Secretory function
of the digestive
system
1
Objectives
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Describe the location of the salivary glands,
composition and function of saliva.
Outline the composition of the pancreatic juice.
Understand regulation of pancreatic juice secretion.
Explain function of bile.
Describe the role of CCK in causing release of bile
from the gall bladder, including the effects on the
sphincter of Oddi.
Describe the mechanisms by which chyme from the
stomach is neutralized in the duodenum.
Explain functions of the liver
Define enterohepatic circulation
List the functions of gastric acid
2
Major Salivary Glands
3
Composition of the saliva

99.5% water

0.5% solutes
 ions
= Na+, K+, Cl-, HCo3, Po4 Organic substances = urea, uric acids, mucus,
immunoglobulin A
 Enzymes : lysozyme
salivary amylase

Saliva is slightly acidic (pH 5.8 -7.4)
4
Secretion of the saliva
Daily secretion: 1-2L, basal flow rate 0.5 mL/min →
5mL/min (max.).
Two main secretions:
Serous: contains enzyme α-amylase
Mucous: contains mucin for lubrication
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Parotid gland secretes watery fluid (serous) contains salivary
amylase
Submandibular glands secrete mixture of watery and mucus
secretion
Sublingual glands secrete mainly mucus
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Secretion of the saliva
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Saliva stimulated by
 the presence of food in the mouth
 Smell, sight, sound, or thought of food
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It is under control of the autonomic nervous system.
 Parasympathetic stimulation causes vasodilatation
and profuse secretion of watery saliva
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Sympathetic stimulation causes vasoconstriction and
secretion of small amount of saliva ( mouth dryness)
e.g. stress
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Secretion of the saliva
 Dehydration,
 Salivary
glands stop secreting saliva ,
resulting in mouth dryness and
contribute to the sensation of thirst.
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Functions of Saliva
 Lubricating effect thanks to water and mucin which:
– Moistens food
– Facilitates swallowing
– Facilitates speech
 Protective effect
– keeps the mouth and teeth clean
–IgA and lysozymes (defense against bacteria)
– HCO3- prevents dental carries
–Diluting noxious substances and the corrosive gastric
acid and pepsin → esophagus and mouth during
vomitting
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Functions of Saliva cont…
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Aids digestion of:
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Carbohydrates: α-amylase
Amylase breaks down starch into smaller molecules
(pH 6.8)
Amylase stops functioning at pH 1.5 (stomach)
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Dietary lipid: lingual lipase
Good messenger for dehydration
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Gastric secretion
H+
H+
H+
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Gastric Juice
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About 2-3 liters/day
Contents:
 Water & mineral salts
secreted by gastric glands
 Mucus secreted by goblet
cells
 HCl & intrinsic factor
secreted by parietal cells
 Enzyme precursors (
pepsinogens) secreted by
chief cells.
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Functions of the gastric juice
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water change into a liquid the food swallowed
HCl
acidifies the food and stop the action of salivary
amylase
 kills ingested microbes
 provides acidic environment for the activation of
pepsinogens.
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Acid secretion
H+ ions are pumped into the
lumen in exchange with K+
by H+/K+ pump (ATPase)
K+ taken up by the cells is
recycled by the K+ channels.
HCO-3 is added to blood in
exchange with Chloride ions
(Cl -HCO3-) exchanger).
ALKALINE TIDE (↑ pH after
a heavy meal)
Cl- diffuses into the lumen
through chloride channels
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Acid Secretion
Parietal cells have
receptors for Histamine,
Ach and Gastrin
1.Histamine It works in a
paracrine fashion.
2.Acetylcholine: released from
the enteric and vagus nerve
endings.
3. Gastrin
Produced by G cells in the pyloric
antrum and duodenum
Gastrin secretion is suppressed
when pH in the pyloric antrum falls
to 1.5
Gastrin and Ach stimulate Histamine release. 14
Secretion of gastric juice
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Functions of the gastric juice cont…
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Pepsinogens (inactive pro-enzyme)
Secretd by chief cells
 activated to pepsin by HCl
 breaks protein to smaller molecules
 most effective at pH 1.5-3.5
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What keeps pepsin from digesting the protein in
stomach cells along with the food?
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Functions of the gastric juice cont…
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Mucus
Prevents
 Mechanical injury to the stomach by lubricating the
contents
 Chemical injury by acting as barrier between the
stomach wall and the corrosive gastric juice
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Functions of the gastric juice cont…
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Gastric Lipase
-Splits the short chain triglycerides in fat
molecules into fatty acids and
monoglycerides.
-Limited role in the adult stomach
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Functions of the gastric juice cont…
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Other functions of the stomach
Temporary storage allowing time for digestive
enzymes
 Mechanical breakdown of the food
 Absorbs water, some lipids and certain drugs
 Production of intrinsic factor (Vitamin B12
absorption)

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Pancreas
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Pancreas
Endocrine pancreas (1%
of its mass), Islets of
Langherans :Glucagon,
Insulin hormones
Exocrine pancreas
(99%), acini:
enzymes and
HCO321
Exocrine pancreas
 Exocrine pancreas secretes pancreatic juice consisting
of enzymes and aqueous alkaline (NaHCO3) secretions.
The duct & centroacinar cells
actively secrete the aqueous
NaHCO3.
The acinar cells secrete 3
types of pancreatic enzymes:
─ Proteolytic enzymes
─ Pancreatic amylase
─ Pancreatic lipase
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 Exocrine pancreas secretes pancreatic
juice consisting of:
1. Aqueous alkaline secretions
2. Enzymes
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Pancreatic aqueous alkaline-secretion
• Large volume of juice with high
concentration of NaHCO3.
•The largest component of
pancreatic secretion: 1-2 L/day
• The bicarbonate component is
important in:
─ Neutralizing the acidic chyme
emptied into the duodenum.
─ Providing optimal pH for
digestive enzyme activity
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 Exocrine pancreas secretes pancreatic
juice consisting of:
1. Aqueous alkaline secretions
2. Enzymes
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Pancreatic enzymes
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Pancreatic proteolytic enzymes
 The protein digesting
enzymes are secreted as
inactive proenzymes:
zymogens.
In the duodenum: zymogens
are converted to active
enzymes when
enteropeptidases converts
trypsinogen to trypsin
 Trypsin, in turn, activates
the other pancreatic zymogens
 Trypsin: central enzyme
catalysing the activation of all
other pancreatic enzymes
Chemotrypsinogen
Procarboxypeptidase
Why proteolytic enzymes secreted in
Proelastase
inactive form?
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Regulation of pancreatic secretion
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Secretion of aqueous alkaline
component is determined by the amount
of acid in the duodenum.
Secretion of enzymes depends on the
amount of fat & proteins in the
duodenum
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Bile
-Bile is a yellow, brownish or olive –
green liquid secreted by hepatocyte
(0.5L/day)
-It has a pH of 7.6-8.6
-It is stored and concentrated in the gall
bladder
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Composition of Bile
1. Bile acids
Are synthesized from cholesterol
Exist mainly as conjugated (bile salts)
2. Phospholipids
Major ones are the lecithins
3. Cholesterol
4. Bile pigments (bilirubin, breakdown product of RBCs)
5. Inorganic ions (mainly Cl- and HCO3-)
6. Water
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Function of bile
Fat emulsification and excretion of cholesterol
(Emulsification is the process by which fat globules
are broken into smaller pieces by the detergent
action of bile salts and especially lethicin
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Bile secretion and enterohepatic
circulation
• Total amount of bile salts is
smaller than what is needed
daily to promote lipid
digestion and absorption.
• The enterohepatic
circulation allows the
recycling of bile salts
Body can recycle the same
bile salts
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Regulation of bile secretion
Relaxation of sphincter of
Oddi
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Functions of the Liver
Carbohydrate Metabolism
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Important in maintaining normal blood glucose
Blood glucose low --- liver breakdown of glycogen
to glucose and release to blood
Blood glucose high--- liver converts blood
glucose into glycogen and triglycerides for
storage
Lipid Metabolism
 Stores some triglycerides
 Synthesize cholesterol
 Use cholesterol to form bile salts
 Breakdown fatty acids to generate ATP
 Synthesize lipoproteins
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Functions of the Liver cont..
Protein Metabolism
Deamination of amino acids
 Synthesis of most plasma proteins such as
albumin, globulin, fibrinogen and prothrombin
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Processing of drugs and hormones
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Detoxification and excretion of some drugs
and hormones
Excretion of bilirubin
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Functions of the Liver cont..
Synthesis of bile acids
Storage
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Glycogen
Vitamins (A, B12, D, E, K)
Minerals (copper and iron)
Phagocytosis
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Reticuloendothelial (kupffer) cells phagocytosed aged
RBCs, WBCs and some bacteria
Activation of Vitamin D
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