Pancreas and Liver Flashcards
(16 cards)
Describe the functional unit of the pancreas
consists of acinar cells that secrete enzymes and NaCl rich secretions (stored in zymogen granules that accumulate during fasting) into a system of intercalated and intralobular ducts, the duct cells of which secrete a bicarbonate rich solution
Describe the blood supply to the pancreas
Supplied by branches of the celiac and superior mesenteric arteries and is drained by the hepatic portal vein.
Describe the innervation of the pancreas
preganglionic parasympathetic vagal fibres synapse with cholinergic postganglionic fibres within the pancreas. Pancreatic blood vessels are innervated by sympathetic fibres from the celiac and superior mesenteric plexuses.
How is the aqueous component of pancreatic juice formed?
Pancreatic juice is secreted mainly by columnar epithelial cells lining ducts. Resting secretion comes from intercalated and intralobular ducts; during stimulation, interlobular ducts also contribute. The fluid is slightly hypertonic, rich in HCO3–, and has Na+ and K+ concentrations similar to plasma.
How are ions and water transported during pancreatic secretion?
H+ ions are transported from duct cells to plasma in exchange for Na+ or K+. HCO3– ions are secreted into the duct lumen in exchange for Cl–. Na+ diffuses paracellularly to maintain electroneutrality, and water follows osmotically into the lumen.
How is the aqueous component of pancreatic secretion regulated?
Secretion of the aqueous component is regulated by cAMP, which prolongs the opening of apical Cl– channels and may stimulate proton pumps at the basolateral membrane.
How does the ionic composition of pancreatic juice change with stimulation?
Unstimulated, Intralobular duct cells transfer ions from the plasma to produce a secretion of similar ion content to plasma. Low levels of aqueous secretion and no modification in the interlobular ducts. Stimulated, secretion occurs in the interlobular ducts, water and bicarbonate secretion is stimulated by secretin - low Cl-
How are pancreatic proteolytic enzymes secreted and activated?
The pancreas stores trypsin, chymotrypsins, and carboxypeptidases as inactive zymogens (trypsinogen, chymotrypsinogen, procarboxypeptidase) inside acinar cells. Activation occurs in the small intestine by enteropetidase or by trypsin itself. Trypsin and chymotrypsins are endopeptidases that hydrolyze peptide bonds; carboxypeptidases (activated by trypsin), elastase, and aminopeptidases further digest peptides into smaller units
How is premature activation of trypsinogen prevented?
Acidic environments within zymogen granules and the presence of trypsin inhibitors in pancreatic juice prevent premature activation. If trypsinogen activates prematurely, it can cause acute necrotizing pancreatitis, a life-threatening condition involving self-digestion of pancreatic tissue.
What is the role of pancreatic amylase?
Pancreatic amylase, secreted in active form, digests starch in the duodenum. It works optimally at pH 6.9 and can digest both cooked and uncooked starch into oligosaccharides like maltose and maltotriose within 10 minutes. Brush-border enzymes then convert these into glucose. Elevated pancreatic amylase levels can signal pancreatic injury.
What are the main pancreatic lipolytic enzymes and their activation?
Pancreatic juice contains lipases (including colipase, cholesterol esterase, and phospholipase A2) secreted as inactive zymogens. Trypsin activates them in the duodenum. Pancreatic lipase breaks down triglycerides at the oil–water interface into free fatty acids and monoglycerides, assisted by colipase anchoring the enzyme to fat droplets.
Explain the effects of Cystic Fibrosis on pancreatic function
Chloride and bicarbonate ions are normally secreted into the ductal lumen by CFTR - driving force for the fluid movement maintaining solubility of ductal enzymes from acinar. In CF, impaired CFTR leads to lower volumes of more acid secretion, causing enzyme precipitation, duct obstruction, reduced efficiency, insufficiency and damage.
Describe the cephalic phase of pancreatic secretion
Less than 20% total secretion. Sight, smell, chewing cause parasympathetic vagal nerve endings to secrete synergistic Ach and Vasoactive intestinal peptide (VIP), causing release of zymogen granules and increased local blood flow. Aside from direct action, vagal stimulation causes antral gastrin secretion mediating a small part of the cephalic phase of secretion
Describe the gastric phase of pancreatic secretion
relatively small component chiefly mediated by gastrin, secreted in response to distension of the stomach and presence of amino acids and peptides in the antrum. Vagovagal reflex also elicits secretion
Describe the intestinal phase of pancreatic secretion
More than 70% total secretion. Chemical content of food and low pH stimulates the upper intestinal mucosa to secrete CCK and secretin respectively. CCK stimulates enzyme rich fluid production from the acinar cells and enhances the secretory effects of secretin, which increases the rate of bicarbonate rich-fluid production by ductal cells.
How does secretin decrease duodenal pH
via three means: it increases pancreatic bicarbonate secretion, decreases gastric emptying and decreases gastric secretion
