretion.
It occurs in response to excitation of the
long vagovagal reflex, local enteric reflex
& gastrin mechanism.
3- The intestinal phase:
It occurs due to the presence of food in
the duodenum, which leads to release of
gastrin.
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Pancreatic secretions
The pancreas is a compound gland (endocrine
& exocrine).
The endocrine portion secretes insulin &
glucagon.
The exocrine portion secretes digestive
enzymes, bicarbonate & water, they pass
through the pancreatic duct to the common bile
duct which opens into the duodenum through
the ampulla of vater which is surrounded by the
sphincter of Oddi.
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Exocrine secretions
1- Enzymes secreted by the acini:
a- Proteolytic enzymes:
The most important are trypsin, chymotrypsin &
carboxypeptidase, & less important are
elastases & nucleases.
They are concerned with the digestion of
proteins.
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b- Pancreatic amylases:
They are concerned with digestion of
starch, glycogen & other CHO except
cellulose.
c- Pancreatic lipases:
They are concerned with the digestion
of fat.
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The pancreatic enzymes are secreted in an
inactive forms (trypsinogen, chymotrypsinogen
& procarboxypeptidase), they are activated
when they enter the duodenum, they don’t
digest the pancreatic cells because the
pancreas secretes trypsin inhibitor with the
secretion of trypsin.
Trypsinogen is activated by enterokinases
secreted by the intestinal mucosa when it
contacts the chyme.
Chymotrysinogen & procarboxypeptidase are
activated by trypsin.
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2- Bicarbonate & water:
They are secreted by the ducts &
ductules.
They have 2 important functions;
a- They neutralize the acidic chyme which
enters the duodenum from the stomach.
b- They provide appropriate pH for the
action of pancreatic enzymes in the
duodenum.
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