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53 Cards in this Set
- Front
- Back
WHat bile does
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Bile is responsible for the principal digestive function of the liver. Bile is required for
the digestion and absorption of lipids. Bile emulsifies fats so that they can be digested by lipases. Bile solubilizes fatty acids into micelles, which are essential for absorption. Also, bile is a vehicle for the elimination of toxic substances and wastes from the body. These include cholesterol and bile pigments, drugs and heavy metals. |
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secretion of bile draws with it ions and water, and duct cells add more ions and water similar to pancreatic method
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yep
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bile secretion increaes hydrosatic pressure
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in the bile duct which causes the gall bladder to fill.
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bile in the gallbladder must be concentrated
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water and ions are reabsorbed by the interstitium
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release of bile is controlled
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by CCK and acetylcholine
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Most bile aids are reabsorbed by
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a secondary active process in the illeum
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bile acids are synthesized in the liver from
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cholesterol
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they have a side chain of 3-9 carbons
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ending in cooh groups, there are 4 main acids
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2 main bile acids
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cholic and chenodeoxycholic acid.
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in the gut bacteria dehydroxylate primary bile acids to
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in the lumen of the gut, bacteria dehydroxylate the primary bile
acids to deoxycholic and lithocholic acids. |
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bile acids are returned to the liver through the portal system and are secreted
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back into bile
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Liver conjugates bile acids with glycine and taurine to lower their pKa
to about 3.7 and 1.5 respectively. WHY |
This increases bile acid solubility at the neutral pH of
the intestine. |
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purpose of bile
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This increases the surface area of fatty acids and
facilitates digestion by lipases. |
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Above a certain concentration, amphipaths form micelles
(40-70 A in diameter). |
This is known as the critical micellar concentration or CMC.
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Phospholipids - mainly lecithins.
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Account for 30-40% of organic composition of bile.
They become solubilized in micelles and facilitate bile acid formation of micelles |
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Cholesterol - Bile is the primary excretory pathway for cholesterol (about 4% of bile
organics)...what if more cholesterol |
If there is more cholesterol than can be solubilized, the excess will form crystals,
which serve as nidus for gall stones. |
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Bile Pigments - about 2% of organics. Bilirubin is a byproduct of hemoglobin
degradation. |
Bilirubin is insoluble so the liver conjugates it with glucuronic acid. Bilirubin is
secreted as the soluble salt, bilirubin glucuronide, which is not found in micelles |
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Bile flows
toward the periphery of the organ, countercurrent to blood flow |
This minimizes the
concentration gradient between blood and bile, which increases the efficiency of bile formation. |
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bile canaliculi
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lie between adjacent hepatocytes
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while the illeum actively absorbs conjugted bile salts. the colon and jejunum
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passively absorb ionized unconjugated and conjugated bile salts
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Total bile acid pool in the body is about 2.5 g.
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about 15-30g day (recylced)
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The rate of BA synthesis is inversely proportional to the return of
BAs via the portal vein. |
Normally, 600 mg of BA are synthesized/day. Returning BAs are a
negative feedback on the rate limiting enzyme in BA synthesis from cholesterol. IMPORTANT |
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bile active reabsorb is mediated through a
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Na+ gradient
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In the duodenum,
100% of BA are conjugated and fully dissociated. Therefore, they are charged and not readily absorbed. |
SOLVED BY Intestinal bacteria deconjugate BA, increasing their lipid solubility 9
times. Bacteria also dehydroxylate BA increasing lipid solubility another 4 fold. |
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The active reabsorption mechanism
positioned in the distal ileum insures that critical micelle concentration is maintained throughout most of the intestine and that BA are not fully reabsorbed until all fatty acid digestion products are absorbed. |
Almost all fatty acids are absorbed by the late jejunum.
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Some BA pass into the large intestine and are deconjugated/dehydroxylated by
bacteria. |
These may be passively reabsorbed or excreted in stool.
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Following extraction from the portal blood, primary and secondary BA are
reconjugated and some are rehydroxylated. |
IMPORTANT
A loss of excess BA into the colon leads to diarrhea. Excess BA in the colon inhibits reabsorption of sodium and water. |
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Phospholipids and cholesterol
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their mechanism of secretion by hepatocytes is
unclear but the quantities secreted are directly related to the secretion of BA. |
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making Bilirubin -
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Hemoglobin is degraded by reticuloendothelial cells where porphyrin is
converted to bilirubin. |
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Insoluble bilirubin is carried through the blood by albumin.
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yep
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Hepatocytes extract bilirubin from the blood
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and conjugate it with glucuronic acid, which increases the soluability of the bilirubin
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uptake of bilirubin
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Uptake is mediated by an active anion transport
mechanism that is different from BA uptake mechanisms. |
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Failure to extract bilirubin from blood leads to jaundice.
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Most bilirubin glucuronide is
excreted with the feces and is not reabsorbed in the intestine. |
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Some is unconjugated and
reduced to urobilinogen in the distal SI and colon. Some of this is reabsorbed. |
The liver
can extract some urobilinogen from the portal circulation. This occurs by active transport. |
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However, some urobilinogen enters the systemic circulation and is filtered and not
reabsorbed by the kidney. |
It is excreted in urine.
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Urobilinogen is colorless
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but it can be
oxidized to urobilin and stercobilin, which are yellow pigments. |
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The higher the rate
of return of BA through portal circulation |
the faster they are secreted into bile, the more
water and electrolytes follow, and the larger the volume of bile. they= water and electrolytes |
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Bile ducts contribute to bile as pancreatic ducts contribute to pancreatic juice.
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Secretin stimulates secretion of HCO3 and water by duct cells. HCO3 concentration in bile
duct fluid can be 2-3X plasma. |
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During interdigestive periods
gallbladder (GB) is flaccid and sphincter of oddi is closed. Bile flows into the GB. |
GB
capacity ranges between 20-60 ml. |
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The GB can concentrate bile by
active reabsorption of Na, Cl, HCO3. |
h20 flows down this gradient
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In humans, the GB lumen is -8 mV indicating
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Na+ must be reabsorbed
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Bile remains isotonic with plasma despite the increase in organic concentration. How
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Bile salts,
cholesterol, phospholipids are present in osmotically inactive micelles. Since bile acids are anions many inorganic cations are also bound to micelles and osmotically inactive. |
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The gallbladder contracts
rhythmically and expels bile within 30 min of a meal. |
cck is main stimulus.CCK contracts the smooth
muscle of the GB and relaxes the sphincter of Oddi. |
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Bile secretion is
affected by duodenal contractions. |
It
enters in spurts when duodenum relaxes and not at all when duodenum contracts. |
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Gallstones form when the organic materials stored in the gallbladder precipitate out of
solution. |
80% are cholesterol
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pigment stones
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Pigment stones result
from precipitation of unconjugated bilirubin and calcium. |
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Native Americans have a genetic predisposition to secrete high levels
of cholesterol in bile. |
They have the highest rate of gallstones in the U.S. Most Native
American men have gallstones by the age of 60. |
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Cholesterol-lowering drugs -
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These drugs raise the cholesterol level in bile,
increasing the risk for stone formation. |
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Rapid weight loss
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As the body metabolizes fat during rapid weight loss, it
causes the liver to secrete extra cholesterol into bile. |
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Fasting
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Fasting decreases gallbladder movement, causing the bile to become
overconcentrated with cholesterol. |
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lictholic acid is a secondary bile acid and
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Lithocholic acid is an extremely toxic chemical that causes DNA strand
breaks and mutations. VITamin D prevents |
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There is little or no affect of neural stimulation on bile production. and
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CCK and gastrin have no affect on bile production.
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butt cck and neural vagal
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cause contraction of gb
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