Secretory functions of pancreas and liver Flashcards
Exocrine secretions of pancreas
-acini are arranged in anastomosing tubular network
-acini surrounded by myoepithelial cells that can contract to expel contents of the acini into the duct
What stimulates exocrine secretions?
**stimulated by CCK in acini
**stimulated by secretin in ductal system
Pancreas exocrine cell functions in ducts
-Centroacinar and proximal duct cells produce HCO3-, Cl, Na, K ions and water
-intercalated duct cells-minimal secretory function
-interlobular duct cells secrete HCO3, Cl, Na, K, water
-main collecting duct- bicarbonate is exchanged for chloride
pH of pancreas
-typically 7.8
-if the pH in duodenum is low, secretin is released from duodenum causing increased Cl removal and pH rises to ~8.2
Regulation and function of pancreatic exocrine secretion
1.Secretion of HCO3 to neutralize the gastric contents entering the duodenum from stomach (stimulated by secretin in ductal system but CCK in acini
2.Secretion of enzymes (most zymogens) for the luminal digestion of carbohydrates, fat, proteins . Controlled by CCK that is released by fat and protein presence
3.Cholinergic stimulation also enhances the effect of secretin on the pancreas. During cephalic and gastric phases the pancreatic secretions are controlled by nerves
>both enzymes and electrolytes can be stimulated by cholinergic stimulation however it is species dependent
Pancreatic flow and bicarbonate
-as flow increases, bicarbonate also increases
*mediated by secretin
Pancreatic secretion during different phases
-intestinal phase (60-70%; high volume, low proteins)
-cephalic phase (20-25%)
-gastric phase (10%; low volume, high protein)
CCK regulation of pancreatic secretion
-stimulator of acinar secretion. Acts via monitor peptide produced by the pancreatic acinar cells
-synthesized and stored by endocrine cells, predominantly in the duodenum and released in response to digesta
CCK-RP
-CCK releasing peptides
-derived from cells in duodenum and released in response to fatty acids and hydrophobic amino acids
Secretin regulation of pancreatic secretion
-responsible for post meal secretion of HCO3-
-S cells in duodenal mucosa release secretin when pH decreases (due to gastric acid)
-Secretin binds to receptors on pancreatic ducts
**If pH increases secretin release decreases
Function of biliary secretion
-provides source of bile acids needed for fat digestion and absorption
-provide excretory route for certain metabolites and drugs
-provides buffer secreted into duodenum
Structure of liver lobule
-cords of hepatocytes arranged around a central vein
-sinusoids (specialized capillaries) present between the hepatic cords
-portal triad: portal vein, hepatic artery, bile duct
Bile secretion
-bile exits the hepatocytes into the ductules and eventually the bile duct of the triad
Sinusoid regulation
-macrophages (Kupffer cells)- important for RBC removal
-large pores that allow albumin into the extravascular space. Hepatocytes can metabolize/filter
Bile acid/salts
-bile acids are steroid acid (derived from cholesterol) comprising about 80% of the organic compounds in bile (others are phospholipids and cholesterol)
Function of bile acids
-facilitate digestion of dietary fats and oils; serve as micelle-forming surfactants which encapsulate nutrients and facilitate absorption
How are bile acids created?
-created from cholesterol via hydroxylations, oxidations, isomerization, saturations, and reductions
*7alpha-hydroxylase (rate limiting step)
Primary bile acids
-cholic and chenodeoxycholic acid
Bile salt synthesis
-cholic and chenodeoxycholic acid conjugated to taurine and glycine to form taurocholic and glycocholic acids forming bile salts
>this reduced the pKa from around 6 to 4 for glycine conjugates and 2 for taurine acids
»increases solubility and makes them impermeable to cell membranes so high concentrations persist in bile and intestinal contents
Cats and Bile acids
-cats are obligate carnivores because they only use taurine (from protein) to make bile salts. Do not use glycine
Secondary bile salts
-arise from the deconjugation and dihydroxylation of bile acids by gut microbes
>these are deoxycholic and lithocholic acids
>at an increase in pH, they become insoluble and are excreted in the feces
Bile salt absorption
-occurs in intestines
-90% occurring in ilium
Bile salt recycling
-recycled to increase the amount in the GI for proper digestion of fat
*turnover: 6-8x in 24 hrs
-if this is impaired, then fat and fat soluble vitamin malabsorption will occur
Blood bile acids during fasting
0-10 umol/L
Blood bile acids post meal
0-20 umol/L
Extrahepatic biliary tract components
-bile duct
-common bile duct
-gallbladder & sphincter of Oddi
Ruminant and pig bile secretion
-almost continuous secretion of bile into intestines
-sphincter is poorly defined
Horse bile secretion
-continuous flow
-no gallbladder
Other species (humans, moose, elephant, rat, elk, deer) bile secretion
-intermittent flow depending on eating patterns
-sphincter closes when animal not eating and gallbladder is relaxed
-sphincter opens (relaxes) and gallbladder contracts when CCK released
Hepatic excretory function
-hepatocytes perform many detoxifying functions
-bilirubin (end product of hemoglobin) is a very water-insoluble carried in blood bound to albumin
-hepatocytes conjugate bilirubin with glucuronic acid to form bilirubin diglucuronide (more soluble and can be excreted in canaliculi)
**when not excreted=jaundice
