PHYS: Bile Secretion and Gall Bladder Function Flashcards
Why is bile necessary?
Bile is necessary for the digestion and absorption of lipids in the small intestine
What are the 4 major components of bile (and their relative percentages)?
Bile salts (50%)
Phospholipids (40%)
Cholesterol (4%)
Bile pigments (2%)
What are bile salts?
bile acids + conjugated forms of bile acids
What is the molecule used to make bile salts?
cholesterol
What enzyme acts on cholesterol to form bile salts?
7-alpha-hydrozylase
What is the most common bile acid (primary)?
cholic acid
What happens when cholic acid (primary bile salts) are secreted into the lumen?
primary bile acids are dehydroxylated at C7 (by bacteria) to become secondary bile acids
How do secondary bile acids become bile salts?
they get coupled to glycine or taurine
What is the characteristic of bile salts that allows them to emulsify lipids and solubilize products of digestion into micelles?
amphipathic nature
What is a micelle?
core of hydrophobic lipid products with a lining of phospholipids, lecithin, cholesterol, and bile salts
What is the major role of bile salts?
osmotic driving force for water and electrolytes into the bile duct
What is the major bile pigment? What is its color?
bilirubin (yellow pigment)
How is bilirubin formed?
Hb metabolism
What protein stimulates the secretion of HCO3-, Na+ and water into the bile?
secretin
What organ synthesizes bile?
liver
What is the roll of the gallbladder?
store bile
concentrate bile
eject bile
What protein stimulates the gallbladder to contract?
CCK
What does CCK do to cause the gallbladder to contract?
CCK triggers a vagovagal reflex→ Ach released to contract the gallbladder and relax the sphincter of Oddi
Into what does the gallbladder secrete bile?
bile duct
What is the name of the sphincter though which bile must pass to get into the duodenum?
sphinter of Oddi
How do components of bile salts get reabsorbed?
- passively reabsorbed in dudenum
- actively transported in the ileum
What characteristic of molecules determines if they will get passively reabsorbed out of the duodenum lumen?
lipophillic molecules (ex. unionized bile salts) get passively reabsorbed
What characteristic of molecules determines if they will get actively transported into the duodenum lumen?
hydrophillic molecules (ex. cholic acid with tons of -OH groups or conjugated bile salts that are ionized) are more likely to go to the ileum before being absorbed
What is responsible for the active transport of bile salts into the ileum?
sodium dependent active transporters
What happens after bile salts are absorbed in the ileum?
recirculated to the liver via enterohepatic (portal) circulation
How do hepatocytes reabsorb bile salts from the portal blood?
sodium dependent bile acid uptake that works to pull bile salts and sodium into the cell (along the concentration gradient set up by Na+/K+ ATPase)
How do bile salts in hepatocytes get transported into the bile canaliculi?
facilitated diffusion (sets up osmotic gradient to pull electrolytes in)
What is a major inhibitor of 7-alpha-hydroxylaase?
bile salts
How often do bile salts travel via the enterohepatic circulation?
system is repeated around 12-15 times in a day
Why is the enterohepatic circulation (and ileum) important?
and vastly decreases the amount of bile salts that the liver must replace in a day (only the 5% or 500mg lost in stool)
Where is hemoglobin borken down?
reticuloendothelial system
What is bilirubin bound to in blood?
albumin
What does the liver do with bilirubin?
extracts it and conjugates to to glucoronic acid (forming bilirubin glucoronide)
What happens to bilirubin glucoronide after formation?
bilirubin glucuronide is secreted into bile, and in the duodenum, bacteria can deconjugate it back to bilirubin.
After bilirubin is re-formed int the duodneum, what is its fate?
reduced to urobilinogen
What happens if urobilinogen stays in the intestine?
gets oxidized to stercobilin (give feces its brown color)
OR gets recirculated via the portal blood to the liver.
What happens if urobilinogen enters the systemic blood?
travels to the kidney where it is converted to urobilin (gives urine its dark color)
How does bile get concentrated in the gallbladder?
epithelial cells of gallbladder absorb ions and water in an isosmotic fashion (micelles are negatively charged and bind up excess Na+ in bile to keep it isosmotic–even though it is concentrated)
When does ejection of bile occur?
around 30 minutes after ingestion of a meal
What cells release CCK?
I cells
Does the gallbladder release all the bile in a continuous movement?
no, occurs in pulsatile “spurts”
True or false: all bile produced is supersaturated with cholesterol.
TRUE
Why are obese people more susceptible to cholesterol stones?
high cholesterol levels
What acts as the nidus for crystal formation and precipitation of cholesterol stones?
bilirubin
What is the cause of pigment stones?
bile is saturated with unconjugated bilirubin→ not water soluble→ becomes stones
Why would you get increased levels of unconjugated bilirubin (which leads to pigment stones)?
Gall bladder wall is damaged by bacteria → leads to the increased beta glucuronidase (enzyme that deconjugates bilirubin glucuronide back to unconjugated bilirubin)
