5.1- Liver and pancreas Flashcards
What happens in the duodenum?
- the stomach empties chyme, 3 burts per minute into the duodenum
- Chyme is highly acidic, hypertonic and partly digested
- therefore the chyme needs to be neutralised
Describe the conditioning of chyme’s acidity and hypertonicity
- Acidity corrected by HCO3- secreted from pancreas, liver and duodenal mucosa
- Hypertonicity is corrected by osmotic movement of water across the permeable duodenal wall
-As chyme enters the duodenum, it draws water from ECF and dilutes it down; it lots of water leaves ECF and enters gut
What are the pancreas’ endocrine and exocrine functions?
- Endocrine function: Islets of Langerhans secrete insulin, glucagon and somatostatin ie regulation of blood sugar
- Exocrine function: have acini that secrete proteases, lipases, amylases
What is the exocrine function of the pancreas?
exocrine gland with acini and ducts:
1) Acini secrete enzymes
2) Ducts secrete alkaline juice
Describe how acinar secretions are formed in the pancreas?
- acini secrete enzymes in a neutral isotonic fluid
- enzymes are synthesised on ribosomes as inactive precursors eg trypsinogen
- packaged into condensing vacuoles by golgi complex
- form zymogen granuoles secreted by exocytosis
- these are activated in intestine by enzymatic cleavage to form trypsin
How do enzymes cause pancreatitis?
- happens when peptidases are inappropriately activated in pancreatic ducts and cause inflammation
- In pancreatitis, these enzymes, especially amylase find their way into the blood stream producing a clear diagnostic sign.
What stimulates acinar secretions?
CHOLECYSTOKININ (CCK): signal to secrete enzymes at the right time ie the intestinal phase
- secreted from I cells in the duodenum in response to hypertonicity and presence of peptides in lumen
- acinar cells are also stimulated by ACh from postganglionic parasympathetic nerves activated via the vagus
What do duodenal APUD cells do?
DNES cells act on D cells to release somatostatin; antagonist of gastrin ie turns off acid secretion
- Somatostatin inhibits gastrin
- decreases acid secretions
- therefore decreases HCl
What is CCK stimulated by?
- hypertonicity
- fats
- gastrin
How does duct secretion from the exocrine pancreas work?
- stimulated by secretin, released from S cells in jejunum
- in response to low pH, to neutralize acid
NEGATIVE FEEDBACK: more acid gets pushed out of the stomachh, more HCO3-
action of secretin is facilitated by CCK
together CCK and secretin neutralise chyme
What are the functions of the liver?
1) Metabolism & Endocrinology: -energy metabolism
- makes clotting factors
- components of blood proteins
2) Exocrine function: -makes bile and contains alkaline juice to neutralize stomach acid
- secretes bile pigments and bilirubin
What ducts surround the liver?
- 2 main lobes, both made up of 8 segments (containing lobules)
- lobules are connected to small ducts which connect w larger ducts to form the COMMON HEPATIC DUCT
this transports bile made by liver cels to gallbladder and duodenun via the COMMON BILE DUCT
What are the two sources of blood supply to the liver?
Hepatic artery: oxygenated blood flows in from here
Hepatic Portal Vein: Nutrient-rich blood flows in from the HPV
Blood from HPV enters sinusoids lined with hepatocytes
Drains into central vein
Bile secreted into canaliculi
Bile flows to branch of bile duct
What is the meaning of a portal triad?
- Hepatic Portal Vein
- Hepatic Artery
- Bile Duct
What are the two components of bile?
1) Bile acid dependent:
- secreted into canaliculi by hepatocytes
- contains bile acids and pigments
2) Bile acid-independent:
- secreted by duct cells
- alkaline HCO3-, controlled by secretin
What are the 2 main functions of bile?
1) Elimination of bilirubin, cholesterol and non-water soluble wastes
2) Bile salts promote emulsification and absorption of dietary fat
What are bile acids?
critical for digestion and absorption of fat
-Related to cholesterol
- cholic acid
- chenodeoxycholic acid
- conjugated to amino acids
-travel in bile as micelles; bile acids, cholesterol, phospholipids
How does digestion of fats occur?
lipid digestion is hard bc of the hydrophobic nature of fats
form large globules as stomach acid breaks down natural emulsionlow
- surface area for enzymes to act ( bc of globules)
- bile acids emulsify fat into much smaller globules
- increases SA for lipases to cleave FA’s and glycerol
- colipase links bile acids and lipases to spread them over surface
What is the purpose of micelles in fat emulsification?
- they serve as a vehicle to carry hydrophobic molecules, after lipases and bile acids have acted on fat, through aqueous luminal contents
- carry them into “unstirred layer” of mucous next cells next to epithelial cells
- fatty acids are released slowly and enter cells by diffusion
How are fatty acids processed in epithelial cells?
happens after micelles carry hydrophobic molecules into unstirred layer next to epithelial cells
- fatty acids are used to re-synthesis lipids in epithelial cells
- lipids get exported to lymphatics as chylomicrons
- exist as a lipid with polar protein and phospholipid coat( protective covering)
- get transported in blood and tissues
How are bile acids recovered?
- released into the lumen after fatty acids are absorbed
- continue to terminal ileum
- absorbed actively by epithelium of terminal ileum
- return in hepatic portal blood to hepatic sinusoids
- hepatocytes take up actively and re-secrete into canaliculi
What is the entero-hepatic circulation?
- substances are metabolised in the liver and excreted into bile and passed into intestinal lumen.
- they are reabsrobed across the mucosa and returned to the liver via portal circulation.
What is the meaning of a bile acid pool?
- bile acids remain in gut ie most are recovered, absorbed in the terminal ileum and returned to the liver via the portal vein, from which they are re-secreted.
- but some are unconjugated by bacterial action, in gut and lost
- enterohepatic circulation preserves a pool of bile acids. Losses are replaced by hepatic synthesis. ( in hepatocytes)
What problem can the bile acid pool cause?
- Bile acids return to the liver and are secreted into bile between meals.
- The bile acid dependent fraction of bile therefore, must be stored in the GB till the next meal
- secreted by cannalicular cells well before needed next
- volume stored is reduced by removal of salts and water( transport across GB epithelium)
Concentration process increases risk of precipitation ie GALL STONES
