Role of Pancreas in digestion
Pancreas has both endocrine and exocrine portions. The endocrine portion of the pancreas is made up of islets of Langerhans, secrets hormones like glucagon (alpha cells), insulin (beta cells) and somatostatin (delta cells) into the blood. The major portion of pancreas is exocrine in function.
Secretions of Pancreas
The enzymes of the pancreatic juice are synthesized and secreted by the acinar cells. The small duct system contain high level of carbonic anhydrase is responsible for the secretion of water and electrolytes.
In most species, the pancreatic duct empties directly into duodenum. Horse, dog and fowl usually have two pancreatic ducts. but In sheep, pancreatic duct joins with common bile duct before entering into duodenum.
Composition of pancreatic juice
Pancreatic juice is a clear alkaline fluid made up of the secretions of two separate phases , aqueous and organic phases
The aqueous phase causes a higher concentration of HCO3 and lesser amounts of Cl. The HCO3 is important for partial neutralization of the acid chyme from the stomach and maintenance of H+ ion concentration suitable for digestive activities of pancreatic enzymes.
The organic phase produces more of pancreatic enzymes. The amount of juice secreted in Horse – 10 to 12, Cattle – 3 to 5, Sheep – 0.5 to 1 L /100kg body weight/day.
Pancreatic enzymes
There are three major groups of enzymes namely proteases, lipases and amylase capable of digesting proteins, fats and carbohydrates respectively.
- The following proteolytic enzymes of pancreas are secreted as zymogen granules or proenzymes-
- Trypsinogen
- Chymotrysinogen
- Procarboxy peptidase A and B
- Proelastase
Trypsingoen is converted to active trypsin either by the action of an enzyme enterokinase present in duodenal and jejunal juice or by autocatalysis.
Chymotrysingoen, procarboxy peptidases and proelstases are activated by trypsin.
Trypisn, elastase and chymotrypsin are endopeptidases; carbodxy pepetidase is a exopeptidase.
Pancreatic amylase (amylopsin) is secreted as an active state. It acts on starch and produces oligosacharides and maltose. It requires an optimum pH of 6 to 9 and chloride ions for this action.
Pancreatic lipase (steapsin) is secreted as an active form.It splits fats into free fatty acids and glycerol. Calcium ions, polypeptides, peptidees and bile salts enhance lipase activity.
Cholesterol esterase and phospholipase act on choleterol esters and phospholipids producing non-esterified fatty acids, cholesterol and lysophospholipid.
Ribonuclease and deoxy ribonuclease present in pancreatic juice reduce ribose nucleic acid and deoxy ribonucleic acids to mononucleotides.
Regulation of pancreatic secretion
Nervous regulation of Pancreas
Nervous regulation of Pancreas is in three phases- Cephalic phase, Gastric phase and Intestinal phase.
1. Cephalic phase
The cephalic phase of gastric secretions , simultaneously transmit the impulses along the vagi to pancreas; results in pancreatic secretion rich in enzymes, but little water and electrolytes.
2. Gastric phase
Distension of stomach due to entry of food into the stomach causes reflex stimulation of pancreatic secretion through vagovagal reflex.
3. Intestinal phase
Distension of intestine following the entry of food into the small intestine, stimulates the production of pancreatic enzymes and their secretion. This phase is controlled by both intrinsic nerves of the intestine and vagus. Acetylcholine released by the vagal action and local nerve reflexes sensitize the pancreas for the action of secretin and CCK.
Hormonal Regulation of Pancreas
Secretin and CCK are the important hormones regulate pancreatic secretion.
Secretin is the gastro intestinal hormone secreted by the duodenal mucosa of small intestine. Presence of acid ingesta, peptides, soaps and amino acids, HCl in the stimulate the release of secretin. When the duodenal pH falls below 4.0 secretin causes increased secretion of thin watery pancreatic juice with high concentration of HCO3 with less or no enzyme referred as hydrolytic secretion. This type of HCO3 release is to neutralise the acidity of duodenal contents. It inhibits gastric secretion.
Cholecystokinin (CCK) stimulates the pancreatic secretion rich in enzymes referred as ecbolic secretion identical to the action of vagal stimulation. CCK also causes contraction of gall bladder and also delays gastric emptying.
Vasoactive intestinal polypeptide (VIP) another intestinal hormone, stimulates pancreatic HCO3 secretion, whereas the pancreatic polypeptide, inhibits pancreatic HCO3 and enzyme secretions.