Bile and bilirubin metabolism

Question 1

Where is bile generated?

Answer 1

liver

Question 2

where is bile stored

Answer 2

gallbladder

Question 3

what are the components of bile?

Answer 3

o Bicarbonate
o Cholesterol
o Phospholipids
o Bile pigments
o Bile salts

Question 4

What are the two important functions of bile?

Answer 4

Bile acids are critical for digestion and absorption of
fats and fat-soluble vitamins in the small intestine.

Many waste products, including bilirubin, are
eliminated from the body by secretion into bile and
elimination in faeces.

Question 5

What can be excreted into bile?

Answer 5

• cholesterol
  Free cholesterol is virtually insoluble in aqueous solutions, but in bile, it is made soluble by bile acids and lipids like lecithin.

Question 6

What are gallstones?

Answer 6

o Gallstones, most of which are composed predominantly of cholesterol, result from processes
that allow cholesterol to precipitate from solution in bile

Question 7

How are bile pigments formed?

Answer 7

Produced when RBC are broken down; generated from breakdown of haem group from haemoglobin

Question 8

Where are bile pigments formed?

Answer 8

Occurs in macrophages of the reticulo-endothelial (RE) system* in the spleen/bone marrow/liver

Question 9

What happens to the products of RBC breakdown?

Answer 9

 Globin is broken down to constituent amino acids and recycled
 Fe2 + is recycled
 The porphyrin ring is converted to bilirubin for transport to the liver for modification and
excretion

Question 10

What is the reticular endothelial system?

Answer 10

 Part of theimmune system; consists of the phagocytic cells located inreticular connective tissue, primarily monocytes andmacrophages.
(Also called the mononuclear phagocytic system)

Question 11

What are the reticuloendothelialcellsfound inthebloodcavitiesoftheliver called?

Answer 11

Kupffer cells

Question 12

What is the function of Kupffer cells?

Answer 12

Thesecellsareconcernedwithbloodcellformationanddestruction,storage
offattymaterials,metabolismofironandpigment; also playaroleininflammationandimmunity

can transform the haemoglobin from disintegrated RBC into bile pigment

Question 13

How is haem broken down into bilirubin?

Answer 13

 Haemoxygenase breaks up the Hb ring to form biliverdin
 Biliverdin is acted on by biliverdin reductase, reducing the double bond to form bilirubin
 At this point the bilirubin is
unconjugated and hydrophobic
 Must bind to serum albumin for transport from the RE system to the liver for biotransformation and
excretion in bile

Question 14

What is bilirubin?

Answer 14

 Bilirubin is an orange-yellow pigment
 Normal plasma samples will be roughly this colour
 Icteric samples (contain excess bilirubin) are dark greenish

Question 15

How is bilirubin conjugated in the liver?

Answer 15

 Unconjugated bilirubin is bound to albumin and transported to hepatocytes in the blood
 In the liver, bilirubin is conjugated to make it water soluble via UDP glucuronyl transferase
 Conjugated bilirubin is transported into bile canaliculi and accumulates in bile within the gallbladder
 From the gallbladder, it can be excreted into the duodenum

Question 16

How is bilirubin metabolised in the intestines?

Answer 16

In the small intestine, beta-glucoronidase converts bilirubin back to its unconjugated form

Intestinal microflora act on the unconjugated form to convert it to metabolites
o Mesobilinogen
o Stercobilinogen
o Urobilinogen

Question 17

W£hat happens to the metabolites of bilirubin?

Answer 17

acted on by flora in the large intestine to form:
o Mesobilin
o Stercobilin - gives faeces brown colour
o Urobilin - gives urine yellow colour

Question 18

Why are bile salts recycled?

Answer 18

Bile salts present in the body are not enough to fully process the fats in a typical meal = they need to be recycled by the enterohepatic circulation

Question 19

How does the pH effect bile salts?

Answer 19

Due to the pH of the small intestine, most of the bile acids are ionized and mostly occur as their sodium
salts - primary conjugated bile salts

Question 20

How do bile salts differ in the lower small intestine?

Answer 20

In the lower small intestine andcolon, bacteria dehydroxylate some of the primary bile salts to form
secondary conjugated bile salts (which are still water-soluble).

Question 21

Where are primary bile salts reabsorbed?

Answer 21

Along the proximal and distal ileum, primary bile salts are reabsorbed into the portal circulation.

Question 22

Where are secondary bile salts reabsorbed from?

Answer 22

Secondary bile acids are reabsorbed predominantly in the colon.

Question 23

How are bile acids reabsorbed from the blood?

Answer 23

Hepatocytes actively extract bile acids from the blood

o This is very efficient, and little escapes the healthy liver into systemic circulation.

Question 24

What is the function of bile salts?

Answer 24

Act as biological emulsifiers, promote emulsification

Question 25

What are bile salts made from?

Answer 25

Consist of cholesterol plus associated acids |  Bile acids are derivatives of cholesterol synthesized in the hepatocyte.

Question 26

How is cholesterol used in the production of bile salts?

Answer 26

 Cholesterol, ingested as part of the diet or derived from hepatic synthesis, is converted into the bile acids cholic and chenodeoxycholic acids  These are then conjugated to an amino acid (glycine or taurine) to yield the conjugated form that is actively secreted into canaliculi.

Question 27

Describe the chemical feature of bile acids

Answer 27

Bile acids are facial amphipathic - contain both hydrophobic and hydrophilic faces. o Cholesterol-derived portion of a bile acid has one face that is hydrophobic (that with methyl groups) and one that is hydrophilic (that with the hydroxyl groups) o Amino acid conjugate is polar and hydrophilic.

Question 28

What is significant about bile acids amphipathic nature?

Answer 28

two important functions: - emulsification of lipid aggregates - solubilisation and transport of lipids in an aqueous environment

Question 29

How does bile acids help break down fat?

Answer 29

Bile acids have detergent action on particles of dietary fat which causes fat globules to break down or be emulsified into minute, microscopic droplets. o Hydrophobic portion binds to and disperses large triglyceride lipid droplets o Hydrophilic portion prevents large droplets reforming o Increases surface area on which triglyceride lipase can act

Question 30

How to bile acids help transport lipids?

Answer 30

Bile acids are lipid carriers and can solubilize many lipids by forming micelles - aggregates of lipids such as fatty acids, cholesterol and monoglycerides - that remain suspended in water.  Bile acids are also critical for transport and absorption of the fat-soluble vitamins

Question 31

What are micelles?

Answer 31

``` very small (4-7 nm) lipid aggregates with hydrophilic (polar) head groups on outside and hydrophilic (non-polar) tails pointing in ``` Bile salts promote micelle formation

Question 32

What is the function of micelles?

Answer 32

Results in efficient digestion and adsorption of lipids and lipid-soluble vitamins (A, D,E,K)

Question 33

What are micelles composed of?

Answer 33

Made of bile salts, fatty acids, monoglycerides, phospholipids, cholesterol and fat soluble vitamins

Question 34

Why do micelles continuously breakdown and reform?

Answer 34

each time the contents are released and some can diffuse across the intestinal lining

Question 35

How do TAGs reform?

Answer 35

in epithelial cells and are packaged into chylomicrons, which enter the blood via the lymph

Question 36

What are the steps in fat absorption?

Answer 36

1) Bile salts emulsify the fat globules in the intestines 2) Bile salts form micelles with FFA produced 3) The absorbed FFAs form TGs and are packaged into chylomicrons for secretion in lacteals

Question 37

Describe the regulation of bile secretion in the interdigestive period

Answer 37

 The flow of bile is lowest during fasting, and most that is diverted into the gallbladder for concentration.  During this period (between meals), the sphincter of Oddi is contracted (closed)  Sphincter of Oddi: a muscular valve that controls flow of bile and pancreatic fluid into the duodenum  Because the sphincter is shut, pressure increases in the common bile duct, and bile flows into the gallbladder  Epithelial cells reabsorb water and electrolytes, thus concentrating the bile

Question 38

Describe the regulation of bile secretion in the digestive period

Answer 38

 Fatty acids and amino acids entering the duodenum (after a meal) stimulate endocrine cells to release cholecystokinin (CCK)  CCK stimulates contraction of gallbladder smooth muscle and relaxes the sphincter of Oddi o The name of this hormone describes its effect on the biliary system - cholecysto = gallbladder and kinin = movement. o The most potent stimulus for release of cholecystokinin is the presence of fat in the duodenum.  Acidic chyme in the duodenum stimulates other endocrine cells to release secretin o Secretin stimulates duct cells in the liver to release bicarbonate into the bile o Secretin also stimulates bile production

Question 39

What are the three groupings of jaundice?

Answer 39

o PRE-HEPATIC – elevated haemolysis o HEPATIC – liver damage o POST-HEPATIC – blockage of bile ducts

Question 40

How are different grouping of jaundice distinguished?

Answer 40

proportion of unconjugated:conjugated bilirubin

Question 41

What is prehepatic jaundice?

Answer 41

(Overproduction of bilirubin - above the capacity of the normal liver to metabolise it)

Question 42

What causes prehepatic jaundice?

Answer 42

``` - Present in several conditions associated with elevated haemolysis 1) More RBC breakdown 2) More Haemoglobin 3) More bilirubin produced (UCB) ``` - Liver cannot cope with increased levels of unconjugated bilirubin o Tropical diseases - Yellow Fever, Malaria o Side-effect of quinine-based anti-malarial drugs o Genetic disorders associated with increased haemolysis (e.g. sickle cell anaemia)

Question 43

What are the causes of neonatal jaundice?

Answer 43

1) Physiological Jaundice of the newborn | 2) Haemolytic disease of the newborn

Question 44

Describe the pathophysiology of physiological jaundice of the newborn

Answer 44

 After birth, newborns must destroy foetal haemoglobin and replace it with adult haemoglobin  The undeveloped liver (with lack of glucuronyltransferase) has insufficient capacity to cope with this elevated haemolysis  On top of that, there are inhibitors of conjugation in breast milk  Bilirubin peaks at 3-5 days and lasts < 14 days  Condition can be treated with phototherapy o Blue light changes the unconjugated bilirubin to a water soluble form that can be excreted

Question 45

Describe Haemolytic disease of the newborn

Answer 45

 alloimmune condition that develops in a foetus, when maternal IgG passes through the placenta. o immune response to nonself antigens from members of the same species (alloantigens)  Rh incompatibility between mother and foetus may also cause haemolysis as antibodies attack antigens on the red blood cells in the foetal circulation, breaking down and destroying the cells  Maternal Rh- blood sensitized by previous pregnancy with RH+ foetus or Rh+ blood transfusion  Can cause very high bilirubin concentration  Risk of kernicterus – bilirubin crosses immature blood brain barrier, deposition of bilirubin in basal ganglia and brainstem nuclei resulting in brain damage if untreated  Treatment – high dose phototherapy and blood transfusion

Question 46

What is hepatic jaundice?

Answer 46

failure of a damaged liver to conjugate bilirubin produced in normal amounts

Question 47

What causes hepatic jaundice?

Answer 47

results from a problem at any point in system: - Impaired uptake of unconjugated bilirubin - Impaired conjugation of bilirubin e. g. Gilberts (reduced glucuronyl transferase activity) - Impaired transport of conjugated bilirubin into bile canaliculi (out of hepatocytes) e. g. primary biliary cholangitis (PBC) (autoimmune destruction of small bile ducts) - Present in conditions that result in liver damage causing cholestasis due to swelling and oedema resulting from inflammation

Question 48

What is the effect of hepatocellular disease on bilirubin metabolism?

Answer 48

there is usually interference in all major steps of bilirubin metabolism—uptake, conjugation and excretion. However, excretion is the rate-limiting step, and usually impaired to the greatest extent. As a result, conjugated hyperbilirubinaemia predominates

Question 49

What is post-hepatic jaundice?

Answer 49

Obstruction of the excretory ducts of the liver that preventing the excretion of bilirubin

Question 50

How can post-hepatic jaundice occur?

Answer 50

Present in conditions associated with obstruction of hepatic, cystic or common bile duct.  Prevents bile from being released into the small intestine = Cholestasis

Question 51

What conditions are common causes of post-hepatic jaundice

Answer 51

- gallstones - pancreatitis - pancreatic tumours

Question 52

How do gallstones contribute to jaundice?

Answer 52

– small pebbles made of cholesterol that move from gall bladder to block ducts. o Arise if capacity of bile salts and phospholipids to solubilise cholesterol is exceeded. o Usually removed surgically o Oral ursodeoxycholic acid treatment (dissolves small gallstones). o Lithotripsy (ultrasonic shock waves) used to break up large stones into smaller stones that can be excreted

Question 53

How does pancreaitis contribute to jaundice?

Answer 53

acute or chronic inflammation of the pancreas following infection or damage. o Swelling can block bile flow.

Question 54

How does pancreatic tumours contribute to jaundice?

Answer 54

tumour growth can | block bile flow

Question 55

What is the link between location of gallstones and symptoms?

Answer 55

Gallstones can be asymptomatic Become increasingly problematic when gallbladder contractions cause stones to move further along the bile ducts

Question 56

What symptoms are present with cystic bile duct gallstones?

Answer 56

(joins gallbladder to common bile duct)  Painful contractions

Question 57

What symptoms are present with common bile duct gallstones?

Answer 57

(links hepatic and cystic duct to the duodenum)  No bile secretion into gut  Steatorrhea  Grey faeces (as no bile pigments - stercobilin)  Post-hepatic jaundice (as reduced excretion of bilirubin)

Question 58

What symptoms are associated with gallstones at the duodenal papilla

Answer 58

(opening of pancreatic duct into duodenum, surrounded by sphincter of Oddi)  No bile or pancreatic secretion into gut  Malnutrition (can’t digest chyme)  Acute pancreatitis

Question 59

Describe the implications Post-hepatic jaundice and has on surgery

Answer 59

Bile salts are required for efficient digestion and adsorption of lipids and lipid-soluble vitamins (A,D,E,K) Vitamin K is required for efficient coagulation factor production a course of Vitamin K is administered parenterally to post-hepatic jaundice patients prior to surgery to prevent haemorrhage

Question 60

How is conjugated bilirubin tested

Answer 60

1. Add diazo reagent to serum. 2. Conjugated bilirubin is converted to blue/purple diazo derivative of bilirubin (Azobilirubin). NB: unconjugated bilirubin is bound to albumin so it does not react with diazo reagent 3. Measurement of absorbance @ 530-545 nm proportional to conjugated bilirubin (not unconjugated). 4. Compare to known standards to calculate concentration.

Question 61

How is total bilirubin tested?

Answer 61

1. Add diazo reagent with caffeine to serum. 2. Caffeine displaces unconjugated bilirubin from albumin. -Free unconjugated bilirubin can now react with the diazo reagent 3. Both conjugated and unconjugated bilirubin converted to blue/purple diazo derivative of bilirubin (Azobilirubin). 4. Measurement of absorbance @ 530-545 nm proportional to BOTH conjugated and unconjugated bilirubin 5. Compare to known standards to calculate concentration.

Question 62

What are the reference ranges of bilirubin in a normal adult?

Answer 62

 Conjugated Bilirubin (adult) < 7 µmol/L |  Total bilirubin (adult) < 21 µmol/L

Question 63

How are bile salts measured in the urine?

Answer 63

Achieved by MultiStix urinalysis - commercially available test strips  Dip in urine, wait, and check colour change

Question 64

What is the significance of finding bilirubin on a multistix urinalysis?

Answer 64

Conjugated bilirubin is water soluble but not detected in urine in health (low concentration) Hyperbilirubinuria (detection of Conjugated bilirubin in urine) is always pathological as it means conjugated bilirubin has leaked back into the blood stream (hepatic or post-hepatic jaundice)

Question 65

What is the implications of urobilinogen results from a urine sample?

Answer 65

Urobilinogen – test area impregnated with p-dimethylaminobenzaldehyde – forms a pink azo dye in the presence of urobilinogen (colourless)  Normally present in low concentrations: high concentrations can indicate increased haemolysis or liver disease (pre-hepatic/hepatic jaundice)  Absence of raised urobilinogen in a jaundiced patient can indicate biliary obstruction (post-hepatic jaundice)

Question 66

How are bile pigments measured in the faeces?

Answer 66

Visual inspection of faeces (colour) is easy and free  Absence of stercobilin is obvious – pale faeces o Indicates biliary obstruction (post-hepatic jaundice)