Download B. Pancreatic digestive Enzymes

Lec.7
19 Apr 2016
Pancreas
Objectives
1. Physiological anatomy of pancreas &functions
2. Exocrine Function of pancreatic




Functional Anatomy
Composition.
Regulation.
Phases of pancreatic juice.
Physiological anatomy of pancreas and functions
The pancreas is a large compound gland, which lies parallel to and
beneath the stomach. The pancreas consists of a head located within the
curvature of the duodenum, and a body and tail which extends to the
spleen .It has both endocrine and exocrine functions.a.Exocrine portion
which secretes pancreatic juices. endocrine portion that secrete insulin
and glucagon which are not secreted by the same pancreatic tissue that
secretes intestinal pancreatic juice .Instead, insulin is secreted directly
into the blood by islets of Langerhans that occur in islet patches
throughout the pancreas.
Exocrine function of pancreas
Functional Anatomy
The structure of the exocrine pancreas resembles a cluster of grapes with
most of its internal structure similar to that of the salivary
.
glands .Digestive enzymes are secreted by the pancreatic acinar cells
and bicarbonate by duct cells. The pancreatic juice flows into long
pancreatic duct that joins the common bile duct immediately before it
empties into the duodenum through the papilla of vater surrounded by
the sphincter of Oddi . The pancreatic duct usually connects with bile
duct from liver and gallbladder
1
Composition of pancreatic juice
About 1500ml of pancreatic juice is secreted per day. 1. the two
important components of pancreatic juice, are bicarbonate ions and
water. 2. Pancreatic digestive enzyme. 3.ions such as Na+, K+ , Ca+ ,
Mg++ , HCO3- , Cl - , SO4 2 - , HPO4 2 –.
A. Bicarbonate Ions (HCO3-): bicarbonate is secreted by duct cells. The
two important components of pancreatic juice, are bicarbonate ions and
water. Pancreatic juice is alkaline due to the presence of high
concentration of bicarbonate (HCO3 - ) approximately 113meq/L.. Its pH
8 a. which provides a favorable condition for the action of digestive
enzymes and helps to neutralize the acidic chyme as it arrives from the
stomach. b.at the same time , the alkaline condition in the small intestine
blocks the action of pepsin which might damage the duodenal wall.
B. Pancreatic digestive Enzymes .The pancreas supplies the principal
enzymes for the digestion of all major foodstuffs, proteins ,carbohydrates
and fats. All pancreatic enzymes are synthesized and secreted by the
acinar cells.a. digestive enzymes for carbohydrates are pancreatic
amylase which hydrolyze carbohydrates such as starches, glycogen, and
most other carbohydrates (except cellulose ) to form mostly disaccharides
and a few disaccharides.
b. The main enzymes for fat digestion are (1) pancreatic lipase, which
is capable of hydrolyzing neutral fat into fatty acids and monoglycerides;
(2) cholesterol esterase, which causes hydrolysis of cholesterol esters;
and (3) phospholipase, which splits fatty acids from phospholipids.
Pancreatic lipase and amylase are secreted as active enzymes.
c. the protein splitting ( proteolytic ) enzymes are : trypsin ,
chymotrypsin , carboxypolypeptidase . When first synthesized in the
pancreatic cells, the proteolytic digestive enzymes are in the inactive
forms trypsinogen, chymotrypsinogen, and procarboxypolypeptidase,
which are all inactive enzymatically. They become activated only after
they are secreted into the intestinal tract. Trypsinogen is activated by an
enzyme called enterorkinase which is secreted by the intestinal mucosa
into the trypsin when chyme comes in contact with the mucosa. Also,
trypsinogen can be autocatalytically activated by trypsin that has already
been formed from previously secreted trypsinogen. Chymotrypsinogen is
activated by trypsin to form chymotrypsin, and procarboxypolypeptidase
is activated by trypsin into d carboxypolypeptide .
Trypsin Inhibitor It is important that proteolytic enzymes of the
pancreatic juice not become activated after they have been secreted into
2
the intestine because the trypsin and other enzymes would digest the
pancreas itself. Fortunately, the same cells that secrete proteolytic
enzymes secrete another enzyme called trypsin inhibitor which prevents
activation of trypsin and prevents digestion of the pancreas itself. And
because it is trypsin that activates other pancreatic proteolytic enzymes,
trypsin inhibitor prevents activation of the others as well.
C..ions such as cations Na+, K+ , Ca+ , Mg++ , and anions such as HCO3, Cl - , SO4 2 - , HPO4 2 –.
Regulation of Pancreatic Secretion
1. Hormonal control
2.Neura control
Hormonal control: Pancreatic secretion is mainly under hormonal
control.
a. CCK which is released from I – cells in the duodenum and jejunum. It's
secreted in response to fat and protein products that enter the duodenum
from stomach. CCK, passes by way of the blood and stimulate the acinar
cells of pancreas causing production of large quantities of pancreatic
digestive enzymes accounting for 70-80% and but relatively small
quantities of water and bicarbonate to go with the enzymes.
b. Secretin: is secreted from S-cell in the duodenum and jejunum in an
inactive form pro-secretin. Secretin is secreted when highly acid food
enters the small intestine. Its absorbed into the blood and stimulates
secretion of large quantities of pancreatic juice rich in bicarbonate HCO3
and water but little amount of pancreatic enzyme,. Neutralization of acid
chyme that enters into the duodenum by HCO3 - . This is a protective
mechanism against the development of duodenal ulcers.. When acid
chyme with pH less than 4.5 to 5.0 (about 3) enters the duodenum from
the stomach, it causes duodenal mucosal release and activation of
secretin, which is then absorbed into the blood. The one truly potent
constituent of chyme that causes this secretin release is the hydrochloric
acid from the stomach.
2. Neural control
Acetylcholine, which is released from the parasympathetic vagus nerve
endings and from other cholinergic nerves in the enteric nervous
system.ACH stimulates secretion of large quantities of pancreatic
digestive enzymes and small amounts water and HCO3.
3
Phases of Pancreatic Secretion
1. Cephalic phase 2. Gastric phase 3. Intestinal phase
1.Cephalic phase : During the cephalic phase of pancreatic secretion, the
same nervous signals from the brain that cause secretion in the stomach
also cause acetylcholine release by the vagal nerve endings in the
pancreas. . This causes secretion moderate amounts of pancreatic
enzymes after a meal, and also small amounts of HCO3 – and water are
secreted.
2. Gastric phase: Distension of either the proximal or distal regions of
the stomach stimulates pancreatic secretion by initiating vagovagal
reflexes. Because these are mediated by acetylcholine, secretion is
primarily release of small amounts of pancreatic enzymes after meal.. No
role of gastrin on pancreatic secretion.
3. Intestinal phase: After the chyme leaves the stomach and enters the
small intestine, a large amount of pancreatic secretion rich in bicarbonate
is secreted, mainly in response to the hormone secretin.
.
4