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Term Paper on Pancreas
Term Paper # 1. Introduction to Pancreas:
The pancreas is both an exocrine and endocrine organ. It consists of three part- the head, body and tail. The head lies in the concavity of the duodenum and continues as the body which forms the major part of the gland. It ends in a pointed tongue-like tail which lies in close contact with the spleen.
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The portion of the pancreas that subserves the exocrine function is made up of compound alveolar tissue. It is made up of numerous secretory units or acini which form the parenchyma of the gland. The acini empty through the duct systems which coalesce to form the main duct of the pancreas—the duct of Wirsung.
The main pancreatic duct joins with the common bile duct to form a dilatation called the ampulla of Vater and opens into the duodenum through the duodenum papilla. The opening is guarded by a smooth muscle sphincter called the sphincter of Oddi. The duct of Santorini is the accessory pancreatic duct which also opens into the duodenum.
The pancreas receives parasympathetic nerve supply through the vagus. The postganglionic fibers innervate both the acinar cells and smooth muscles of the duct. The sympathetic fibers supply the blood vessels and are vasomotor in function. Pain fibers travel in the sympathetic afferents.
Term Paper # 2. Composition of Pancreatic Juice:
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Volume: 1000 to 1500 ml/day
Specific gravity: 1.010 to 1.018
pH: 7.8 to 8.4
Water: 98.5%
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Solids: 1.5%
Electrolytes: Cations: Na+, K+, Ca++, Mg++, Zn++
Anions: HCO3–, CI–, and traces of SO4–, HPO4–.
Enzymes:
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1. Pancreatic α-amylase
2. Pancreatic lipase
3. Pancreatic phospholipase A
4. Pancreatic cholesterol hydrolase
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5. Pancreatic colipase
6. Pancreatic proteolytic enzymes ―
These are powerful protein splitting enzymes that are secreted as inactive proenzymes:
i. Trypsinogen
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ii. Chymotrypsinogen
iii. Procarboxypeptidase A and B
iv. Ribonuclease
v. Deoxyribonuclease
vi. Proelastase
The pancreatic enzymes can act only in an alkaline medium. The high bicarbonate content of the pancreatic juice (>100 mEq/L) neutralizes the acid chyme which enters the duodenum, raising the pH to 6 to 7, which ensures the alkalinity in the intestines.
Activation of the Proenzyme:
Trypsinogen is converted into the active form trypsin by the enteropeptidase or enterokinase secreted in the small intestine. Trypsin thus formed autocatalyzes the process of activation of trypsinogen.
Trypsin then converts the other proenzymes to their active forms:
a. Chymotrypsinogen to chymotrypsin
b. Proelastase to elastase
c. Procarboxypeptidase to carboxypeptidase
d. Colipase to active colipase
e. Phospholipase A to active phospholipase A
The pancreatic acinar cells contain a trypsin inhibitor which prevents the auto digestion of the pancreas by the activated trypsin.
Action of Pancreatic Enzymes:
1. Pancreatic α-Amylase:
Acts on starch and converts it to maltose, maltotriose and a-limit dextrins. It acts on uncooked starch.
2. Pancreatic Lipase:
Hydrolysis the neutral and long chain fats (triglycerides) to mono-and diglycerides, fatty acids and glycerol. Pancreatic lipase is water soluble and acts in the presence of colipase at the lipid water interphase. This lipid water interphase is provided by the emulsifying action of the bile salts.
3. Pancreatic Cholesterol Hydrolase:
Hydrolysis cholesterol esters to form cholesterol.
4. Pancreatic Colipase:
Enables pancreatic lipase to act on emulsified fat by binding to the carboxy terminal of lipase, opens the lid which covers the active site of action.
5. Trypsin and Chymotrypsin:
Acts on proteoses, peptones and peptides and converts them to polypeptides. Some amino acids are also formed. Both the enzymes are endopeptidases and act on the interior peptide bonds at the carboxyl side. Chymotrypsin coagulates milk.
6. Carboxypeptidases A and B:
Carboxypeptidases A and B are exopeptidases that act on the C-terminal of peptides. Type A acts on aromatic amino acids while type B acts on basic chains (arginine, lysine).
7. Ribonuclease and Deoxyribonuclease:
Act on RNA and DNA respectively to form nucleotides.
8. Elastase:
Digest the elastin fibers in meat. It is an endopeptidase.
Term Paper # 3. Formation of Pancreatic Juice:
There are three steps in the formation:
i. Synthesis of the enzyme in the granular endoplasmic reticulum of the acinar cells.
ii. Storage of the enzyme in zymogen granules in Golgi apparatus.
iii. Discharge of the granules by exocytosis.
Bicarbonates and other electrolytes are secreted by the centroacinar cells, and cells lining intercalated ducts. When the secretion passes through the larger ducts the bicarbonate is absorbed in exchange for chloride. Hence, when the rate of secretion is low, more bicarbonate gets absorbed and so it’s content in the excreted juice is less.
Term Paper # 4. Regulation and Phases of Pancreatic Secretion:
i. Nervous regulation
ii. Humoral regulation
i. Nervous Regulation:
Stimulation of vagus causes the secretion of pancreatic juice rich in enzymes mediated via acetylcholine.
ii. Hormonal Regulation:
Two GI hormones secretin and cholecystokinin-pancreozymin (CCK-PZ) stimulate the secretion of pancreas.
Secretin causes the secretion of pancreatic juice which is watery and alkaline, rich in bicarbonates. It acts on the pancreatic ducts where bicarbonates are absorbed in exchange for chlorides. CCK-PZ causes the secretion of thick viscous juice small in volume but rich in enzymes. It acts on the acinar cells and causes the release of zymogen granules.
Phases of Pancreatic Secretion:
i. Cephalic Phase:
It forms about 15 to 20% of the total secretion. The sight smell and thought of food causes the secretion of enzyme rich pancreatic juice.
ii. Gastric Phase:
It forms 5 to 10% of the secretion. Presence of food in the stomach causes distension, leading to the secretion of small volume of pancreatic juice rich in enzymes. This is mediated via the gastropancreatic reflex through the vagus.
iii. Intestinal Phase:
It contributes to 75% of the total secretion. Entry of food into the duodenum causes the secretion of the hormones secretin and CCK-PZ which is responsible for this phase of the secretion.
Term Paper # 5. Pancreatic Function Tests:
i. Estimation of Serum Amylase:
Normal: 50 to 120 units/L. Increased in acute pancreatitis.
ii. Fecal Fat Excretion Test:
Fat is split mainly by the pancreatic lipase. Therefore, in the pancreatic insufficiency, the fat content of stools markedly increases. Normal fat excretion: 5 to 6 gm/day. In pancreatic insufficiency, it is increased to 40 to 50 gm/day.
iii. Lundh Test:
This assesses the function of trypsin in the pancreatic juice. Pancreatic juice is obtained by duodenal intubation following a meal. The average trypsin activity of less than 6 IU/L indicates pancreatic insufficiency.
iv. Secretin and CCK-PZ Stimulation Test:
This is done by aspirating the duodenal contents following the injection of secretin intravenously followed by CCK-PZ. The total volume of the juice aspirated, the pH, HCO3– and enzyme concentration is measured and analyzed.
v. Cytological Examination:
Fresh uncontaminated duodenal aspirate is collected and cytological analysis done for cancer cells.
Applied Aspects:
Acute Pancreatitis:
Here there is activation of the pancreatic enzymes within the gland resulting in auto digestion and chemical inflammation of the gland. There is marked elevation of amylase and other enzymes along with impairment of the endocrine functions of the gland.
Hypo function of the gland can occur following surgical resection of the pancreas, chronic pancreatitis, and neoplastic tumors of the pancreas and fibrocystic disease of the pancreas.
Decreased secretion of pancreatic enzymes results in steatorrhea (excessive excretion of fat in the feces) characterized by bulky, frothy, foul-smelling stools.
Fibrocystic disease of the pancreas is a rare condition which occurs in childhood. It affects other glands like the sweat glands and shows changes in the electrolyte composition of sweat.
Carcinoma of the head of the pancreas can obstruct the bile duct resulting in obstructive jaundice.