Lecture 5.1: Gastrointestinal Physiology Flashcards
What are zymogen granules?
Specialised storage organelles in the exocrine pancreas that allow the sorting, packaging and regulated apical secretion of digestive enzymes
What pancreatic enzymes are produced in their inactive form? (2)
- Trypsin
- Chymotrypsin
Why are some pancreatic enzymes produced in their inactive form?
If these proteases were released in their active form, they would end up digesting the pancreas causing massive destruction of the pancreas
What enzyme cleaves trypsinogen into trypsin?
Enteropeptidase
What is the role of Cholecystokinin (CCK)? (4)
- Stimulates the gallbladder to contract and release
stored bile into the intestine - Stimulates the secretion of pancreatic juice
- Relaxation of sphincter of Oddi to allow bile into
duodenum - May induce satiety
What hormone stimulates release of enzyme rich secretions from the pancreas?
Gastrin
What hormone stimulates release of hydrogen-carbonate rich secretions from the pancreas?
Secretin
What are the 5 main roles of the Liver?
- Bile production
- Carbohydrate, protein and lipid metabolism
- Protein synthesis
- Vitamin D synthesis
- Detoxification
What is bile comprised of? (6)
- Bile Salts
- Phospholipids
- Cholesterol
- Conjugated Bilirubin
- Electrolytes
- Water
Name 2 bile acids
- Cholic Acid
- Chenodeoxycholic Acid
What cells produce Secretin? Why is secretin produced?
- Released from S cells of duodenal mucosa in response
to acidic chyme - Resulting in release of HCO3- rich solution
What do I-Cells of the duodenal and upper jejunal mucosa produce?
Cholecystokinin (CCK, pancreozymin)
What is the consequence of reduce bile/pancreatic lipase secretion?
Reduced fat emulsification and digestion and uptake and thus there is fat in the stool causing diarrhoea
What are the 4 anatomical lobes of the liver?
- Right
- Left
- Caudate
- Quadrate
What are the functional lobes of the liver? (2)
- True Right
- True Left
What 2 ligaments help suspend the liver in the abdomen?
- Falciform ligament
- Coronary ligament
What is a Hepatic Lobule?
The anatomic unit of the liver
What is a Hepatic Acinus?
A unit that contains a small portal tract at the center and terminal hepatic venules at the periphery
How is bilirubin conjugated in the liver?
Bilirubin is conjugated within the hepatocyte to glucuronic acid by uridine-diphosphoglucuronic glucuronosyltransferase (UDPGT)
What is conjugated bilirubin?
The form of bilirubin which has been conjugated with glucoronic acid and is excreted in the bile
What is unconjugated bilirubin?
Bilirubin that is bound to a certain protein (albumin) in the blood
Where is bile made?
Liver
Where is bile stored?
Gallbladder
What are the functions of bile? (4)
- Aid in the digestion of fat via fat emulsification
- Absorption of fat and fat-soluble vitamins
- Excretion of bilirubin and excess cholesterol
- Provides an alkaline fluid in the duodenum to
neutralise the acidic pH of the chyme that comes from
the stomach
How much of pancreatic tissue is exocrine?
95-98%
Pancreatic juice flows via the………. and enters the…..
Pancreatic juice flows via the main pancreatic duct and enters the duodenum
Bile and main pancreatic duct converge near to the duodenum at the…..?
Hepatopancreatic ampulla
What controls the entry of contents into the duodenum?
Sphincter of Oddi
Functional anatomy of the exocrine pancreas
- It consists of lobules of acinar cells that secrete
enzymes/fluid into microscopic ducts - Intercalated ducts
- The intercalated ducts are lined with epithelial cells
(duct cells) that secrete a HCO3- rich solution - Intercalated ducts drain into intralobular ducts and in
turn into interlobular ducts - Interlobular ducts empty into the main pancreatic duct
(this runs along the length of the pancreas) - The main pancreatic generally fuses with the bile duct
before draining into the duodenum
What is the smaller pancreatic duct that empties directly into the duodenum called?
Duct of Santorini
How ml of Pancreatic Juice produced everyday?
~1500ml
What are the 2 components of Pancreatic Juice?
- Aqueous
- Enzyme
What does the aqueous component of pancreatic juice contain?
- HCO3- rich
- pH ~ 8
- Neutralizes acidic chyme (along with intestinal
secretions)
What does the enzyme component of pancreatic juice contain?
Proteases, carbohydrase’s, lipases – all contained within the aqueous component
What cells are primarily involved in the formation of alkaline pancreatic fluid?
- The source is primarily the columnar epithelial cells
that line the ducts - At rest from the intercalated and intralobular ducts
- When stimulated interlobular ducts as well
How is alkaline pancreatic fluid formed?
- HCO3- ions (from CO2) exchanged for Clions in the
lumen and H+ ions exchanged for Na+in the interstitial
fluid - Cl- ions cross the apical membrane via Cl- channels
including the cystic fibrosis transmembrane
conductance regulator (CFTR) - Na+ ions diffuse from the interstitial fluid via a
paracellular pathway (maintains electroneutrality) and
water follows by osmosis (can be either transcellular
or paracellular)
What enzyme is responsible for the majority of the digestion of polysaccharides? How does it work?
- Pancreatic amylase
- It breaks 1,4 glycosidic bonds (not 1,6 bonds that link
the chains of glucose molecules)
Main products of pancreatic amylase activity (3)
- Disaccharides
- Trisaccharide’s
- α-limit dextrins
Fat within chyme undergoes…?
Emulsification (essentially large aggregations of lipids are dispersed to become more numerous smaller aggregations)
Why is emulsification important?
Increase the surface area for pancreatic lipases to act
Pancreatic lipase can be inhibited by bile salts, how it its activity restored?
By combining with colipase
How is Pancreatic Secretion regulated?
Neuroendocrine Control
Regulation of Pancreatic Secretion: Parasympathetic
- Acinar and ductal cells express M3 receptors
- PANS activation results in increased secretions of both
cells - SANS inhibits secretion – probably via decreasing
blood flow
Regulation of Pancreatic Secretion: Endocrine
- Secretin and CCK stimulate the production of the
HCO3- rich fraction of pancreatic juice (maintains
duodenal pH)
Control of pancreatic secretion can be divided into 3 phases, name them
- Cephalic
- Gastric
- Intestinal
What cells produce bile?
Hepatocytes
What is Cholagogue?
A substance which promotes gallbladder contraction
Bile salts are amphipathic, what does this mean?
They have both hydrophobic (orange) and hydrophilic (blue) regions
How do bile salts emulsify fats?
The bile salts insert themselves into fat globules in chyme – this disrupts the globule forcing regions of it to bud off as lipid droplets
What are two important aspects to the formation of bile acids?
- Bile acid dependent component
- Bile acid independent component
What are two important aspects to the formation of bile acids?: Bile Acid Dependent Component
- The first depends upon how quickly bile salts (BS) are
returned back to the hepatocytes via the
enterohepatic circulation - Active removal from blood with active secretion into
bile
What are two important aspects to the formation of bile acids?: Bile Acid Independent Component
- Secretion of water and electrolytes –active transport of
Na+and passive movement of Cl- and water - Similarly, active transport of HCO3- and passive
movement of water and Na+ B
What percentage of of bile slats that enter the intestine are recycled?
c.95%
How are bile salts that enter the intestine recycled?
- Via an active transport mechanism (ASBT apical
sodium bile transporter - OAT basolateral organic anion transporter) in the distal
ileum - Many return to the liver unaltered
- Some deconjugated in the intestinal lumen
- Some are converted to secondary bile acids
- Some secondary bile acids are insoluble and are
excreted in faeces
