Session 6 - Physiology of the liver, biliary tree and pancreas Flashcards
What are the key properties of chyme leaving the stomach?
- Acid
- Hyper tonic
- Partially digested
Where is chyme secreted into?
The duodenum
How is acidity regulated and corrected?
Acidity corrected by HCO3- secreted from pancreas, liver and duodenal mucosa
How is hypertonicity regulated and corrected?
• Hypertonicity corrected by osmotic movement of water across duodenal wall
How is digestion completed in the small intestine?
- Digestion completed by enzymes from pancreas and small intestinal mucosa
- Bile acids from liver
What is Bile made up of?
• Bile acid dependent and bile acid independent
Where is bile acid dependent portion of bile acid secreted from?
• Cells lining canaliculi
Outline the constituent part of bile acid dependent
• Bile acids (salts) - Cholic acid/chenodeoxycholic acid
○ Bile salts conjugated to amino acids, travelling as micelles in biles. Play a major role in digestion and absorption of fats
• Cholesterol
• Bile pigments (majority is bilirubin)
Where is the bile acid independent portion of bile acid secreted from?
• Intrahepatic bile duct
What is the bile acid independent portion of bile?
• Alkaline juice (HCO3-), like that from pancreatic duct cells
What is the basic functional unit of the liver?
• A lobule surrounding a central vein, which drains blood from the liver to the systemic veins
Outline course of blood through liver
Blood from hepatic portal vein and arteries enter vessels at the periphery of the lobule and flow through sinusoids lined by hepatocytes to the central vein
Where is bile formed in the liver?
• Canaliculi, flow towards periphery in bile ducts
What happens after gastric emptying?
- The duodenum secretes cholecystoinin
- This stimulates contraction of the gall bladder, ejecting concentrated bile acids together with enzymes from the pancreas
- Alkali from the pancreas and liver is also released in response to secretin
Outline the path of bile through the body
- Bile acids are released through the ampulla of vater and aid with the digestion of fats.
- They continue to the terminal ileum where actively absorbed by the epithelium
Venous return from the gut enters hepatic portal blood, where hepatocytes actively take up bile acids and re-secrete them into canaliculi
How come not all bile acids are reabsorbed?
• Some are unconjugated from amino acids through the action of gut bacteria and are lost
Hepatocytes subsequently replace them
Where do bile acids return to when they are not needed?
- Gall bladder
* Secreted by canaliculi wall cells a long time before they’re needed
How is the volume of bile acids which need to be stored reduced?
• Concentrated by transport of salt and water across the gall bladder epithelium
What does the concentration process of bile acids lead to?
• Gall stones
What can gall stones cause in the body?
- Move into biliary tree, causing biliary colic
* Obstruction causes inflammation (cholecystitis) and infection of the gall bladder
Why is pain from gall stones worse after eating?
Due to the release of CCK, which causes gall bladder to contract
Name 4 things secreted from pancreas?
- Amylases
- Lipases
- Proteases
- Alkaline juice
Give four types of proteases secreted from pancreas
- Trypsinogen
- Chymotrypsin
- Elastase
- Carboxypetidase
What is the function of having multiple proteases?
• Cleave proteins at different points to break down into amino acids
What is the basic structure of exocrine glands?
• Made up of acini and ducts
What do acini secrete in the pancreas?
Enzymes
What do ducts secrete in the pancreas?
• Alkaline juice (relatively large volumes involved)
Outline the process of acinar secretion
- Enzymes synthesised on ribosomes
- Packaged into condensing vacuoles by golgi complex
- Form zymogen granules
- Zymogen granules secreted by exocytosis
Activated in intestine by enzymatic cleavage
What enzyme can be assayed in the blood to assess pancreatic damage?
Amylase
What hormone stimulates pancreas to secrete enzymes?
• Cholecytokinin
What is CCK?
• Hormone closely related to gastrin
• Released from duodenal APUD cells
○ Stimulated by hypertonicity and fats in the intestinal phase
How does CCK reduce stomach acid conc?
• Competes with gastrin for place on receptors
Give three methods of stimulating pancreatic secretion
- Cephalic phase - Vagus nerve releases Ach
- Intestinal phase - CCK released
Gastrin
How do duct cells secrete HCO3-?
Draw out cell
How is HCO3- secretion stimulated from duct cells
- Hormone secretin
- Released from jejunal cells in response to low pH
- Action of secreting amplified by choleocystokinin
Why is HCO3- high in blood?
Due to gastric acid secretion
Outline process of secretion of HCO3- into ducts of exocrine pancreas
- Na-K-ATPase sets up an Na+ concentration gradient
- Hydrogen ions are exported from the duct cell into ECF using the Na+ concentration gradient
- H+ ions combine with HCO3- to form H20 and CO2 which are taken up into the cell
- H20 and CO2 reform H+ and HCO3- inside the cell
- HCO3- is exported into the duct lumen
- H+ ion is recycled, ‘going around in a circle’ to carry more HCO3- from the ECF to the lumen
What is duct secretion of alkaline juice stimulated by, and where is the substance released from?
- Secretin
* Released from jejunal cells in response to low pH and CCK
Describe the control of acinar pancreatic secretions
- Stimulated by CCK
- Released from duodenal APUD cells
- Stimulated by hypertonicity and fats
Describe the control and release of pancreatic duct secretions
- Stimulated by secretin
- Released from Jejunal cells
- Stimulated in response to low pH
What stimulates biliary secretion?
- CCK
- Released from duodenal APUD cells
- In response to gastric emptying
What are the main difficulties surrounding digestion of fats?
- Tend to form large globules as stomach acid breaks down natural emulsions
- Low surface area for enzymes to act
How do bile acids assist in the digestion of fats?
- Emulsify fats into much smaller globules
- Increase surface area for lipases to cleave fatty acids and glycerol
- Colipase links bile acids and lipases to spread them over surface
How are micelles of fat digested?
- Glycerol cleaved from fatty acid and absorbed through unstirred layer of mucosa on gut wall
- Absorbed as TAGs and re-expelled as chylomicrons, structured small particles made up of lipids covered in phospholipids
What do chylomicrons do?
Facilitate the transport of fat in the lymphatic system from the gut to systemic veins
What is steatorrhoea?
• If bile acids or pancreatic enzymes are not secreted in adequate amounts, fat appears in faeces. Pale, floaty and smelly
What is jaundice?
- Bile pigments are excretory products
- Most common bile pigment in bilirubin, produced as a product of haemoglobin breakdown.
- If bile can be excreted, it accumulates in blood causing jaundice
What are the two main roles of the liver
• Blood related
Gut related
Give three blood facing functions of the liver
- Energy metabolism
- Detoxification
- Plasma proteins
What is the main function of the gut liver?
- Secretion of bile
- Bile acids and alkaline juice for digestion
- Excretion of bile pigments
What are the three parts of the hepatic triad?
- Hepatic portal vein - enters sinusoids lined with hepatocytes
- Branch of hepatic artery
- Bile canaliculi
What lines the ducts of the hepatic triad?
• Hepatocytes
What are two components of bile?
- Bile acid dependent
* Bile acid independent
Where does the bile acid dependent portion of bile come from, and what is it secreted into?
- Canaliculi by hepatocytes
* Contains bile acids and pigments
Where does the bile acid independent portion of bile come from?
- Secreted by duct cells
* Alkaline juice like that from pancreatic duct cells
Give two types of bile acid?
- Cholic acid
* Chenodeoxycholic acid
What happens to bile acids to make them soluble?
• Conjugated to amino acids
• Travel in bile as micelles
○ Bile acids
○ Cholesterol
Phospholipid
