Session 5: Chyme, Pancreas, and the Liver Flashcards
What protects the duodenum from the low pH, hypertonic and partially digested chyme from the stomach?
Brunner’s glands which secrete alkaline mucosa.
Is the chyme hypertonic once exiting the duodenum?
No, it’s isotonic
Why is the chyme in the stomach hypertonic?
Because the stomach wall is highly impermeable to water. This means that water cannot move into stomach to dilute the chyme.
Why is the chyme exiting the duodenum isotonic?
Because the duodenum is relatively permeable to water allowing water to move in.
The food is only partially digested once entering the duodenum from the stomach. What aids further digestion?
Juices from the pancreas and the liver.
What stimulates the secretion of these juices?
The presence of chyme which in its turn stimulates secretin and CCK.
Also autonomics where sympathetic inhibits secretion and parasympathetic vagal stimulation stimulates the secretion.
Explain the role of secretin.
Acts on pancreas to stimulate the release of an aqueous bicarbonate component to neutralise the acidity of the chyme.
It also acts on the liver to stimulate bicarbonate release there as well.
Explain the role of CCK.
Acts on the pancreas to stimulate the release of enzyme component of the pancreatic secretion.
CCK also acts to contract the gallbladder as well as relax the sphincter of oddi.
Explain the pancreatic structure.
The pancreas has acina which are all connected to ducts.
Magnifying on the acina you can find acinar cells, duct cells, centroacinar cells and a terminal duct.
What do the acinar cells secrete?
Amylases
Lipases
Proteases (inactive) like:
Trypsinogen
Chymotrypsinogen
Proelastase
Procarboxypeptidase
What stimulates the acinar cells?
CCK and vagus stimulation
Why are pancreatic proteases released in their inactive forms?
The proteases are inactive and stored in zymogen granules. They are not released in their active form because they would then start to digest the linings of the pancreas.
What do duct cells secrete?
The aqueous component of the pancreatic juice as well as bicarbonate.
This is in order to neutralise the acidic chyme
Explain the route of the pancreatic juice.
Secreted via acinar cells and duct cells into the small ductules. Travel via the small ductules into larger ducts and then into a long pancreatic duct.
The long pancreatic duct joins with the common bile duct to then empty out into the duodenum via the ampulla of vater and its sphinctor of Oddi.
What activates trypsinogen?
Enterokinase secreted by the intenstinal mucosa
Trypsin itself can also convert trypsinogen into trypsin.
What activates chymotrypsinogen?
Trypsin
What activates procarboxypolypeptidase?
Trypsin
Role of trypsin
Activating other zymogens as well as breaking down protein into smaller constituents like peptides.
It does not digest proteins into amino acids.
Explain the role of chymotrypsin
Breaking down proteins into peptides.
Explain the role of carboxypolypeptidase.
Breaking down peptides into tripeptides, dipeptides and individual amino acids.
What secretes bile?
The liver
What does bile consist of?
Bile acids and bile pigments
as well as
alkaline solution.
Main role of bile.
Emulsifying fat and assist in absorption of the fatty acids as well.
Triad of structures in the liver lobule.
Portal vein
Hepatic artery
Bile duct
What is the functional area of the lobule called?
The acinus
There are three zones of the acinus.
Which zone is most vulnerable to ischaemic damage?
The third zone as blood enters the first zone first.
There are three zones of the acinus.
Which zone is most vulnerable to toxins.
Zone 1
Where is bile produced?
In the hepatocytes and duct cells of the liver.
It is produced continously be only needed intermittently.
Bile is produced continuously but only needed at times. How is this sorted?
By storing the bile in the gallbladder.
How does bile in the gallbladder differ from bile in the liver?
Bile in the gallbladder is much more concentrated and does not have a lot of water and ions.
Which part of the bile is secreted by the hepatocytes?
The bile salts and the pigments
Which part of the bile is secreted by the duct cells.
The alkaline juices.
Explain the production of bile salts.
The precursor is cholesterol which is converted into cholic acid and chenodeoxycolic acid. The acids then combine with glycine and to a lesser extent also taurine to form conjugated bile acids more commonly known as bile salts.
Briefly explain the structure of bile salts.
A hydrophilic end (water soluble) and a hydrophobic (lipid soluble) end.
Explain the role of bile salts
Fat globules have a small surface area so it is hard for the pancreatic lipase to efficiently act on them.
The bile salts emulsify the globules into smaller units which increases the surface area.
This allows the lipases to more efficiently digest the smaller units into free fatty acids and monoglycerides.
What is the problem with free fatty acids in the duodenum?
The hydrolysis of triglycerides into free fatty acids is reversible. This means that as more free fatty acids form the newly formed free fatty acids will start to inhibit the hydrolysis of triglycerides.
How is the inhibition of hydrolysis of triglycerides by free fatty acids solved?
By the bile salts forming micelles and trapping the fatty acids in the micelles.
What is another function of the micelles formed by bile salts?
Acts as a transporter for hydrophobic molecules to allow the fatty acids and monoglycerides to be absorbed by the intestinal epithelial cells.
The contents of the micelles are released into the cytosol of the enterocytes and the bile salts are then recycled.
Explain the entero-hepatic circulation of bile salts.
The bile salts remain in the gut as the micelles do not enter the enterocytes. They are then reused until they are reabsorbed in the terminal ileum. Once absorbed they return to the liver via the portal vein.
The liver recycles the bile salts so it doesn’t have to re-synthesise large volumes.
Once the fat has entered the cytosol of the enterocytes, what happens next?
The fatty acids and the monoglycerides are picked up by the smooth ER. Here they are rebuilt into triglycerides, cholesterol and phospholipids once again.
They are then packed into a new vehicle which is a chylomicron with apoproteins.
The chylomicrons are then exocytosed from the basolateral membrane of the gut into the lympathics.
What is steatorrhea?
Undigested fats in faeces.
Signs of steatorrhea
Pale, floating and foul smelling faeces
Broad causes of steatorrhea
No emulsification (bile)
No digestion (pancreas)
Explain the route of bile.
Produced in the liver and enters the bile caniculi. Then enter the bile duct. The bile duct then becomes the left or right hepatic bile duct.
The left and right hepatic bile ducts then join to form the common hepatic duct.
If there is no stimulation to secrete bile the bile salts will enter into the gallbladder.
The common hepatic duct joins with the cystic duct from the gallbladder to form the common bile duct.
The common bile duct then joins with the pancreatic duct. Enter the duodenum via the ampulla of vater.
Causes of gallstones
Too much absorption of water from bile
Too much absorption of bile acids from bile
Too much cholesterol in bile
Inflammation of epithelium
Explain the relationship between cholesterol and gallstone formation.
Cholesterol is not soluble in water so bile salts and lecithin accompanies the cholesterol and forms micelle to carry it.
Usually as concentration of cholesterol changes the concentration of bile salts and lecithin changes along with it.
However under abnormal conditions if the cholesterol concentration increases too much cholesterol can accumulate in the gallbladder and form crystals.
Explain the relationship between diet and gallstones.
The amount of cholesterol in the bile is dependent on how much fat a person eats.
In the case of a person being on a diet with a lot of fat over a period of years they are prone to develop gallstones.
The pancreatic enzymes are firstly activated in the small intestine to prevent the digestion of the pancreas itself.
What else helps with the inhibition of activation before the enzymes reach the duodenum?
Trypsin inhibitors
Explain trypsin inhibitors.
Formed in the cytoplasm of the glandular cells and prevent the activation of trypsin in the pancreas
Explain how acute pancreatitis can come about.
Pancreatic obstruction or damage to an area of the pancreas can lead to accumulation of pancreatic secretions in that area.
This can lead to the trypsin inhibitors being overwhelmed by an increasing concentration of pancreatic secretions.
This leads to activation of trypsin and the enzymes start to digest the pancreas in just a few hours.
Complications of acute pancreatitis.
Death due to circulatory shock
If not lethal can lead to pancreatic insufficiency
