Pancreas

Pancreas v Itis an accessory structures in the digestive system. v Ithas both endocrine & exocrine functions. v Endocrine partsmaller than exocrine part. ü Endocrine functions are performed by islets of Langerhans (pancreatic islets). ü This portion contains some cells; the main cells are β cells thatsecrete insulin and α cells thatsecrete glucagon. ü The majority of pancreas is the exocrine cells (pancreatic acini) ü Pancreatic acinar cell secretes pancreatic juice (pancreatic secretion) and its two parts: I. fluid secretions II. enzymes Ø Pancreatic secretions (juice): § Enzyme (is secreted by acinar cell(inactive enzymes)) ð Note Zymogen granules: granules that contain inactive enzymes. Ø Fluid secretions: § Water secretions rich in bicarbonate (HCO3-), then its alkaline secretions.why?! ð The enzymes thatreleased by pancreatic cells are importantin digestion, and these enzymes work in neutral - alkaline environment, then we mustneutralize the chyme (the mix of food and stomach secretion) in duodenum (Into the pancreatic ducttoward the duodenum.) Ø Digestive enzymes: § Pancreatic amylase thatdigest carbohydrates. § Pancreatic lipase thatdigestlipids. § Proteolytic enzymes: A group of protein digestive enzymes (the main one is trypsin thatsecreted in an inactive form “trypsinogen” and then activated by enterokinase). Other enzymes: chymotrypsinogen & proelastase & Procarboxypeptidase ð Remember: Pancreatic amylase & Pancreatic lipase (thatdigestcarbohydrates and lipids respectively) are active. ð Butthe enzymes thatdigestproteins mostly secreted in an inactive form (zymogens). ð They will be activated in duodenum § The main enzyme is trypsinogen in addition to many other inactive enzymes (zymogens). o The enterokinase which is brush border enzyme ,itactivate trypsinogen converted itinto trypsin. o Trypsin in turn activates / converts other inactive pancreatic enzymes (zymogens) into active form. ð brush border enzyme = located in microvilli of small intestines) v Trypsin is the activated form of trypsinogen and its activated by enterokinase v Trypsin digests proteins by cleavage of internal peptide bonds between amino acids. v The long chains of proteins are digested into free amino acids, 2 amino acids or 3 amino acid maximum to be absorbable. Ø Other enzymes such as chymotrypsin, elastase and Carboxypeptidase are activated by trypsin (the activator). v Chymotrypsin is the activated form of chymotrypsinogen and itcleaves internal peptide bonds. v Elastase is the activated form of proelastase and it cleaves internal peptide bonds. v Carboxypeptidase is the activated form of Procarboxypeptidase itcleaves lastamino acid from carboxyl-terminal end of polypeptide v Pancreatic lipase (digestlipids) thatcleaves triglyceride into free fatty acids and glycerol. v Pancreatic amylase digeststarch (carbohydrates) to maltose and shortchains of glucose molecules (dextrins). v digestive system, and in addition to the secretion of enzymes, acids and mucus, there are enteroendocrine cells thatsecrete certain types of hormones thatplay an importantrole in regulating gastric functions. Ø like Gastrin thatsecreted by G cells in stomach: Ø Stimulates parietal cells to secrete HCl. Ø Ø Stimulates chief cells to secrete pepsinogen. Ø Gastrin Affects and maintains structure of gastric mucosa by the stimulation of proliferation of gastric mucosa. v The cells of small intestine secrete different types of hormones: ü Like Secretin thatsecreted from S cells in duodenum and jejunum: ü Secretin Stimulates water and bicarbonate secretion in pancreatic juice (from pancreas it affects the exocrine portion, itaffects the cell in the ducts). ü Secretin Potentiates of cholecystokinin actions on pancreas. ü Cholecystokinin (CCK) thatsecreted by (CCK )cell in small intestine: ü Cholecystokinin its functions is potentiated by secretin. ü Stimulates contraction of gallbladder that contains bile (the bile synthesized in liver and stored in gallbladder). ü After eating the (CCK)is secreted and stimulates contraction of gallbladder to secrete and release the bile, then bile moves via bile ducts to duodenum ü The bile is importantin digestion, because it contributes in fatdigestion. ü Affects the exocrine portion of pancreas (affect acinar cells) then itstimulates secretion of pancreatic juice enzymes. ü Secretin affectin duct(water and bicarbonate secretion) ü Cholecystokinin Inhibits gastric motility and secretion. ü ð CCK is secreted by small intestine when the small intestine is full with material (chyme), then the small intestine work and digestmaterials thatin it, and there mustnotbe any moving of food from stomach into small intestine because itwill be more overwhelmed. So, itinhibits gastric motility (inhibits digestion and empting). ü Maintains structure of exocrine pancreas (acinl). v Gastric inhibitory peptide (GIP): glucosedependentinsulinotropic peptide Ø Inhibits gastric motility and secretion. Ø Stimulates secretion of insulin from pancreatic islets. • The Action of Pancreatic Amylase • Mostcarbohydrates in the dietare starch (a polymer of glucose). • The digestion of carbohydrates begins in the oral cavity by salivary amylase (partial digestion). • carbohydrates digestion continues in the small intestine (in duodenum) by pancreatic amylase thatconverts starch (polysaccharides) into short oligosaccharides (dexstrins & maltriose & maltose) thataren’tcapable to be absorbed, (not complete digestion). • The carbohydrates to be digested itmustbe as monomers (monosaccharides). • These substances thatformed because of salivary amylase digestions & pancreatic amylase (Oligosaccharides) digestions are digested more and more by sI brush border enzymes. (SI = Sucrase-isomaltase) • Brush border enzymes thatresponsible in carbohydrate digestion and digestall those components into monosaccharides (glucose & galactose & fructose) thatare absorbable., o The absorption of glucose and galactose occurs by secondary active transportwith Na+. (symport) o Na+ enters with its electrochemical gradient with glucose and galactose. o The absorption of fructose occurs by facilitated diffusion (carrier protein that facilitates the fructose diffusion into the enterocytes). o The entrance of glucose & galactose is different. than thatin fructose (different mechanism). o Butthey leave enterocytes in small intestine to the circulation atthe same mechanism (by facilitated diffusion). Ø The protein is polypeptides (sequence of amino acids). Ø The digestion of proteins begins in the stomach by pepsin & HCl (partial digestion). Ø The digestion of polypeptides in small intestine occur by the pancreatic enzymes. Ø Some pancreatic enzymes called endopeptidase (trypsin, chymotrypsin, elastase). Ø And the other called exopeptidase (carboxypeptidase). Ø These enzymes break down the polypeptides and peptides bonds giving tripeptides (3 amino acids together), dipeptides (2 amino acids) or single / free amino acids. And all of them are absorbed. Ø The transportation of them from the enterocytes into circulation occurs as single amino acids Ø . Then, inside the epithelial cells of small intestine there are enzymes convertthe tripeptides & dipeptides into single amino acids. Ø The enzyme thatacts upon the tripeptides is tripeptidase converted itto single amino acids . Ø The enzyme thatacts upon the dipeptides is dipeptidase converted itto single amino acids. Ø These single amino acids transported in certain mechanism to the circulation. v The single amino acids are transported from lumen into the enterocytes occurs by active transportor secondary active transportwith Na+ (using energy indirectly). v Dipeptides & Tripeptides are transported into the enterocytes by secondary active transportwith H+. ü Dipeptides in the enterocytes are broken down by dipeptidase into single amino acids. ü Tripeptides in the enterocytes are broken down by tripeptidase into single amino acids. ü Single amino acids transported to the circulation by diffusion.