GI S6 - Liver & Bile Flashcards
What are the functions of the pancreas
Endocrine secretions
Exocrine secretions
What happens to chyme when it leaves the stomach?
Acidity is corrected by the addition of alkali secretions in bile
Hypotonicity is corrected by mass water movement in the duodenum
Digestion is continued by enzyme secretions
Describe the endocrine function of the pancreas
In the islets of langerhans
Secrete insulin and glucagon in response to changes in plasma glucose concentration
Describe the exocrine function of the pancreas
In acini and ducts
Acini produce enzymes in neutral isotonic solution which are carried to the gut lumen via the ducts
The enzymes are transported as inactive precursors, zymogens
Name some enzymes secreted by the acini
Trypsinogen Carboxypeptidases Elastase Chymotrypsin Amylases Lipases
Why are enzymes secreted as zymogens?
Inappropriate activation in the ducts causes severe inflammation and so pancreatitis
What is a clear diagnostic test of pancreatitis?
When pancreatic enzymes (especially amylase) are found in the blood, it means pancreatic cells are damaged
How is acinar secretion controlled?
Stimulated by CCK (cholecystokinin), which is secreted from I cells in the duodenum in response to hypertonicity and peptides in the lumen
Also stimulates by ACh via the parasympathetic postganglionic fibres of the vagus nerve
What is the function of ducts?
Carry pancreatic enzymes to the duodenum
Pump HCO3 into the lumen from the ECF
Describe the mechanism by which HCO3 is secreted
H+ ions are expelled across the basolateral membrane using energy derived from the inward movement of Na
These combine with HCO3 to form H2O and CO2
CO2 enters the cell, where HCO3 and H+ are regenerated
HCO3 is secreted into the lumen and H+ is free to restart the cycle
How is duct cell HCO3 secretion controlled?
Jejunal S cells produce secretin in response to low pH
The effect of secretin is potentiated by CCK
How can the functions of the liver be broadly classified?
Those related to the blood
Those related to the gut
What are some blood related functions of the liver?
Energy storage
Production of various blood components
Detoxification
What are some gut related functions of the liver?
The secretion of bile and its components
The excretion of bile pigments, particularly bilirubin
What is bile composed of?
The bile acid dependent and bile acid independent fractions
Describe the bile acid dependent fraction
Secreted by cells lining the canaliculi
Contains bile acids (salts), cholesterol and bile pigments
Give examples of bile acids
Cholic acid
Deoxycholic acid
Describe the bile acid independent fraction
Secreted by cells lining the intrahepatic ducts
Describe bile acids/salts
Cholesterol derivatives conjugated with amino acids
Travel in micelles - micro particles made up of bile acids, phospholipids and cholesterol
Play an important part in lipid digestion and absorption
Why are bile acids so important for lipid digestion and absorption?
Lipids are hydrophobic so aggregate into large globules which decreases surface area available for enzymatic digestion
Micelles increase surface area so increase rate of lipid digestion
They also carry lipids through the “unstirred layer” to the epithelial cells where the fatty acids can diffuse in
What happens to bile acids after the lipids have been digested and absorbed?
Apart from a small proportion de conjugated by bacterial action, the bile salts remain in the gut until the terminal ileum, where they are absorbed.
From there, they are transported back to the liver via the portal vein from where they are resecreted or stored in the gall bladder
How do gall stones form?
Bile acids are stored in the gall bladder, but their volume is reduced as they are concentrated.
Crystals can form and if they move into the neck of the gall bladder, obstruction can occur, causing the intense pain of biliary colic
Obstruction often leads to inflammation and infection
How is bile acid secretion controlled?
CCK induces the release of bile acids from storage by causing contraction of the smooth muscle of the walls of the gall bladder and ducts
What can often be the result of a problem with bile acid secretion?
Improper lipid digestion
Therefore lipid in the faeces
Known as steatorrhoea
‘Pale, light and offensive’
Describe cirrhosis
Often seen in alcoholics
Progressive destruction of hepatocytes and replacement with fibrous tissue
Fibrous tissue surrounds intrahepatic vessels so impedes circulation and makes the liver firm
This increases pressure, causing portal hypertension
What can portal hypertension cause?
Dilation at points of anastomoses causes varicose veins, which may rupture and cause haemorrhage
You can get oesophageal varices and haemorrhoids
Ascites
Caput medusa
