GI - Physiology (Bile/Bilirubin)

Question 1

Of what is the bile composed?

Answer 1

Composed of bile salts (bile acids conjugated to glycine or taurine, making them water soluble), phospholipids, cholesterol, bilirubin, water, and ions

Question 2

What catalyzes the rate-limiting step of bile synthesis?

Answer 2

Cholesterol 7alpha-hydroxylase catalyzes rate-limiting step of bile synthesis

Question 3

What are 3 major functions of bile?

Answer 3

Functions: (1) Digestion and absorption of lipids and fat-soluble vitamins (2) Cholesterol excretion (body’s only means of eliminating cholesterol) (3) Antimicrobial activity (via membrane disruption)

Question 4

What process creates bilirubin?

Answer 4

Product of heme metabolism

Question 5

What removes bilirubin from the blood? With what is it conjugated, and into what substance is it excreted?

Answer 5

Bilirubin is removed from blood by liver, conjugated with glucoronate, and excreted in bile

Question 6

Define direct versus indirect bilirubin. Which is water soluble versus insoluble?

Answer 6

DIRECT BILIRUBIN - conjugated with glucoronic acid; water solube; INDIRECT BILIRUBIN - unconjugated; water insoluble

Question 7

Where does the production of unconjugated bilirubin occur? From what is it produced?

Answer 7

Macrophages; RBCs => Heme => Unconjuaged bilirubin (Indirect bilirubin [water insoluble])

Question 8

What happens to unconjugated bilirubin once in the bloodstream?

Answer 8

Unconjugated bilirubin-albumin complex

Question 9

What enzymes conjugates bilirubin, and where?

Answer 9

UDP-glucuronosyl-transferase; Liver

Question 10

What happens to conjugated bilirubin in the gut?

Answer 10

Gut bacteria convert it to urobilinogen

Question 11

What are the possible fates of conjugated bilirubin once converted to urobilinogen, and what percentage of urobilinogen takes which route?

Answer 11

80% is (1) Excreted in feces as stercobilin, which gives brown color of stool; 20% is either (2) Excreted in urine as urobilin, which gives yellow color of urine (Kidney, 10%) OR (3) Recycled back into enterohepatic circulation (Liver, 90%)

Question 12

What percentage of urobilinogen is excreted in feces? In what form is it secreted, and what effect does this have on stool?

Answer 12

80%; Excreted in feces as stercobilin, which gives brown color of stool

Question 13

What percentage of urobilinogen is excreted in urine? In what form is it secreted, and what effect does this have on urine?

Answer 13

10% of the 20% not excreted in feces (so like 2%); Excreted in urine as urobilin, which gives yellow color of urine (Kidney, 10%)

Question 14

What percentage of urobilinogen is neither excreted in feces nor urine? What route does this urobilinogen take?

Answer 14

90% of the 20% not excreted in feces (so like 18%); Enterohepatic circulation