9.Chyme, Pancreas and Liver Flashcards
What is the pH and tonicity of chyme as it enters duodenum from stomach?
Low pH and hypertonic
Why does chyme become more hypertonic as digestion takes place?
breakdown of larger molecules into smaller one which are more osmotically active
How is the epithelium in the duodenum protected from the acidic chyme?
Brunners glands secrete alkaline mucus
What happens to tonicity of chyme in the duodenum?
Water drawn into the duodenum from the ECF/circulation and becomes isotonic by the time its leaving the duodenum
Why isn’t chyme diluted in the stomach?q
Stomach wall largely impermeable to water whereas duodenum is permeable
why must chryme release into duodenum be controlled?
Too much can overwhelm duodenum (liver/pancreatic secretions)
- cannot correct hypertonicity and acidity
- controlled by pyloric sphincter
How is the acidity of chyme fixed in the duodenum?
Secretions from ◦ Pancreas - Enzymes - Bicarbonate ions ◦ Liver - Bicarbonate ions - Bile
What stimulates release of secretin?
In response to low pH of chyme
What stimulates release of CCK?
In response to hypertonicity/small peptides/fats within the chyme in the duodenum
What is the effect of secretin on the pancreas?
Stimulate release an aqueous bicarbonate (HCO3-) solution from duct cells
What is the effect of CCK on the pancreas?
Stimulates release of enzymes for digestion to neutralise acidic chyme
What is the effect of CCK on gall bladder?
Causes contraction of gallbladder and relaxation of sphincter of oddi
What is the sphincter of oddi?
a muscular valve that controls the flow of digestive juices (bile and pancreatic juice) through the ampulla of Vater
Into which part of the duodenum does the ampulla of vater open into?
Second part
What percentage of pancreas function is exocrine?
90%
What is the exocrine function of the pancreas innervated by?
•Sympathetic -inhibits - reduces blood supply •Parasympathetic -Vagus (stimulates) •Hormones
What is the basic strcuture of pancreas
- Acinus (enzymes)
- Centroacinar (aqueous component)
- Duct (modifies aqueous secretion)
What stimulates the acinus of the pancreas?
vagus & cholecystokinin (CCK)
What does vagus and CCK stimulate pancreas to produce?
Produces enzymes ◦ Amylases/Lipases (active) ◦ Proteases (inactive) - Trypsin - Chymotrypsin - Elastase - Carboxypeptidase
Describe production and storage of enzymes
◦ Formed on Rough Endoplasmic reticulum ◦ Moved to Golgi complex ◦ Condensing vacuoles ◦ Concentrated in zymogen granules to be stored ◦ Released with appropriate stimulus - Parasympathetic / CCK
Where are inactive proteases stored and where are they activated?
Zymogen granules in acinar cells
Activated in intestinal lumen
Why are Pancreatic Proteases Released in Inactive Forms?
Prevent digestion of pancreas by proteases
Describe zymogen granules
Membrane bound
Contain zymogen
◦ Inactive pre-cursor of an enzyme
What is indicated if pancreatic enzymes (amylase/Lipase) appear in blood?
Signifies pancreatic damage
Where is the aqueous bicarbonate component of pancreatic secretion secreted from?
From duct cells
Which main ducts do pancreatic secretion travel to reach duodenum?
Pancreatic duct which joins with common bile duct to form ampulla of vater
summarise two problems of chyme as it enters duodenum and how it is fixed
- only partially digested
◦ Secrete enzymes for digestion (CCK) - Acidic
◦ Neutralise acidic chyme by releasing an aqueous bicarbonate (HCO3-) solution (secretin)
How does a high flow rate affect pancreas?
◦ Increased secretion of HCO3
◦ Increased HCO3 in pancreatic secretion
What are the functions of the liver?
Largest single organ ◦ Energy metabolism ◦ Detoxification ◦ Plasma protein production Secretion of bile
What does the liver secrete into the duodenum?
Bile (250-1000mL/day)
What does bile consist of and what pH is it?
- Bile acids and bile pigments
- alkaline solution
What is the role of bile in digestion?
Emulsifying fat (lipids) in duodenum so that they can be readily digested by lipases
What are the different lobes of the liver?
Left, right, quadrate, caudate
What are the Chief functional cells of the liver?
hepatocytes
What function do hepatocytes have and what adaptations do they have to support it?
Very active at producing proteins/lipids for export
◦ Contain lots of rough/smooth ER
◦ Stacks of Golgi membranes
Contain lots of Glycogen
What is the shape of the lobules in the liver and what is at each corner?
Hexagonal arrangement.
Triad of portal vein, hepatic artery and bile duct at each corner
What is present at the centre of each lobule in the liver?
Central vein
What do the central vein drain into?
hepatic veins which drain int IVC
What is the difference between hepatic veins and portal vein?
hepatic veins drain liver
portal vein brings blood to liver
what is the hepatic acinus and the different zone?
region of adjoining hepatic lobules made up of 3 zones
of the hepatic acinus, which zone is more likely to be affected by toxins?
1 as it closest to blood coming in
of the hepatic acinus, which zone is more likely to be affected by ischaemia?
3 as furthest from blood coming in
where does all the blood from the gut drain into?
into the liver - portal vein
What is bile deposited into from hepatocytes?
Canaliculi, into bile duct to duodenum
describe the blood flowing into the liver
◦ Venous portal blood
◦ Arterial blood (hepatic arteries)
◦ Flowing toward central vein
◦ Which then drains into hepatic veins (vena cava)
What is bile produced by?
Hepatocytes and duct cells in the liver
What are the two components of bile and what is the difference in where they are produced and what they contain?
◦ Bile acid dependant
- 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
What are the 2 primary bile acids?
◦ Cholic acid
◦ Chenodeoxycholic acid
◦ Further bile acids formed in gut
What are bile salts?
Bile salts are bile acids that are conjugated with the amino acids
◦ Glycine
◦ Taurine
Why are bile acids converted to bile salts?
◦ Bile acids are not always soluble at duodenal pHs
Bile salts generally are
◦ bile salts have an amphipathic structure
How does the structure of bile salts allow emulsification of fats?
Bile salts have an amphipathic structure
• Hydrophilic end (water soluble)
• Hydrophobic end (lipid solube)
• Act at oil/water interface
Why is emulsification of lipids important?
Bile acids emulsify fat into smaller units
◦ Help disperse droplets
◦ Increases surface area for lipases to act
What do bile salts form after emulsification of fats and what is the importance of them?
Micelles
- act as vehicle for transporting hydrophobic molecules (product of lipid digestion) towards of enterocytes
What are the products of lipid digestion transported in micelles?
◦ Cholesterol
◦ Monoglycerides
◦ Free fatty acids
What happens to bile salts that form micelles?
Bile salts reabsorbed in the terminal ileum, returned to the liver in the portal blood. Liver recycles the bile salts
What happens to products of lips digestion once inside the enterocytes?
Lipids diffuse down concentration gradient into intestinal epithelial cell
Inside cell re-esterified back to ◦ Tri-glycerides ◦ Phospholipids ◦ cholesterol Reformed lipids packaged with apoproteins ◦ Chylomicrons
Where do chylomicrons go from enterocytes?
Exocytosis from basolateral membrane ◦ Too large to enter capillaries ◦ Enter lymph capillaries (Lacteals) ◦ Travel through lymphatic system ◦ Re-enter vascular circulation ◦ Thoracic duct
where is bile stored?
Gallbladder
What does the gallbladder do to the concentration of bile and what may be the effect of it??
◦ Concentrates bile (removes water/ions)
◦ This can lead to gallstones
WHat is the action of CCK on gallbladder?
◦ CCK released from duodenum stimulates gallbladder contraction to release bile
◦ Relaxes ‘sphincter of Oddi’
What is steatorrhoea and describe its appearance?
If Bile acids (salts) or pancreatic lipases are not secreted in adequate amounts ◦ Fat appears in faeces ◦ Pale ◦ Floating ◦ Foul smelling
Describe how bilirubin is associated with bile?
Breakdown product of haemoglobin
◦ Conjugated in liver
◦ Secreted into bile
◦ Excreted in faeces
◦ Accumulates in blood if cannot be excreted
◦ jaundice
◦ if water soluble, bilirubin also excreted in urine giving it a dark colour
