GIT secretions & hormones
• This include
• Digestive enzymes (mouth-ileum): based on
location and type of food
• Mucus (mouth - anus)
Glands
• They are the structures on the surface of the
epithelium or invaginations of the epithelium
that produce alimentary tract secretions
• Examples are: mucous glands, crypts of
lieberkuhn, tubular glands and complex glands
(lying outside the wall of the tract) like salivary
glands, pancreas, liver…..
Structure of a glandular cell & gland
Functions of mucous secretions
• They have adherent qualities
• Prevent direct contact of food with the
mucosa
• Provides a low resistance for slippage
• Causes fecal particles to adhere to one
another
• Its resistant to digestion by GIT enzymes
• The glycoproteins of mucus have amphoteric
or buffering properties
Stimulation of the glands
• The glands are stimulated by:
• Mechanical presence of food in a part of the GIT
• Local epithelial stimulation that activates the ENS
eg tactile stimulation, chemical irritation, gut
distension…..
• Parasympathetic stimulation
• Sympathetic stimulation has a dual effect
• Regulation: majorly hormonal
Saliva secretion
• Glands
Salivary secretions
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Glands
parotid,
submandibular,
sublingual glands;
small buccal glands.
Secretion rate: 800-1500mls/day
(ave=1000)
• Ph = 6.0 – 7.0
• Composition
• contains two major types of protein secretion
• (1) a serous secretion that contains ptyalin (an
a-amylase), which is an enzyme for digesting
starches,and
• (2) mucus secretion that contains mucin for
lubricating and for surface protective purposes.
• Parotid: majorly serous
• Submandibular/sublingual: serous and mucus
• Buccal: mucus
Mechanism for secretion of saliva
Mechanism
• Salivary secretion is in two stages:
• the first stage involves the acini, and
• The second, the salivary ducts.
• The acini secrete a primary secretion that contains ptyalin
and/or mucin in a solution of ions in concentrations not greatly
different from those of typical extracellular fluid.
• As the primary secretion flows through the ducts, two major
active transport processes take place that markedly
• modify the ionic composition of the fluid in the saliva.
• First, sodium ions are actively reabsorbed from all
the salivary ducts and potassium ions are actively
secreted in exchange for the sodium.
• Therefore, the sodium ion concentration of the saliva
becomes greatly reduced, whereas the potassium ion
concentration becomes increased.
• However, there is excess sodium reabsorption over
potassium secretion, and this creates electrical
negativity of about -70 millivolts in the salivary ducts;
this in turn causes chloride ions to be reabsorbed
passively.
• Therefore, the chloride ion concentration in the
salivary fluid falls.
• Second, bicarbonate ions are secreted by the ductal
epithelium into the lumen of the duct.
• partly caused by passive exchange of bicarbonate for
chloride ions, and
• active secretory process.
• Sodium conc (mEq/L) 145-15
• Potassium conc (mEq/L) 4.5-30
• Bicarbonate ions (mEq/L) ???- 50-70
• Increased rate of production: saliva rich in sodium is
produced (copious)
• Decreased rate of production: saliva rich in potassium is
produced (sticky)
Functions of saliva
• For oral hygiene
• Wash away pathogenic bacteria and food
particles
• Destroy bacteria: proteolytic enzyme and
thiocyanate
Regulation of salivary secretion
• The salivary glands are controlled mainly by
parasympathetic nervous signals all the way
from the superior and inferior salivatory nuclei
in the brain stem.
Secretion is stimulated by
• Taste: acid
• Tactile: smooth objects
• Higher centres: appetite area close to anterior
hypothalamus, amygdala and cerebral cortex
• Gastric and small intestinal reflexes
• Sympathetic stimulation: dual
• Blood vasodilators: kallikrein and bradykinin
Gastric secretion
• Glands
• 1. oxyntic / gastric glands: HCl, pepsinogen,
intrinsic factor and mucus
• location: inside surfaces of the body and
fundus of the stomach
• 2. pyloric glands: mucus and gastrin
• Location: antral portion and distal part of
stomach
Oxyntic/ Gastric glands
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Is composed of three types of cells
Mucus neck cells: mucus
Peptic or chief cells: pepsinogen
Parietal/oxyntic cells: HCl and intrisic factor
Mechanism of acid secretion by the
oxyntic cells
Mechanism of acid secretion
• 1. Chloride ion is actively transported from
the cytoplasm of the parietal cell into the
lumen of the canaliculus
• 2. sodium ions are actively transported out of
the canaliculus into the cytoplasm of the
parietal cell
• 3. a negative potential of -40 to -70 millivolts
is created in the canaliculus
• 4. the negative potential causes diffusion of
small quantities of sodium and potassium
ions back into the lumen
Mechanism (cont’d)
• 5. water inside the oxyntic cell dissociates into
hydrogen and hydroxyl ions
• 6. hydrogen ions are actively secreted into the lumen in
exchange for potasium ions using H+-K+ ATPase
• 7. sodium ions are reabsorbed back into the ECF using
Na+-K+ATPase.
• 8. water moves downhill from the ECF through the cell
to the lumen
• 9. CO2 in the cell combines with OH- in the presence of
CA to form HCO3- which is reabsorbed back into the
ECF
• Final secretion is thus: HCl and water, conc= 150 to
160 mEq/L, KCl= 15 mEq/L
Secretion of pepsinogen by peptic cells
• pepsinogen is secreted (no digestive activity) ,
• comes in contact with HCl
• it is activated to form active pepsin.
• functions
• an active proteolytic enzyme
• necessary for protein digestion in the stomach
Regulation of pepsinogen secretion
• (1) stimulation of the peptic cells by
acetylcholine released from the VN or from the
gastric ENS
• (2) stimulation of peptic cell secretion in
response to acid in the stomach.
Applied Physiology
• In people who have lost the ability to secrete
normal amounts of acid, secretion of pepsinogen
isalso decreased, even though the peptic cells
may otherwise appear to be normal.
Secretion of intrinsic factor by parietal cells
• Intrinsic factor is essential for absorption of vitamin B in the ileum,
• it is secreted by the parietal cells along with the secretion of HCl.
12
• Applied physio
• Destruction of parietal cells caused by acute or chronic gastritis can
cause:
1. achlorhydria (lack of stomach acid secretion)
2. pernicious anemia because of failure of maturation of the RBCs in
the absence of vitamin B stimulation of the bone marrow
12
Secretion of mucus and gastrin
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The pyloric gland secretes mucus and gastrin
Mucus
Its produced by surface mucous cells
Coats the stomach mucosa
Protects the stomach wall
Lubricates food for easy transport
Alkaline in nature
• Gastrin to be discussed later
Stimulation of Gastric Acid Secretion
• the parietal cells operate in close association with
enterochromaffin-like cells (ECL cells), the primary
function of which is to secrete histamine.
• The rate of secretion of HCl by the parietal cells is
directly related to the amount of histamine secreted
by the ECL cells.
• ECL is stimulated/regulated by
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Gastrin
Acetylcholine
Hormones from the ENS
Stimulation by gastrin
• Gastrin is produced by Gastrin (G) cells in
response to presence of meat or other proteincontaining foods in the stomach
• Gastrin stimulates the ECL cells to produce
histamine
• Histamine stimulates the parietal cells to
produce HCl
Phases of Gastric Secretion
• a cephalic phase,
• a gastric phase, and
• an intestinal phase.
Cephalic phase
• The cephalic phase occurs even before food enters the
stomach
• It results from the sight, smell, thought, or taste of food, and
the greater the appetite, the more intense is the stimulation.
• Neurogenic signals that cause the cephalic phase of gastric
secretion originate in the cerebral cortex and in the appetite
centers of the amygdala and hypothalamus.
• They are transmitted through the dorsal motor nuclei of the
vagi and thence through the vagus nerves to the stomach.
• This phase of secretion normally accounts for about 20% of
the gastric secretion associated with eating a meal.
Gastric phase
• Once food enters the stomach, it excites
• (1) long vagovagal reflexes from the stomach
to the brain and back to the stomach,
• (2) local enteric reflexes,and
• (3) the gastrin mechanism
• The gastric phase accounts for about 70% of
the total gastric secretion associated with
eating a meal
Intestinal phase
• The presence of food in the upper portion of the small intestine,
particularly in the duodenum inhibits gastric secretion by
1. Initiation of a reverse enterogastric reflex
2. Release of inhibitory hormones eg secretin, GIP, VIP and
somatostatin in the presence of acid, fat, protein breakdown
products, hyperosmotic or hypo-osmotic fluids, or any irritating
factor in the upper small intestine
3. Reduction of gastric motility
• purpose: to slow passage of chyme from the stomach when the
small intestine is already filled or already overactive.
Pancreatic secretion
Phases of pancreatic secretion
• Cephalic: vagal signals, 20% of total secretion
• Gastric: vagal signals fire on, 5-10%, chyme in
stomach
• Intestinal: chyme in the small intestine: 70-75%
caused by secretin
Pancreatic secretion
• The pancreas is a large compound gland
• Its pancreatic duct joins the hepatic duct
immediately before it empties into the
duodenum through the papilla of Vater,
surrounded by the sphincter of Oddi.
• Its acini produce digestive enzymes
• The epithelial cells of the ductules secrete
bicarbonate and water
Pancreatic secretion
• Stimulated by:
• Presence of chyme in the duodenum
• Its islets of Langerhans secrete insulin
directly into the blood
• Total amount is 1 liter/day
Pancreatic digestive enzymes
• These include:
• Proteins: trypsin & chymotrypsin (proteins to
peptides) , carboxypolypeptidase (peptides to
amino acids)
• CHO: pancreatic amylase (CHO, glycogen to tri n di
saccharides)
• Fat: pancreatic lipase (neutral fat to f.a. &
monoglycerrides), cholesterol esterase,
phospholipase
Activation of the enzymes
• The enzymes become activated only in the intestine.
• Trypsinogen is converted to trypsin in the presence
of enterokinase
• Chymotrypsinogen is activated to form
chymotrypsin in the presence of trypsin
• Note: trypsin inhibitor is formed in the cytoplasm
of the glandular cells, and it prevents activation
of trypsin both inside the secretory cells and in
the acini and ducts of the pancreas.
• trypsin inhibitor prevents activation of the others
as well.
Applied physiology
• Acute pancreatitis: occurs when the effect of
trypsin inhibitor is overwhelmed eg when a
duct is blocked
• Thus the pancreatic secretions rapidly
become activated and can literally digest
the entire pancreas within a few hours,
Bicarbonate ion secretion
• Are secreted by epithelial cells of the ductules
and ducts of the pancreas along with water
• Normal conc is 29mEq/L (can rise to 145 mEq/L,
to neutralize acidic chyme)
• Steps involved:
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2
1
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Regulation of pancreatic
• ACh: more enzymes, less water & electrolytes
• CCK: secreted by I cells of the mucosa of duodenum
and jejunum. more enzymes, less water & electrolytes
• Secretin: secreted by S cells of the mucosa of
duodenum and jejunum. Causes production of large
water & bicarbonate, less enzyme all in response to
acidic chyme.
• An important protective mechanism to prevent the
development of duodenal ulcer.
HO
• HCl +NaHCO3
NaCl +H2CO3
2
CO2
CCK
• Secreted by the I cells of the mucosal of the
duodenum and jejunum
• Release is potentiated by presence of
proteoses, peptones and long chain f.a in the
chyme
Regulation
Bile secretion by the liver
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The liver produces 600-1000ml of bile per day.
Functions of bile
1. fat digestion and absorption
2. means of excretion of waste products like bilirubin
Secretion of bile: secreted in two stages
1. hepatocytes secrete large amounts of bile acids,
cholesterol, other organic constituents into bile
canaliculi
• 2. bile flows in the system of ducts through the cystic
duct into the gall bladder or empties directly into the
duodenum: these ducts add Na+ & HCO3- to the
secretion (secretin effect)
Gallbladder: for storage and
concentrating bile
• water, sodium, chloride, and most other small
electrolytes are continually absorbed through the
gallbladder mucosa, concentrating the remaining bile
constituents that contain the bile salts, cholesterol,
lecithin, and bilirubin.
• Most of this gallbladder absorption is caused by active
transport of sodium through the gallbladder epithelium,
and this is followed by secondary absorption of chloride
ions, water, and most other diffusibleconstituents.
• Bile is normally concentrated in this way about 5-fold,
but it can be concentrated up to a maximum of 20-fold.
Composition of bile
CCK stimulates gallbladder emptying
• Stimulus: food digestion in the stomach and
presence of fatty foods in the duodenum, Ach &
ENS
• Rhythmical contraction of the bladder wall begins
under the influence of CCK
• Sphincter of Oddi relaxes
• gallbladder empties its store of concentrated
bile into the duodenum
Regulation
Role of bile salts in fat digestion and
absorption
• Emulsifying or detergent function: to decrease
the surface tension of the particles
• Help in the absorption of fatty acids,
monoglycerrides, cholesterol and other lipids
from the tract
• They form physical complexes called micelles,
through which the lipids are ferried into the
blood
Applied physio: Gallstone
• Bile salts are formed in the hepatic cells from
cholesterol in the blood plasma.
• Under abnormal conditions, the cholesterol may
precipitate in the gallbladder causing gallstone
formation
• Other causes include
Too much absorption of water from bile
Too much absorption of bile acids from bile
Too much cholesterol in bile
Inflammation of the epithelium
Formation of gallstone
Small intestinal secretions
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A. Brunner’s glands secrete alkaline mucus in response to:
(1) tactile or irritating stimuli on the duodenal mucosa;
(2) vagal stimulation: increased; and SNS inhibits
(3) gastrointestinal hormones, especially secretin.
• Functions
• to protect the duodenal wall from digestion by the
highly acid gastric juice emptying from the stomach
• Neutralizes the HCl entering the duodenum from the
stomach.
Intestinal secretions contd
• B. Crypts of Lieberkühn with villi:
• contains goblet cells that secrete mucus and
enterocytes which secrete water and
electrolytes
• Mainly to aid digestion and absorption
Large intestinal secretions
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Crypts of Lieberkühn without villi:
The epithelial cells secrete only mucus
Regulation
Tactile stimulation
Functions
protects the intestinal wall against excoriation
provides an adherent medium for holding fecal matter
together
• protects the intestinal wall from the great amount of bacterial
activity that takes place inside the feces
• provides a barrier to keep acids formed in the feces from
attacking the intestinal wall.
• Local reflexes: vagal stimulation through pelvic nerves
increases mucus production