Pancreas and liver Flashcards
Describe chyme as it enters the duodenum from the stomach
- Hypertonic
- Hypertonicity increases as digestion takes place
- Acidic
- Partially digested
What mechanisms are used by the GI system to make chyme more suitable for the duodenum?
- Water is added to chyme from ECF or circulation to make it isotonic
- Enzymes + bicarbonate ions secreted from pancreas
- Bicarbonate ions + bile secreted from liver
Why is chyme hypertonic?
- Food produces lots of solutes that are dissolved in gastric juice
- Stomach is largely impermeable to water
- Water cannot dilute solute in chyme
Outline how water is added to chyme in the duodenum
- Duodenum is relatively permeable to water
- Introduction of hypertonic chyme draws water into duodenum from extracellular fluid
- Pyloric sphincter controls how much chyme enters duodenum so it does not get overwhelmed
- Chyme is isotonic when it leaves duodenum
What is the function of Brunner’s glands?
- Secrete alkaline mucus to protect epithelium of duodenum from acidic chyme
How much of the pancreas is exocrine?
- 90%
What is the basic structure of the pancreas?
- Acinus produces enzymes
- Centroacinar cells produce aqueous component of secretions
- Terminal duct modifies aqueous secretions
- Terminal ducts join to form major pancreatic duct
- Major pancreatic duct releases secretions into duodenum when sphincter of Oddi is relaxed
Outline the innervation of the pancreas
- Sympathetic nervous system inhibits exocrine function by reducing blood supply
- Vagus nerve (parasympathetic) stimulates
- Also regulated by hormones
What stimulates an acinus to release enzymes?
- Vagus nerve and cholecystokinin (secreted by I cells)
- These are stimulated when hypertonicity/small peptides/ fats are detected in the duodenum
Which enzymes are produced by acini?
- Amylases/lipases (active)
- Proteases (inactive) e.g. trypsin, chymotrypsin, elastase, carboxypeptidase
Outline how enzymes are produced and packaged in an acinus
- Formed on rough endoplasmic reticulum
- Moved to Golgi complex
- Condensing vacuoles
- Concentrated in zymogen granules
- Released with appropriate stimulus (parasympathetic or CCK)
Describe zymogen granules
- Membrane bound
- Contain zymogen - an inactive pre-cursor of an enzyme e.g. trypsinogen
- Prevents pancreas being auto-digested
How can we tell if the pancreas has been damaged?
- If pancreatic enzymes appear in blood
What are the functions of the liver?
- Energy metabolism
- Detoxification - everything absorbed from gut drains through liver
- Plasma protein production
- Secretes bile into gut
What are hepatocytes?
- Chief functional cell of liver
- Surround vasculature and bile architecture
- Compromise 80% of mass of liver
- Very active at producing proteins/lipids for export
How are hepatocytes adapted to their function?
- Contain lots of rough/smooth ER
- Contain lots of stacks of Golgi membranes
- Contain lots of glycogen
What is meant by the statement that the liver has a dual blood supply?
- Liver has blood supply from hepatic artery and hepatic portal vein
- Artery gives 20% of blood
- Portal vein gives 80% of blood
- Portal vein blood is not as deoxygenated as normal venous blood, so liver obtains O2 from portal vein
- Venous and arterial blood mixes in sinusoids
Describe the structure of a liver lobule
- Hexagon shaped
- Hepatic triad (bile duct, hepatic artery, portal vein) in each corner
- Central vein in middle
Outline the venous drainage of a liver lobule
- Blood travels from portal vein to central vein
- Central veins merge and drain into hepatic vein
- 3x hepatic veins drain into inferior vena cava
Outline the structure of an acinus
- Region where liver lobules join
- Diamond-shaped
- Has 3 zones
- Zone 1 in closest proximity to portal triads so any toxin coming into liver will affect zone 1 the most
- Zone 3 is closest to central vein so is most likely to be affected by ischaemia
How does bile leave the liver?
- Flows along canaliculi until it reaches bile ducts
- Then flows into duodenum when sphincter of Oddi is relaxed
What are the 2 components of bile?
- Bile acid dependent
- Secreted into canaliculi by hepatocytes
- Contains bile acids and pigments - Bile acid independent
- Secreted by duct cells
- Similar alkaline solution to pancreatic duct cells
- Stimulated by secretin
Bile also contains cholesterol
What are the primary bile acids?
- Cholic acid
- Chenodeoxycholic acid
- Further bile acids are formed in the gut
What are bile salts?
- Bile acids that are conjugated with the amino acids glycine and taurine
Why do we need bile salts in addition to bile acids?
- Bile acids are not always soluble at duodenal pHs whereas bile salts generally are
- Bile salts have an amphipathic structure which a hydrophilic end and a hydrophobic end
- They can therefore act at the oil/water interface
- This is crucial for the emulsification of dietary lipids
How does bile help us digest fats?
- Lipids tend to form large globules by the time they reach the duodenum
- This creates a low surface area over which enzymes can act
- Bile acids emulsify fats into smaller units
- Helps disperse droplets
- Creates a larger SA for lipases to act on
- Bile acids then form micelles with products of lipid breakdown
What are micelles?
- Grouping of bile acids surrounding breakdown products of fat
- Allows transport of hydrophobic molecules (cholesterol, monoglycerides, free fatty acids) through an aqueous environment
- Micelles diffuse with products to brush border of epithelial cells
What happens once lipids have entered the intestinal epithelial cells?
- They are re-esterified back to triglycerides, phospholipids and cholesterol
- Then re-packaged with apoproteins into chylomicrons
- These transport fatty products around the body
How do chylomicrons enter circulation?
- Exocytosis from basolateral membrane of intestinal epithelial cells
- Enter lymph capillaries (too large to enter normal capillaries)
- Travel through lymphatic system
- Re-enter vascular circulation at thoracic duct
Outline the enterohepatic circulation of bile acids
- Bile salts remain in gut lumen
- Reabsorbed in terminal ileum
- Return to liver in portal blood
- Liver extracts bile salts and can then reuse them (instead of synthesising more)
Where is the fundus of the gallbladder found?
- In transpyloric plane at level of L1
What is the function of the gallbladder?
- Stores bile
- Concentrates bile by removing water/ions
- This can lead to gallstones
What stimulates gallbladder contraction?
- CCK released from duodenum
- Relaxes sphincter of Oddi
What is steatorrhoea?
- If bile salts or pancreatic lipases are not secreted in adequate amounts, fat appears in faeces
- Faeces are pale, floating and foul smelling
Which components of bile are excreted?
- Bilirubin
- A breakdown product of haemoglobin
- Conjugated in liver
- Secreted into bile
- Excreted in faeces
- Otherwise accumulates in blood and causes jaundice
