Pancreatic Exocrine Secretions and Control

• What are the two main functional components of the pancreas?

  The pancreas has exocrine and endocrine components. The exocrine part secretes digestive enzymes; the endocrine part secretes hormones like insulin and glucagon.
• What are the structural units of the exocrine pancreas?

  Pancreatic acini (clusters of acinar cells) and the ductal system (intercalated, intralobular, and interlobular ducts).
• What type of cells make up the pancreatic acini?

  Pyramidal-shaped acinar cells, which are highly polarised epithelial cells with basal nuclei and apical zymogen granules.
• What is the main function of acinar cells in the pancreas?

  To synthesise, store, and secrete digestive enzymes (e.g., trypsinogen, chymotrypsinogen, amylase, lipase) in zymogen granules.
• What are zymogen granules and where are they found?

  Membrane-bound vesicles in acinar cells' apical cytoplasm that store inactive digestive enzymes before secretion.
• What cells initiate the pancreatic ductal system?

  Centroacinar cells, located at the center of the acinus, are the start of the ductal system and are continuous with intercalated ducts.
• What is the role of the ductal system in the exocrine pancreas?

  To modify and transport acinar secretions. Duct cells add bicarbonate-rich fluid to neutralise acidic chyme from the stomach.
• What are the main duct types in the exocrine pancreas?

  1. Intercalated ducts – drain individual acini, secrete HCO₃⁻
  2. Intralobular ducts – within lobules, transport modified secretions
  3. Interlobular ducts – between lobules, merge into main pancreatic duct
• What do acinar cells primarily secrete?

  Digestive enzymes, including:

  • Proteases (e.g., trypsinogen, chymotrypsinogen)
  • Amylase (for carbohydrates)
  • Lipase (for fats)
  • Nucleases
• Are enzymes secreted by acinar cells active or inactive? Why?

  Mostly inactive zymogens to prevent auto-digestion of pancreatic tissue.
• What do duct cells primarily secrete?

  Bicarbonate (HCO₃⁻) and water – to flush enzymes and neutralise gastric acid.
• How do acinar and duct cells work together in secretion?

  Acinar cells produce enzymes; duct cells dilute and alkalinise the secretion by adding HCO₃⁻ and fluid.
• What are the main phases of pancreatic secretion?

  1. Cephalic phase – via vagus nerve (ACh); sight/smell of food
  2. Gastric phase – gastric distension stimulates secretion
  3. Intestinal phase – major phase, controlled by hormones
• Which hormones control pancreatic exocrine secretion?

  • Cholecystokinin (CCK) – stimulates enzyme secretion from acinar cells
  • Secretin – stimulates HCO₃⁻ secretion from duct cells
• What stimulates CCK release?

  Fatty acids and amino acids in the small intestine stimulate I cells to release CCK.
• What stimulates secretin release?

  Acidic chyme (low pH) in the duodenum stimulates S cells to release secretin.
• What is the role of ACh in exocrine pancreatic secretion?

  Released by vagus nerve; stimulates both enzyme and bicarbonate secretion synergistically with CCK.
• What are the major constituents of pancreatic juice?

  • Digestive enzymes (e.g., amylase, lipase, proteases)
  • Bicarbonate (HCO₃⁻)
  • Water
  • Electrolytes (Na⁺, K⁺, Cl⁻)
• What is the function of bicarbonate (HCO₃⁻) in pancreatic juice?

  • Neutralises acidic chyme entering the duodenum from the stomach
  • Creates an optimal pH (~7-8) for pancreatic enzyme activity
  • Protects the intestinal mucosa from acid damage
• Which pancreatic cells are responsible for bicarbonate secretion?

  Ductal epithelial cells of the pancreas
• What are the three main classes of pancreatic enzymes?

  1. Amylolytic enzymes (e.g., amylase)
  2. Proteolytic enzymes (e.g., trypsinogen, chymotrypsinogen, procarboxypeptidase)
  3. Lipolytic enzymes (e.g., lipase, phospholipase, colipase)
• What is the function of pancreatic amylase?

  Breaks down starch into maltose and other disaccharides
• What is the function of pancreatic lipase?

  • Breaks down triglycerides into monoglycerides and free fatty acids
  • Requires colipase for activity in the presence of bile salts
• What do trypsin and chymotrypsin do?

  • Break peptide bonds in proteins to form smaller peptides
  • Trypsin also activates other protease precursors (zymogens)
• How are pancreatic proteases secreted and activated?

  • Secreted as inactive precursors (zymogens)
  • Trypsinogen is activated to trypsin by enterokinase (enteropeptidase) in the small intestine
  • Trypsin activates other zymogens like chymotrypsinogen and procarboxypeptidase
• Why are pancreatic proteases secreted as zymogens?

  • To prevent auto-digestion of the pancreas
  • Activation occurs only in the small intestine, ensuring safe protein digestion
• What is the function of water in pancreatic juice?

  • Acts as a solvent for enzymes and bicarbonate
  • Facilitates transport of enzymes into the duodenum
  • Helps maintain fluidity of digestive secretions
• What hormone stimulates bicarbonate secretion by ductal cells?

  • Secretin
  • Released from the duodenum in response to acidic chyme
  • Acts on ductal cells to increase HCO₃⁻ secretion
• Which hormone stimulates the secretion of digestive enzymes from pancreatic acinar cells?

  • Cholecystokinin (CCK)
  • Released from the duodenum in response to fats and proteins
  • Stimulates acinar cells to release digestive enzymes
• How is pancreatic exocrine secretion regulated during digestion?

  • Cephalic phase: Vagal stimulation (ACh) triggers mild enzyme secretion
  • Gastric phase: Gastric distension and peptides stimulate pancreatic output
  • Intestinal phase: Secretin and CCK are released in response to chyme in duodenum, leading to major secretion of HCO₃⁻ and enzymes
• How does the cephalic phase influence pancreatic exocrine secretion?

  • Cephalic phase is triggered by sensory stimuli (e.g., food sight, smell).
  • It stimulates the vagus nerve, leading to the release of acetylcholine, which stimulates the acinar cells of the pancreas to secrete digestive enzymes (e.g., amylase, lipase, proteases).
• What is the role of the vagus nerve in pancreatic exocrine secretion during the cephalic and gastric phases? 

  • In the cephalic phase, it stimulates the release of digestive enzymes via acetylcholine.
  • In the gastric phase, it enhances enzyme secretion and also stimulates the release of bicarbonate to neutralise gastric acid.
• How is pancreatic exocrine secretion regulated during the gastric phase? 

  • The gastric phase is initiated by the presence of food in the stomach, which stimulates the stretch receptors.
  • This activates the vagus nerve to further stimulate enzyme and bicarbonate secretion.
  • Additionally, gastrin is released from G cells, which enhances pancreatic enzyme secretion.
• What is the role of secretin in pancreatic exocrine secretion during the intestinal phase?

  • Secretin is released from the S cells of the duodenum in response to acidic chyme entering the small intestine. It stimulates the pancreas to secrete bicarbonate, which neutralises the acidity of the chyme, creating a suitable pH for enzyme action.
• What is the role of cholecystokinin (CCK) in pancreatic exocrine secretion? 

  • Cholecystokinin (CCK) is released from I cells in the duodenum and jejunum in response to fatty acids and amino acids in the chyme.
  • CCK stimulates the pancreas to release digestive enzymes and also promotes gallbladder contraction to release bile, aiding in digestion.
• How do secretin and CCK work together to regulate pancreatic exocrine secretion in the intestinal phase?

  • Secretin stimulates the pancreas to release bicarbonate, while CCK stimulates the release of digestive enzymes.
  • The combination of bicarbonate and enzymes creates an optimal environment for digestion in the small intestine.
• What is the relationship between the vagus nerve, secretin, and CCK in controlling pancreatic exocrine secretion?

  • The vagus nerve stimulates both enzyme and bicarbonate secretion during the cephalic and gastric phases.
  • In the intestinal phase, secretin and CCK take over: secretin stimulates bicarbonate secretion, while CCK stimulates enzyme release.
  • All three contribute to optimizing digestion.
• What are the physiological effects of pancreatic exocrine secretions during digestion? 

  1. Provide digestive enzymes (amylase, lipase, proteases) for macronutrient breakdown.
  2. Release bicarbonate to neutralise stomach acid and provide an alkaline environment for enzyme activity.
  3. Facilitate the digestion and absorption of nutrients in the small intestine.
• What is the primary exocrine function of the pancreas?

  To secrete digestive enzymes and bicarbonate-rich fluid into the duodenum to aid digestion.
• Name the two main components of pancreatic exocrine secretions

  1. Digestive enzymes (proteases, lipases, amylases, nucleases)
  2. Bicarbonate-rich fluid (HCO₃⁻)