Pancreas and Biliary System Flashcards
Does the small intestine secrete digestive enzymes?
no
The small intestine requires secretions (ie. digestive enzymes) from…
pancreas and liver
What does the pancreas supply? (2)
- enzymes that breakdown proteins, carbohydrates, and fats
- alkaline fluid to neutralize acidic gastric chyme
What does the liver supply?
bile (which emulsifies fats)
Where is the pancreas?
immediately below the stomach
What are parts of (or parts connected to) the pancreas? (4)
- pancreatic duct
- common bile duct
- bile duct ampulla (ampulla of Vater)
- sphincter of hepatopancreatic ampulla (sphincter of Oddi)
What are pancreatic exocrine secretions produced by?
cells in the pancreatic acini – acinar cells and duct cells
What are acinar cells?
cells that line the terminal end of the pancreatic acinus, and secrete pancreatic digestive enzymes
What are dust cells?
cells that line the neck of the pancreatic acinus, and secrete pancreatic aqueous alkaline fluid
What do pancreatic proteolytic enzymes do?
hydrolyze peptide bonds:
What are the three different pancreatic proteolytic enzymes?
trypsinogen (active form = trypsin)
chymotrypsinogen (active form = chymotrypsin)
procarboxypeptidase (active form = carboxypeptidase)
Do pancreatic proteolytic enzymes have potential for autodigestion?
yes – enzymes are stored inactivated in zymogen granules
What type of inhibitor does the pancreas produce?
trypsin inhibitor
How are pancreatic proteolytic enzymes activated?
two-step process:
- inactive trypsinogen is cleaved to active form trypsin by enterokinase
- trypsin activates other enzymes
all three activated enzymes act to digest ingested proteins in lumen
What is pancreatic amylase similar to?
salivary amylase
Does pancreatic amylase threaten pancreatic tissue?
no – therefore secreted in active form
What is pancreatic lipase?
the only enzyme that can digest fat
What happens without pancreatic lipase?
ingested fats remain too large and cannot be absorbed
clinically, ~60-70% of ingested fats are excreted in feces – steatorrhea
What type of pH environment do pancreatic enzymes require?
neutral pH
duodenum is not equipped to deal with acid injury
What is the largest component of pancreatic secretions?
pancreatic aqueous alkaline secretion
What controls the release of pancreatic secretions?
both autonomic and hormonal control
Does autonomic or hormonal control of pancreatic secretions dominate?
hormonal influences dominate
Control of Pancreatic Secretions
Describe what sympathetic and parasympathetic input of autonomic control does.
- sympathetic input decreases all secretions
- parasympathetic input increases all secretions
Control of Pancreatic Secretions
What two hormones control secretions?
- secretin
- CCK
What is secretin secreted by?
duodenal mucosal cells
What does secretin do?
increases NaHCO3 production and release
When does secretin get released?
in presence of acidic chyme in duodenum
What is CCK produced by?
duodenal mucosal cells
What does CCK do?
stimulates production and release of lipase and proteolytic enzymes from pancreatic acinar cells
When does CCK get released?
in presence of fat in duodenum
Control of Pancreatic Secretions
Secretin
- acid in duodenal lumen
- increased secretin release from duodenal mucosa
- pancreatic duct cells “activated”
- increased secretion of aqueous NaHCO3 solution into duodenal lumen – neutralized acid
Control of Pancreatic Secretions
CCK
- fat and protein products in duodenal lumen
- increased CCK release from duodenal mucosa
- pancreatic acinar cells “activated”
- increased secretion of pancreatic digestive enzymes into duodenal lumen – digests fat and protein products
What is the liver?
large visceral organ that performs a variety of functions
In the GI system, what do hepatocytes in the liver do?
produce bile salts – secreted into duodenum via common bile duct
What are the two distinct blood sources that the liver receives?
- venous blood from GI tract
- arterial blood via hepatic artery
What are liver sinusoids?
capillary network in the liver that receives both venousand arterial blood
What does the hepatic portal system do?
allows liver to process and detoxify substances absorbed from GI tract
What is the first-pass effect?
the phenomenon which occurs whenever a drug is administered orally, enters the liver, and suffers extensive biotransformation to such an extent that the bioavailability is drastically reduced, thus showing subtherapeutic action
What is the liver organized into?
lobules – in hexagonal arrangements around a central vein
What are the three vessels in each of the 6 ‘corners’ of the hexagonal lobule arrangement of the liver?
- branch of hepatic artery
- branch of hepatic vein
- bile duct
How are hepatocytes arranged in hepatic lobules?
- arranged such that each hepatocyte borders a sinusoid
- thin bile passage (bile canaliculus) also runs between each layer of hepatocytes
What do hepatocytes continually secrete, and to where?
secrete bile into canaliculus
What is the common bile duct formed by?
bile ducts from different lobules converge to form common bile duct
Enterohepatic Circulation
Where is bile delivered?
to duodenum via sphincter of Oddi
Enterohepatic Circulation
When does the sphincter of Oddi close?
in absence of duodenal chyme
Enterohepatic Circulation
Where does bile go if the sphincter of Oddi is closed?
bile is diverted into gall bladder for storage
What is the gall bladder?
muscular sac that stores and concentrates bile, and contracts to expel bile into duodenum
Enterohepatic Circulation
Where is the majority of bile salts reabsorbed?
in terminal ileum
Enterohepatic circulation provides a method of ____ regulation.
cholesterol
Enterohepatic Circulation
% Bile Salts
- secreted bile salts consist of 95% old, recycled bile salts and 5% newly synthesized bile salts
- 95% of bile salts are reabsorbed by terminal ileum
- reabsorbed bile salts are recycled by enterohepatic circulation
- 5% of bile salts are lost in feces
What are the components of bile? (4)
- alkaline solution (duct cells)
- bile salts (hepatocytes)
- bilirubin (hepatocytes)
- lecithin – phosphatidylcholine (hepatocytes)
What is lecithin?
component of cell membranes
What is bilirubin?
yellowish pigment resulting from RBC degradation
associated with jaundice
What is bilirubin derived from?
derived from iron-containing haemoglobin
What are bile salts?
deprotonated molecules (negatively charged)
What are bile salts derived from?
derived from cholesterol, but with a negatively charged hydrophilic tail
Where can water-soluble lipase act?
only on surface of fat aggregates
What is the role of bile?
bile salts act as detergent and emulsify large fat aggregates into smaller pieces
- cholesterol portion of bile salts dissolves in fat
- its hydrophilic tail portion exposed to aqueous chyme
- negatively-charged tail portions of one ‘droplet’ repel other droplets
What does colipase do?
anchors lipase to bile salts at surface of fat droplet
What does bile help form?
help form microdroplets of monoglycerides (micelles) following lipase digestion
What are micelles composed of?
bile salts, cholesterol, and lecithin molecules – which form hydrophobic core with hydrophilic tail ‘shell’
What do micelles do?
shuttle digested fat molecules to epithelial surface
Structure and Function of Bile Salts
- negatively charged H2O-soluble portion (polar groups – all located on one side of the molecule)
- lipid-soluble portion (derived from hydrophobic non-polar cholesterol)
Are small lipid droplets polar or nonpolar?
nonpolar – with bile salt molecules adsrobed on its surface
Control of Bile Secretion
How is the release of bile regulated?
hormonal and autonomic inputs
- CCK triggers contraction of gall bladder, and relaxation of sphincter of Oddi
- vagal efferents augment this response
Control of Bile Secretion
How is the production and secretion of bile regulated?
neuronal – vagal input increases bile production
hormonal – secretin stimulates production and secretion of NaHCO3 component of bile
Control of Bile Secretion
How does bile salt regulate bile production and release?
bile salts in the blood (from reabsorption of bile salts via enterohepatic circulation) is the most potent stimulator of further bile production and release
Control of Bile Secretion
How is the inhibition of bile regulated?
- when fats have left duodenum:
- CCK levels drop
- sphincter of Oddi closes
- bile cannot enter duodenum - hepatocytes continue to manufacture bile until circulating bile salt concentration in enterohepatic circulation declines – this removes the strong activation on hepatocytes, and bile production slows
