S6) Chyme, Pancreas and Liver Secretions Flashcards
Describe 3 properties of chyme
- Hypertonic (lots of osmotically active molecules compared to plasma)
- Low pH
- Partially digested
Explain how hypertonic chyme exits the stomach but isotonic chyme exits the duodenum
- Initial digestion of food in stomach creates solutes that make the chyme hypertonic
- Stomach wall largely impermeable to water so chyme cannot be diluted
- Duodenum relatively permeable to water so hypertonic chyme draws in water from ECF/circulation
Duodenum secretes secretin and CCK in response to presence of chyme.
What is the action of secretin?
Secretin acts on pancreas to stimulate release an aqueous bicarbonate (HCO3-) component of the pancreatic secretion into the duodenum to neutralise the chyme
Duodenum secretes secretin and CCK in response to presence of chyme.
What is the action of CCK?
- Acts on pancreas to stimulate the release of enzyme component of pancreatic secretion
– carries on the incomplete digestion
- Contracts gallbladder
- Relaxes sphincter of oddi
- it is released from I cells
- stimulated by fats in chyme
How are the exocrine secretions of the pancreas controlled?
– approx 90% of pancreas
– via glands
- Stimulated via gut hormones
I. Secretin
II. CCK
- Also, autonomic control:
I. Sympathetic inhibits (by reducing blood supply)
II. Parasympathetic stimulates (vagus)
Describe the structure of the pancreatic acini
Which enzymes are released from the pancreas?
CCK & vagus nerve stimulate acini to produce enzymes:
- Amylases and lipases (active)
- Proteases (inactive - if it were active we would be digesting our own pancreas)
Identify 4 activated proteases which are derived from the secretions of pancreatic acini
- Trypsin
- Chymotrypsin
- Elastase
- Carboxypeptidase
How are pancreatic proteases produced, transported and stored?
- Protease enzymes (inactive) produced within acinar cells
- transported from RER → Golgi → vacuoles → zymogen granules
- are concentrated and stored in zymogen granules
- released with parasympathetic / cck stimulation
What do zymogen granules contain?
Zymogen granules contain zymogen e.g. trypsinogen; which is converted to trypsin in intestinal lumen - role is to break down protein
What happens when pancreatic duct cells are stimulated?
Duct cells secrete aqueous component (isotonic) and bicarbonate which act to neutralise the acidic chyme
How do pancreatic secretions get to the duodenum?
Pancreatic secretions reach the duodenum via pancreatic duct and ampulla of vater
How does the liver respond to the presence of chyme?
Liver secretes bile (stored in gallbladder) into the duodenum
- clean all the chyme
What does bile consist of?
- Bile acid-dependent: bile acids and pigments
- Bile-acid independent: alkaline solution
What is the role of bile in the duodenum?
Bile emulsifies fat (lipids) in duodenum so that they can be readily digested by pancreatic lipases
Describe the micro-anatomy of the liver
- Lobules form the structural units of the liver
- Hexagonal arrangement of cells, bile ducts and blood vessels
Describe the arrangement of structures in the liver lobule
- Triad of structures at each corner: portal vein (branch), hepatic artery (branch), bile duct
- Central vein in middle
Describe the flow of blood and bile in the liver lobule
- Blood flows in towards central vein via sinusoids from the branches of the portal vein (80%) and hepatic artery (20%)
- Bile flows out along canaliculi and bile duct into duodenum
What is the name of the functional area of the liver lobule?
Functional area of the lobule is called the acinus
With references to the liver, what is the significance of blood draining from the periphery of the lobule towards the centre?
- Substances brought to the liver from gut therefore start at periphery and work towards middle (towards central vein)
- Creates a series of zones (1,2,3) corresponding to distance from arterial blood supply
- zone 1 more susceptible to infection due to close supply of blood
- zone 3 more susceptible to ischemia
Describe the production of bile in the liver
- Bile is continuously produced by hepatocytes and duct cells in liver
- It is only needed intermittently and, thus, is stored in the gallbladder
Describe the secretion of bile into the duodenum
- CCK stimulates the contraction of gallbladder
- Bile is secreted via the common bile duct, then Ampulla of Vater into duodenum
Which structure secretes bile acids and pigments? Bile acid dependant component
Bile acids and pigments are secreted into canaliculi by hepatocytes
Why do bile salts need to emulsify fats?
- Lipids form large globules once in duodenum, with a small surface area for lipases to act
- Bile salts emulsify fat into disperse droplets, increasing surface area for lipases
Bile salts then create micelles with products of lipid breakdown.
What do they do?
Micelles act as a vehicle for transporting hydrophobic molecules (products of lipid digestion) towards the luminal membrane of an enterocyte
They have a hydrophilic head and a hydrophobic tail.
What happens after lipids diffuse into intestinal epithelium cells?
- Inside the enterocyte, lipid molecules are re-esterified back to triglycerides, phospholipids and cholesterol
- Then they are packed with apoproteins within enterocyte, forming chylomicrons
What do chylomicrons do?
Chylomicrons carry digested fats via the lymphatic system to ultimately reach blood circulation
Why are chylomicrons exocytosed from the basolateral membrane of the enterocyte?
- Too large to enter capillaries
- Enter lymphatic capillaries through thoracic duct
What is steatorrhea?
Steatorrhea is when undigested fat appears in faeces due to inadequate secretion of bile acids (salts) or pancreatic lipases
