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LECTURE 6Dr. Zahoor Ali Shaikh

LIVER & BILE SECRETION

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Bile is formed in the liver, emptied in the duodenum via bile duct.

Biliary system includes the liver, gallbladder, and ducts.

LIVER & BILE SECRETION

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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.Storing Glycogen, fat, iron, copper.

LIVER FUNCTION

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Storing vitamin A,D, Vit B12.Activating vitamin D.Removing bacteria and old red blood cell by

macrophages.Secreting hormones e.g. insulin like growth

factor 1 [stimulates growth], thrombopietin [stimulates platelet production].

Excreting cholesterol and bilirubin [bilirubin is breakdown product of RBC].

LIVER FUNCTION

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Liver cell is called HEPATOCYTE. ‘HEPATO’ means liver, ‘CYTE’ means cell.HEPATOCYTE performs metabolic and

secretary functions.HEPATOCYTE does not do Phagocytosis.Phagocytosis is done by macrophages present

in the liver called KUPFFER cells.

LIVER FUNCTION

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Two sources 1. Hepatic Artery – supplies arterial blood 2. Portal Vein – from GITBlood leaves the liver via hepatic vein.NOTE – Portal Vein breaks in sinusoids which

exchange with hepatic cells before draining into hepatic vein which joins inferior venaceva.

LIVER BLOOD FLOW

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LIVER BLOOD FLOW

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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 DuctFrom Hepatic artery and Portal vein blood

goes to expended capillary space called SINUSOIDS which runs to central vein.

LIVER BLOOD FLOW

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The Kupffer cells line the sinusoids and 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.

LIVER BLOOD FLOW

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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, then common hepatic duct is formed which combines with Cystic duct (from Gall bladder) to form Common bile duct.

Common bile duct transports bile to the duodenum.

BILE SECRETION

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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

BILE SECRETION

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In the gallbladder bile is stored and concentrated between the meals.

BILE Bile secreted per day --- 500 ml Bile is alkaline fluid PH 8 (it has NaHco3- from

bile ducts) Bile contains Bile salts, cholesterol, lecithin (phospholipid) and Bilirubin (all derived from Hepatocyte activity )

BILE SECRETION

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Bile does not contain any enzyme Bile is important for Emulsification [breaking

fat into small molecules] therefore helping in digestion and absorption if Fats

BILE SALTS AND THEIR ENTROHEPATIC CIRCULATION

Bile salts are derivates of cholesterol. They are actively secreted in bile.

BILE SECRETION

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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 them into the bile. This recycling of bile salt between small intestine and liver is called ‘ENTROHEPATIC CIRCULATION’ [‘Entro’ means intestine, ‘hepatic’ means liver].

BILE SALTS AND THEIR ENTROHEPATIC CIRCULATION

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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.

Last bile salts are replaced by new bile salts synthesized by liver.

BILE SALTS AND THEIR ENTROHEPATIC CIRCULATION

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Bile salts cause lipid emulsification [breaking fat into small molecules], therefore, increasing the surface area, so that pancreatic lipase can act.

These fat globules are triglyceride molecules.To digest fat globules, lipase must act on

them.NOTE – If no bile salt, it will take long time

for lipase to do fat digestion.

BILE SALTS & FAT DIGESTION & ABSORPTION

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MICELLAR 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.

BILE SALTS & FAT DIGESTION & ABSORPTION

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Gall Stones - Cholesterol Gall Stone – 75% - Bilirubin Gall Stone – 25%

GALL STONES

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Bilirubin is waste product excreted in bile.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.

BILIRUBIN

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In intestine, bilirubin is acted by bacteria and converted to urobilinogen and stercobilinogen.

Stercobilinogen is excreted in feces and gives brown color to feces.

If bile duct is obstructed by stone, color of feces turns to grey white.

Small amount of urobilinogen is reabsorbed in the intestine in the blood and excreted in the urine and gives yellow color to urine.

BILIRUBIN

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If increased amount of bilirubin in the blood, it causes Jaundice [yellow coloration of eyes].

Normal bilirubin is less than 2mg /dl.

BILIRUBIN

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Three Mechanism 1. Chemical 2. Hormonal 3. Neural 1. Chemical MechanismBy bile salt, they are most potent stimulus

for increasing bile secretion.Any substance which increases bile secretion

is called ‘CHOLERETIC’.

CONTROL OF BILE SECRETION

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2. Hormonal MechanismSecretin hormone stimulates watery alkaline bile

secretion from the bile ducts.

3. Neural MechanismVagal stimulation plays minor role in bile secretion

during cephalic phase of digestion.

IMPORTANT – Food in duodenum [fat] causes release of CCK. This hormone causes contraction of gallbladder and relaxation of sphincter of ODDI.

CONTROL OF BILE SECRETION

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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 50ml of bile.

GALL BLADDER

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JaundiceWhen bilirubin increases more than 2mg/dL

jaundice occurs. Causes of JaundicePre-hepatic [problem before liver] or Hemolytic

Jaundice – due to increased breakdown of RBC.Hepatic [problem in liver] e.g. Viral HepatitisPost-hepatic [problem after the liver] or

Obstructive Jaundice e.g. stone in the bile duct.

CLINICAL APPLICATION

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Functions of LiverBlood supply to LiverBile SecretionBile Salts and their Entrohepatic CirculationRole of Bile Salt in Lipid Emulsification, Fat

Digestion and AbsorptionBilirubin – Source & ExcretionControl of Bile SecretionJaundice

What You Should Know From This Lecture

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Thank you