Pancreatic Physiology L4

• The pancreas is a large compound gland located beneath the stomach, with exocrine and endocrine functions.

• Exocrine pancreas produces organic secretions (primarily enzymes) and inorganic secretions (electrolytes) in response to food in the duodenum.

• Endocrine pancreas consists of islets of Langerhans, which secrete hormones directly into the bloodstream.

• Pancreatic juice contains a large amount of sodium bicarbonate solution to neutralize acidic chyme in the duodenum.

• Pancreatic enzymes are secreted by acinar cells, which contain zymogen granules.

• The islets of Langerhans produce three hormones: insulin, glucagon, and somatostatin, which regulate glucose, lipid, and protein metabolism.

• HCO3- is formed in the pancreatic ductal cells by the combination of CO2 and H2O, with the help of carbonic anhydrase, and is secreted into the pancreatic juice by an apical membrane transporter.

• The basolateral membrane of the ductal cells uses an Na+/H+ exchanger and Na+/K+ ATPase to transport Na+ into the cell, which is necessary for the HCO3- secretion process.

• The pancreatic juice is an isosmotic bicarbonate solution that plays a crucial role in the digestion and absorption of food in the small intestine.

• The pancreatic secretion rate increases with an increase in secretion velocity, which leads to an increase in the amount of HCO3- secreted and a decrease in the amount of Cl-, Na+, and K+, while maintaining the same concentration in the plasma and pancreatic juice.

• Pancreatic secretions contain various enzymes for digesting proteins, carbohydrates, fats, and large quantities of HCO3 ions.

• Three phases of pancreatic secretion: Cephalic, Gastric, and Intestinal.

• Cephalic Phase: stimulated by smell, hearing, and swallowing. Accounts for 20% of pancreatic secretion.

• Gastric Phase: triggered by gastric distention and presence of proteins, fatty acids, and acidity of the chyme. Accounts for 5-10% of pancreatic secretion.

• Intestinal Phase: initiated by the presence of food in the intestine. Accounts for 70-75% of pancreatic secretion.

• Gastric Phase secretion is reduced in case of vagotomy.

• Pancreatic enzymes for carbohydrate digestion include pancreatic amylase and maltase.

• Pancreatic enzymes for fat digestion include pancreatic lipase, cholesterol esterase, and phospholipase.

• Trypsin, chymotrypsin, and carboxypeptidase are the most important pancreatic enzymes for digesting proteins.

• Trypsinogen, chymotrypsinogen, and procarboxypeptidase are the inactive forms of the proteolytic digestive enzymes.

• Enteropeptidase, an enzyme secreted by the intestinal mucosa, activates trypsinogen.

• Trypsin and chymotrypsin activate carboxypeptidase.

• Trypsin, chymotrypsin, and carboxypeptidase split whole and partially digested proteins into peptides of various sizes without causing release of individual amino acids.

• Carboxypeptidase and carboxyl esterase split some peptides into individual amino acids.

• Trypsin inhibitor, a substance secreted by the pancreatic cells, prevents digestion of the pancreas itself.

• In case of obstruction or other causes, the accumulation of enzymes in the pancreas can lead to autoactivation and acute pancreatitis.

• The total secretion is greater when all stimuli of pancreatic secretion occur at once.

• The pancreas releases secretions in response to multiple stimuli, which "multiply" or "potentiate" each other.

• Vagus nerve stimulation: increases enzyme secretion and relaxes the sphincter of Oddi.

• Secretin: released when acid chyme (stomach content) enters the duodenum, stimulates secretion of large quantities of H2O and NaHCO3 solution from ductal cells.

• CCK: released in response to food in the small intestine, stimulates secretion of large quantities of H2O and NaHCO3 solution and pancreatic digestive enzymes.

• Acetylcholine: released from vagus nerve endings and other cholinergic nerves in the enteric nervous system, stimulates enzyme secretion and relaxes the sphincter of Oddi.

• Secretin and CCK act on different cells in the pancreas: secretin stimulates ductal cells to release a HCO3-rich fluid, while CCK stimulates acinar cells to release digestive enzymes.

• The secreted HCO3- concentration is much higher than normal but has a low concentration of Cl-. H2CO3 formed from this reaction releases CO2 and H2O.

• Secretin and CCK enhance each other's effects, leading to a greater total secretion than the sum of the secretions caused by each stimulus separately.

• Trypsin inhibitors get degraded in the duodenum to allow trypsin to function.

• Secretin is present in an inactive form (prosecretin) and has various effects on the pancreas and digestive system.

• Acinar cells secrete a protein-rich (digestive enzymes) secretion in an isotonic plasma-like fluid, and ductal cells secrete a HCO3-rich fluid to alkalinize and hydrate it.

• The main pancreatic duct joins the common bile duct and both open in the 2nd portion of the duodenum through the papilla of Vater.

• The sphincter of Oddi is regulated by CCK, which can relax it to allow secretions to flow into the duodenum or squeeze the bile and pancreatic ducts to release components into the duodenal lumen.

• Pancreatic secretions have major functions including neutralizing acid in the duodenal chyme, preventing damage to the duodenal mucosa, and providing an optimal pH for action of intestinal enzymes.

• All gastrointestinal secretions are isosmotic except for saliva, which is hypoosmotic.

• Enteropeptidase is a brush border enzyme and is not found in the pancreatic tissue or duct.

Test your knowledge of the phases of pancreatic secretions with this quiz. Learn about the cephalic, gastric, and intestinal phases and the stimuli and mediators involved in each phase.