Download GIT 323

LIVER AND BILE
SECRETION
DR. AMEL EASSAWI
DR. SHAIKH MUJEEB AHMED
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OBJECTIVES
The student should be able to:
 Enlist the functions of Liver.
 Know the blood supply to Liver.
 Describe bile secretion.
 Discuss bile salts and their Entrohepatic
Circulation.
 Know the role of bile Salt in lipid emulsification,
fat digestion and absorption.
 Discuss control of bile secretion.
 Discuss Jaundice.
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LIVER AND BILE SECRETION
• Bile is formed in the liver, emptied in the
duodenum via bile duct.
• Biliary system includes:
- Liver
- Gallbladder
- Ducts
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LIVER FUNCTION
• Liver cell is called HEPATOCYTE ‘HEPATO’ means liver,
‘CYTE’ means cell.
• Metabolism of carbohydrate, protein, and fat, after their
absorption from the digestive tract.
• Detoxification of drugs, hormones, body waste products.
• Plasma protein synthesis.
• Blood clotting factors synthesis.
• Excreting cholesterol and bilirubin [bilirubin is breakdown
product of RBC].
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LIVER FUNCTION
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Store glycogen, fat, iron, copper.
Store vitamin A, D, Vit B12.
Activating vitamin D.
Secreting hormones e.g. insulin like growth factor 1
[stimulates growth], thrombopietin [stimulates platelet
production].
• Removing bacteria and old red blood cell by macrophages.
Hepatocyte does not do Phagocytosis. Phagocytosis is
done by macrophages present in the liver called KUPFFER
cells.
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LIVER BLOOD FLOW
Two sources:
1. Hepatic Artery – supplies arterial blood.
2. Portal Vein – from GIT.
3. Hepatic Vein – carries blood away from the liver.
 NOTE – Portal Vein breaks in sinusoids which exchange
with hepatic cells before draining into hepatic vein which
joins inferior venaceva.
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LIVER BLOOD FLOW
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LIVER BLOOD FLOW
• Liver has functional units called lobules, they are hexagonal
with central vein.
• Each lobule has three vessels:
- Branch of Hepatic Artery
- Branch of Portal Vein
- Bile Duct
• From Hepatic artery and Portal vein blood goes to expended
capillary space called SINUSOIDS which runs to central vein.
• The Kupffer cells line the sinusoids destroy old RBC and
bacteria that pass through them.
• Central veins of all liver lobules converge to form hepatic vein,
which carries blood away from the liver.
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BILE SECRETION
• The bile is formed and continuously secreted by liver
cells [Hepatocyte].
• Bile goes to bile duct.
• Bile ducts from different lobules form right and left
Hepatic duct, it combines with Cystic duct (from Gall
bladder) to form Common bile duct.
• Common bile duct transports bile to the duodenum.
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BILE SECRETION
• Bile is continuously secreted by liver and sent to the
gallbladder between meals.
• Opening of the bile duct in the duodenum is guarded by
the sphincter of Oddi. It allows the bile to pass in the
duodenum during meals.
• When food is not taken sphincter of Oddi is closed ,
therefore bile secreted by the liver goes to the gallbladder.
• In the gallbladder bile is stored and concentrated between
the meals.
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BILE SECRETION
•
Bile secreted 500 ml per day.
• Bile is alkaline fluid pH 8 (has NaHCO3 from bile ducts).
• Bile contains: Bile salts, cholesterol, lecithin (phospholipid)
and bilirubin (all derived from Hepatocyte activity).
• Bile does not contain any enzyme.
• Bile is important for emulsification and helping in digestion
and absorption of fats.
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BILE SECRETION
BILE SALTS AND THEIR ENTROHEPATIC CIRCULATION
• Bile salts are derivates of cholesterol. They are actively
secreted in bile.
• Most of the bile salts [95%] are reabsorbed from terminal
ileum [small intestine] into the blood by active transport.
• Bile salt are returned to liver by hepatic portal system.
• Liver re-secretes bile salts into bile. This recycling of bile
salt between small intestine and liver is called
‘ENTROHEPATIC CIRCULATION’ [‘Entro’ means
intestine, ‘hepatic’ means liver].
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BILE SECRETION
BILE SALTS AND THEIR ENTROHEPATIC CIRCULATION
• On average bile salts cycle between liver and small
intestine twice during one meal.
• Only 5% of bile salt are lost in the feces per day.
• Lost bile salts are replaced by new bile salts synthesized
by liver.
• Bile salts cause lipid emulsification [breaking fat into small
molecules], therefore, increasing the surface area, so that
pancreatic lipase can act.
 NOTE – If no bile salt, it will take long time for lipase to
digest fat.
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BILE SALTS AND FAT
DIGESTION AND ABSORPTION
Micelle Formation
• Micelle is small lipid particle 3-10 nm [emulsified fat
droplet is 1000 nm].
• bile salt, cholesterol and lecithin help in Micelle
formation.
• Micelle carry Monoglycerides and free fatty acids.
Gall Stones
- Cholesterol Gall Stone – 75%
- Bilirubin Gall Stone – 25%
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BILE SALTS AND FAT
DIGESTION AND ABSORPTION
Bilirubin
• It is bile pigment, derived from the breakdown of RBC.
• It does not play any role in digestion.
• Bilirubin is taken from the blood by Hepatocyte and
actively excreted into the bile.
• Bilirubin is yellow pigment, therefore, gives yellow color
to bile.
• In intestine, bilirubin is acted by enzyme and converted
to biliverdin, which gives brown color to feces.
• If bile duct is obstructed by stone, color of feces turns to
grey white.
• Small amount of bilirubin is reabsorbed in the intestine in
the blood and excreted in the urine and gives yellow
color to urine.
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CONTROL OF BILE SECRETION
Three Mechanism:
1. Chemical
2. Hormonal
3. Neural
1. Chemical Mechanism:
• By bile salt, they are most potent stimulus for increasing
bile secretion.
• Any substance which increases bile secretion is called
‘CHOLERETIC’.
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CONTROL OF BILE SECRETION
2. Hormonal Mechanism:
• Secretin hormone stimulates watery alkaline bile
secretion from the bile ducts.
• Food in duodenum [fat] causes release of CCK. This
hormone causes contraction of gallbladder and
relaxation of sphincter of Oddi.
3. Neural Mechanism:
• Vagal stimulation plays minor role in bile secretion during
cephalic phase of digestion.
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GALL BLADDER
• It stores and concentrates bile between meals and
empties bile into the duodenum during meals.
• As bile secretion occurs continuously in liver,
therefore, bile secreted between the meals is send
to the gall bladder, where it is stored and
concentrated.
• In gall bladder, bile is concentrated 5-10 times due
to absorption of bile salt and water.
• Gall bladder can hold 50 ml of bile.
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CLINICAL APPLICATION
Jaundice:
• When bilirubin increases more than 2mg/dL jaundice
occurs.
Causes of Jaundice
• Pre-hepatic [problem before liver] or Hemolytic Jaundice
– due to increased breakdown of RBC.
• Hepatic [problem in liver] e.g. Viral Hepatitis
• Post-hepatic [problem after the liver] or Obstructive
Jaundice e.g. stone in the bile duct.
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REFERENCES
• Human Physiology by Lauralee Sherwood, seventh
edition.
• Text book Physiology by Guyton &Hall,11th edition.
• Text book of Physiology by Linda S. Contanzo, third
edition.
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