Pancreas

Pancreas

Pancreas is a dual organ having two functions, namely endocrine function and exocrine function. Endocrine function is concerned with the production of hormones. The exocrine function is concerned with the secretion of digestive juice called pancreatic juice.

The pancreas is a pale grey gland weighing about 60 grams. It is about 12 to 15 cm long and is situated in the epigastric and left hypochondriac regions of the abdominal cavity. It consists of a broad head, a body and a narrow tail. The head lies in the curve of the duodenum, the body behind the stomach and the tail lies in front of the left kidney and just reaches the spleen. The abdominal aorta and the inferior vena cava lie behind the gland.

Blood supply-

The splenic and mesenteric arteries supply arterial blood to the pancreas and the venous drainage is by the veins of the same names that join other veins to form the portal vein.

Nerve supply-

 As in the alimentary tract, parasympathetic stimulation increases the secretion of pancreatic juice and sympathetic stimulation depresses it.

Pancreatic Juice-

Volume - 500 to 800 mL/day

 Reaction - Highly alkaline with a pH of 8 to 8.3

Specific gravity - 1.010 to 1.018

Function of Pancreatic Juice-

Digestive Function- Pancreatic juice plays an important role in the digestion of proteins and lipids. It also has mild digestive action on carbohydrates.

1. Digestion of Proteins-

Pancreatic juice plays an important role in the digestion of proteins and lipids. It also has mild digestive action on carbohydrates.

Trypsin-

Trypsin is a single polypeptide with a molecular weight of 25,000. It contains 229 amino acids.

It is secreted as inactive trypsinogen, which is converted into active trypsin by enterokinase. Enterokinase is also called enteropeptidase and it is secreted by the brush­bordered cells of duodenal mucus membrane.

Actions of trypsin-

• Digestion of proteins- Trypsin is the most powerful proteolytic enzyme. It is an endopeptidase and breaks the interior bonds of the protein molecules and converts proteins into proteoses and polypeptides
•   Curdling of milk- It converts caseinogen in the milk into casein
•  Blood clotting- It accelerates blood clotting
•  It activates the other enzymes of pancreatic juice, viz.

1. Chymotrypsinogen into chymotrypsin
2. Procarboxypeptidases into carboxypeptidases
3. Proelastase into elastase
4. Procolipase into colipase

• Trypsin also activates collagenase, phospholipase A and phospholipase B
•  Autocatalytic action-Once formed, trypsin itself converts trypsinogen into trypsin.

Chymotrypsin-

 Chymotrypsin is a polypeptide with a molecular weight of 25,700 and 246 amino acids. It is secreted as inactive chymotrypsinogen, which is activated into chymotrypsin by trypsin.

Actions of chymotrypsin-

• Digestion of proteins: Chymotrypsin is also an endopeptidase and it converts proteins into polypeptides
• Digestion of milk: Chymotrypsin digests caseinogen faster than trypsin. Combination of both enzymes causes rapid digestion of milk
• On blood clotting: No action.

Carboxypeptidases-
 Carboxypeptidases are carboxypeptidase A and carboxypeptidase B. Carboxypeptidase A is derived from the precursor procarboxypeptidase A. Carboxypeptidase B is derived from procarboxypeptidase B. Procarboxypeptidases are activated into carboxypep tidases by trypsin.

Actions of carboxypeptidases-

 Carboxypeptidases are exopeptidases and break the terminal bond of protein molecules. Exopeptidases split the polypeptides and other proteins into amino acids.

 Carboxypeptidase A splits the proteins into amino acids having aromatic or aliphatic side chains. Carboxypeptidase B converts the proteins into amino acids having basic side chains.

Nucleases-

 Nucleases of pancreatic juice are ribonuclease and deoxyribonuclease, which are responsible for the digestion of nucleic acids. These enzymes convert the ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) into mononucleotides.

Elastase-

 Elastase is secreted as inactive proelastase, which is activated into elastase by trypsin. Elastase digests the elastic fibers.

 Collagenase-

Collagenase is secreted as inactive procollagenase, which is activated into collagenase by trypsin. It digests collagen.

2. Digestion of Lipids-

Pancreatic lipase-

 Pancreatic lipase is a powerful lipolytic enzyme. It digests triglycerides into monoglycerides and fatty acids. Activity of pancreatic lipase is accelerated in the presence of bile. Optimum pH required for activity of this enzyme is 7 to 9.

• Bile salts, which are responsible for the emulsification of fat, prior to their digestion
• Colipase, which is a coenzyme necessary for the pancreatic lipase to digest the dietary lipids.

Cholesterol ester hydrolase-

 Cholesterol ester hydrolase or cholesterol esterase converts cholesterol ester into free cholesterol and fatty acid by hydrolysis.

Phospholipase A-

 Phospholipase A is activated by trypsin. Phospholipase A digests phospholipids, namely lecithin and cephalin and converts them into lysophospholipids. It converts lecithin into lysolecithin and cephalin into lysocephalin.

 Phospholipase B-

Phospholipase B is also activated by trypsin. It converts lysophospholipids (lysolecithin and lysocephalin) to phosphoryl choline and free fatty acids.

Colipase-

Colipase is a small coenzyme, secreted as inactive procoli pase. Procolipase is activated into colipase by trypsin. Colipase facilitates digestive action of pancreatic lipase on fats.

 Bile-salt-activated lipase-

Bile­salt­activated lipase is the lipolytic enzyme activated by bile salt. It is also called carboxyl ester lipase or cholesterol esterase. This enzyme has a weak lipolytic action than pancreatic lipase. But it hydrolyses a variety of lipids such as phospholipids, cholesterol esters and triglycerides. Human milk contains an enzyme similar to bile-salt-activated lipase.

3. Digestions of Carbohydrates-

Pancreatic amylase is the amylolytic enzyme present in pancreatic juice. Like salivary amylase, the pancreatic amylase also converts starch into dextrin and maltose.

4. Neutralization action of pancreatic juice-

When acid chyme enters intestine from stomach, pancreatic juice with large quantity of bicarbonate is released into intestine. Presence of large quantity of bicarbonate ions makes the pancreatic juice highly alkaline. This alkaline pancreatic juice neutralizes acidity of chyme in the intestine. Neutralizing action is an important function of pancreatic juice because it protects the intestine from the destructive action of acid in the chyme.

Enzyme	Activator	Acts on (substrate)	End products
Trypsin	Enterokinase Trypsin	Proteins	Proteoses and polypeptides
Chymotrypsin	Trypsin	Proteins	Polypeptides
Carboxypeptidases	Trypsin	Polypeptides	Amino acids
Nucleases	Trypsin	RNA and DNA	Mononucleotides
Elastase	Trypsin	Elastin	Amino acids
Collagenase	Trypsin	Collagen	Amino acids
Pancreatic lipase	Alkaline medium	Triglycerides	Monoglycerides and fatty acids
Cholesterol ester hydrolase	Alkaline medium	Cholesterol ester	Cholesterol and fatty acids
Phospholipase A	Trypsin	Phospholipids	Lysophospholipids
Phospholipase B	Trypsin	Lysophospholipids	Phosphoryl choline and free fatty acids
Colipase	Trypsin	Facilitates action of pancreatic lipase	-
Bile­salt­activated lipase	Trypsin	Phospholipids  Cholesterol esters Triglycerides	Lysophospholipids Cholesterol and fatty acids Monoglycerides and fatty acids
Pancreatic amylase	-	Starch	Dextrin and maltose

 

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