Download Pancreatic secretion

)GIT) physiology
‫المرحلة الثانية‬
‫المحاضرة السادسة‬
Pancreas
Anatomy:The pancreas, which lies parallel to and beneath the stomach is a large
compound gland with most of its internal structure similar to that of the salivary glands.
The pancreas is composed of both exocrine cells and endocrine cells. The exocrine cells are
located within the acini. These cells are responsible for the digestive enzymes produced
by the pancreas, and bicarbonate ions.
The pancreatic digestive enzymes are secreted by pancreatic acini, and large volumes
of sodium bicarbonate solution are secreted by the small ductules and larger ducts leading
from the acini, The combined product flows through a long pancreatic duct that normally
joins the hepatic duct immediately before it empties into the duodenum through the
papilla of Vater, surrounded by the sphincter of Oddi.Scattered among the acini are the
pancreatic islets. Within the islets are located endocrine cells which produce insulin and
glucagon. These hormones play an important role in blood glucose homeostasis.
Anatomy of pancreas
pancreas acini, is the blind end of a branching duct system is lined with aciner cells,
that secrete the enzymatic portion of pancreatic secretion. The ducts are lined with ductal
cells, ductal epithelial cells extend into the acinus' into a special region of centroacinar
cells,which secrete aqueousHC03.
pancreatic acini
Pancreatic secretion: The exocrine part of pancreas secretes pancreatic juice about
I.5 litter I day of an aqueous component high in HCO3¯( to neutralize the H+ from stomach)
and an enzymatic component to digest carbohydrates, proteins and lipid. aqueous
secretion rich in bicarbonate ,Na+, K+, CL -,water its responsible for washing out the acini
and duct to pare enzymes secretion into the duodenum figure (10).
Figure(10) pancreatic secretion
The pH is alkaline 6-7 which helps to neutralize the acid chyme and also this range
of pH is essential for pancreatic enzymes activity. Pancreatic juice is secreted most
abundantly in response to the presence of chyme in the upper portions of the small
intestine, and the characteristics of the pancreatic juice are determined to some extent by
the types of food in the chyme.
Exocrine pancreas is innervated by both parasympathetic and sympathetic nervous
system, Parasympathetic activity stimulates pancreatic secretion, and sympathetic activity
inhibits pancreatic secretion ( in contrast with the salivary gland, in which both
sympathetic and parasympathetic activity are stimulatory).
Pancreatic Digestive Enzymes:
1- The proteolytic enzymes for proteins digestion are trypsin, chymotrypsin, and
carboxypolypeptidase.. Trypsin and chymotrypsin split digested proteins into
peptides,and carboxypolypeptidase does split some peptides into individual amino
acid.
2- The digestive enzyme for carbohydrates is pancreatic amylase which hydrolyzes
starches, glycogen and most other carbohydrates (except cellulose) to form disaccharides
and a few trisaccharides..
3- The main enzyme for fat digestion is(1) pancreatic lipase which is capable of
hydrolyzing neutral fat into fatty acids (2) cholesterol esterase, which causes
hydrolysis of cholesterol esters; and (3) phospholipase, which splits fatty acids from
phospholipids
Pancreas protects itself from auto digestion:by proteolytic enzymes by the following ways:
A- When proteolytic enzymes synthesized in the pancreatic cells, they are in the
inactive forms, These become activated only after they are secreted into the intestinal
tract, they are activated by an enzymes are secreted by the intestinal mucous called
enterokinase.
B- The same cells that secrete the proteolytic enzymes into the acini of the pancreas
secrete another substance called trypsin inhibitor. This substance is stored in the
cytoplasm of the glandular cells surrounding the enzyme granules and it prevents
activation of trypsin both inside the secretary cells and in the acini and ducts of the
pancreas.
Regulation of Pancreatic Secretion
Three basic stimuli are important in causing pancreatic secretion:
1. Acetylcholine, which is released from the parasympathetic vagus nerve endings
and from other cholinergic nerves in the enteric nervous system
2. Cholecystokinin, which is secreted by the duodenal and upper jejunal mucosa
when food enters the small intestine
3. Secretin, which is also secreted by the duodenal and jejunal mucosa when highly
acid food enters the small intestine
The first two of these stimuli, acetylcholine and cholecystokinin, stimulate the acinar
cells of the pancreas, causing production of large quantities of pancreatic digestive
enzymes but relatively small quantities of water and electrolytes to go with the enzymes.
Secretin, in contrast to the first two basic stimuli, stimulates secretion of large quantities
of water solution of sodium bicarbonate by the pancreatic ductal epithelium. pancreatic
secretion normally results from the combined effects of the multiple basic stimuli, not
from one alone.
Phases of Pancreatic Secretion
Pancreatic secretion occurs in three phases:1- Cephalic phase:
The thought, smell, and taste of food produces the cephalic phase of pancreatic
secretion. Both acinar and to less extent the ductal cell secretion are enhanced by
vagal stimulation. Most the enzymes are stored in the acini .
2- Gastric phase: Pancreatic secretion is enhanced during the gastric phase by :
A- Distention of the antrum and the body of the stomach which initiates avagovagal
reflex resulting in a low volume of pancreatic secretion containing both bicarbonate ions
and enzymes.
B- Food breakdown products (amino acid and peptides) stimulate pancreatic
secretion by release gastrin ( from G cells of antrum) which produces a low-volume, high
enzyme pancreatic secretion.
3 - Intestinal phase: The most important for pancreatic secretion are hormone CCK(
and secretin. They are released from endocrine cells in the duodenum and jejunum during
the intestinal phase in response to the entrance of chyme into the small intestine.
The amino acids, fatty acids are the major stimuli for (CCK) secretion from I cells
which found in upper small intestine . CCK( in addition to its effect on the gallbladder, is a
potent stimulant of pancreatic enzyme secretion ,by its effect on the acinar cells which
trigger the second messenger inisitol tri phosphate(IP3) and then cells makes the
enzymes. Secretin present in S cells that found in upper small intestine this cells triggers
cAMP in the ductal cells to stimulates the HCO3¯ secretion. Low pH(<4.5) due to presence
of gastric acid in the intestine, stimulate for the release of secretin. This causes large
quantities of pancreatic juice containing HCO3 . Figure(11)
Phenylalanine, methionine, tryptophan
small peptides
Fatty acids
Figure(11) regulation of pancreatic secretion)
Disorders of the pancreas
Pancreatic Failure
is failure of the pancreas to secrete pancreatic juice into the small intestine.
Lack of pancreatic secretion frequently occurs (1) in pancreatitis (2) when the
pancreatic duct is blocked by a gallstone at the papilla of Vater, or (3) after the head
of the pancreas has been removed because of malignancy.Loss of pancreatic juice
means loss of trypsin, chymotrypsin, carboxypolypeptidase, pancreatic amylase,
pancreatic lipase, and still a few other digestive enzymes.Without these enzymes, as
much as 60 per cent of the fat entering the small intestine may be unabsorbed,
as well as one third to one half of the proteins and carbohydrates. As a result, large
portions of the ingested food cannot be used for nutrition, and copious,
fatty feces are excreted.
The gallbladder
is a thin walled green muscular sac on the inferior surface of the liver. The
gallbladder stores bile that is not immediately needed for digestion and concentrates
it. When the muscular wall of the gallbladder contracts bile is expelled into the bile
duct.
Bile secretion:
Bile is necessary for digestion and absorption of lipids in the small intestine. Bile is a
mixture of bile acids, bile pigments and cholesterol. Bile is produced and secreted by
liver, stored in gallbladder, and ejected into the lumen of small intestine. It emulsify
lipids to prepare them for digestion.
When chyme reaches the small intestine CCK is secreted which stimulates
contraction of gallbladder and relaxation of sphincter of Oddi, to flow bile into the
lumen of the duodenum. When lipid absorption is complete, bile acids are
recirculated to liver. figure(12)
Figure (12)bile secretion
The function of the gall bladder is :1-to store bile
2- to concentrate bile ( epithelial cells of gall bladder absorb ions and water)
3 -to eject bile into the lumen of the small intestine.
Ejection of bile from gall bladder begins within 30 minutes after a meal.
Bile is constituents of *bile acid (50%) *cholesterol (4%) *and phospholipids
(40%).*Bile contains electrolytes and water..
The main stimulus for ejection of bile is CCK which is secreted by I cell in response to
amino acids and fatty acid. 95% of secreted bill acids are recirculated to the liver, bile
acid excreted in feces 5% , must be replaced by synthesis of new bile acids.
Role of bile secretion
A-detergent action :
Bile secretion decrease the surface tension of the substance so it will brake them
into smaller particle called micelles to increase the surface area to about 1000 times
original one in which lipase will do it’s effect .
B –hydrotropic effect :
Which means makes lipid water soluble to pass easily via epithelial cell membrane
bile secretion regarded as amphipathic molecule which means that it has 2 poles
one is the hydrophilic pole which is formed from dissociate carboxyl group or
hydroxyl group and the other is the methyl group which represent the hydrophobic
pole of the bile salts .bile salts arrange themselves around the lipid molecule in such
away that the hydrophilic group in the surface and the hydrophobic toward the lipid
particles so the surface of lipid will be water soluble.
Gallstone Formation:Bile salts are formed in the hepatic cells from cholesterol in the blood plasma. In the
process of secreting the bile salts, about 1 to 2 grams of cholesterol are removed
from the blood plasma and secreted into the bile each day. Cholesterol is almost
completely insoluble in pure water, but the bile salts and lecithin in bile combine
physically with the cholesterol to form ultramicroscopic micelles in the form of a
colloidal solution. When the bile becomes concentrated in the gallbladder, the bile
salts and lecithin become concentrated along with the cholesterol, which keeps the
cholesterol in solution. Under abnormal conditions, the cholesterol may precipitate
in the gallbladder, resulting in the formation of cholesterol gallstones. The
amount of cholesterol in the bile is determined partly by the quantity of fat that the
person eats, because liver cells synthesize cholesterol as one of the products of fat
metabolism in the body. For this reason, people on a high-fat diet over a period of
years are prone to the development of gallstones.