19 Secretions of pancreas and small intestine Flashcards
(40 cards)
Pancreatic secretion characteristics/ what factors increase secretion
- High HCO3- (isotonic) increased by:
• Secretin
• Cholecystokinin (CCK) - potentiates secretin - Pancreatic lipase, amylase and proteases increased by:
• Parasympathetic supply
How much does the exocrine pancreas secrete per day into the duodenum?
1 L of fluid per day
What is pancreatic secretion composed of?
Aqueous solution containing:
- Enzymes (digest carbohydrates, proteins and lipids)
- High HCO3- (neutralise stomach H+)
What is the innervation of the exocrine pancreas?
- Parasympathetic - from vagus (stimulates secretion)
2. Sympathetic - inhibits secretion
Where are pancreatic enzymes stored?
Stored in condensed zymogen granules until release
What is the formation of pancreatic secretion
Aqueous component of secretion released from centroacinar cells and ductal cells:
- Pancreatic fluid is an isotonic fluid containing Na+, K+, Cl- and HCO3-
- Modification of the composition by ductal cells results in a fluid secretion rich in HCO3-
Enzymatic component of pancreatic secretion?
Released from acing cells:
- Pancreatic amylase and lipases are secreted as active enzymes
- Pancreatic proteases are secreted in an inactive form and activated in the duodenum (enterokinase)
Regulation of pancreatic secretion (3 phases)
- Cephalic phase:
• Initiated by taste, smell and conditioning
• Mediated by the vagus nerve (mainly enzymatic secretion) - Gastric phase:
• Initiated by distention of the stomach and
• Mediated by vagus nerve (mainly enzymatic secretion) - Intestinal phase:
• ~80% of pancreatic secretion
• Both enzymatic and aqueous secretions are stimulated
How are acinar cells regulated?
- Duodenal I cells secrete CCK - in response to the presence of amino acids, small peptides and fatty acids in the intestinal lumen
- Vagal release of ACh potentiates CCK action
=> Enzymes
How are ductal cells regulated?
- Secretin released by the S cells of the duodenum - major stimulus for aqueous rich HCO3- secretion
- Secretin release is triggered by the arrival of acidic chyme in the duodenum
- ACh and CCK potentiate secretin action
=> Aqueous secretion (Na+, K+, Cl-, HCO3-)
What is the function of bile?
Essential for the digestion and absorption of lipids (water insoluble)
Where is bile produced and stored?
- Produced and secreted by liver
* Stored in gall bladder
What is bile secretion a mixture of?
- Bile salts (50%)
- Bile pigments - e.g bilirubin (2%)
- Cholesterol (4%)
- Phospholipids (40%)
What is the function of bile salts?
Emulsify lipids to prepare them for digestion and solubilise the products of digestion into ‘packets’ called micelles
What stimulates biliary secretion?
- CCK (gallbladder contraction, relaxation of the sphincter of Oddi
- Parasympathetic supply
Functions of the gallbladder (3)?
- Reservoir for bile:
• Stores the bile which is continuously produced by the hepatocytes and flows –> gallbladder through the bile ducts - Concentration of bile:
• Epithelial cells lining the gallbladder absorb ions and water isosmotically - Ejection of bile:
• Begins 30 mins after a meal
• Major stimulus is the release of cholecystokinin from the I cells in the duodenum and jejunum
What is cholecystokinin? Secreted from?
- A 33 amino acid peptide hormone related to gastrin
- Secreted from the I cells of the duodenal and jejunal mucosa in response to the presence of monoglycerides, fatty acids, small peptides and amino acids
Hormonal actions of cholecystokinin?
- Contraction of gall bladder and relaxation of sphincter of Oddi - eject bile (emulsification and solubilisation of dietary fat)
- Secretion of pancreatic enzymes (both lipase and proteases)
- Secretion of pancreatic HCO3
- Growth of exocrine pancreas and gall bladder
- Inhibition of gastric emptying
Control of biliary system?
- Hepatocytes synthesise and secrete the constituents of bile (bile salts, cholesterol, phospholipids, bile pigments, ions and water)
- Bile flows from liver in bile ducts and is stored and concentrated in gall bladder
- Chyme in small intestine triggers CCK release:
• Stimulates gall bladder contraction
• Stimulates relaxation of sphincter of Oddi - When lipid absorption is complete bile salts are recirculated to the liver by the enterohepatic portal circulation
- Bile salts are extracted from the portal blood by hepatocytes (20% loss per day)
What are mixed micelles? Structure?
• Products of lipid digestion (cholesterol, monoglycerides, lysolecithin and free fatty acids) are solubilised in mixed micelles (diameter 5nm)
- Core contains the products of lipid digestion
- Surface coating of bile salts which are amphipathic (soluble in water)
What is digestion?
Chemical breakdown of ingested food into absorbable molecules
What is absorption?
Movement of nutrients, water and electrolytes from the lumen of the intestine to blood
What are the 2 pathways involved in absorption?
- Cellular
2. Paracellular
Structure of the intestinal mucosa:
• Arrangement of SI surface?
• Structure of villi?
• Apical surface of epithelial cells covered by?
• Surface of the small intestine is arranged in circular folds of Keckring (valvulae conniventes)
• Villi project from the folds
- surface of villi are covered with epithelial cells (enterocytes) with mucus secreting cells (goblet cells)
• Apical surfaced of epithelial cells covered by microvilli - brush border
