BIOL 2420 Lecture Notes - Lecture 9: Common Bile Duct, Bile Acid, Hepatocyte
26 Apr 2018
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Unit 9 – Lecture 9
The Liver Secretes Bile
- Bile
o Non-enzymatic solution
o Secreted from hepatocytes (liver cells)
o Key components:
▪ Bile salts – facilitate enzymatic fat digestion
▪ Bile pigments – bilirubin – waste products of hemoglobin degradation
▪ Cholesterol – excreted in the feces
o Drugs and other xenobiotics are cleared from the blood by hepatic processing and are
also excreted in bile
▪ Bile salts – act as detergents to make fats soluble during digestion, made from
steroid bile acids combined with amino acids
o Bile secreted by hepatocytes travels in hepatic ducts to the gallbladder (stores and
concentrates the bile solution)
▪ During a meal that includes fats, contraction of the gallbladder sends bile into the
duodenum through the common bile duct
▪ Gallbladder – an organ that is not essential for normal digestion
• If the duct becomes blocked by hard deposits known as gallstones the
gallbladder can be removed without creating long-term problems
o Bile salts are not altered during fat digestion
▪ When they reach the terminal section of the small intestine (ileum) they
encounter cells that reabsorb them and send them back into the circulation
▪ From here, bile salts return to the liver, where the hepatocytes take them back up
and re-secrete them
▪ Recirculation of bile salts is essential to fat digestion because the body’s pool of
bile salts is essential to fat digestion because the body’s pool of bile salts much
cycle from 2-5 times for each meal
▪ Bilirubin and other wastes secreted in bile cannot be reabsorbed and pas into the
large intestine for excretion
Most Digestion Occurs in the Small Intestine
- Intestinal, pancreatic, and hepatic secretion of enzymes and bile is essential for normal digestive
function
o A significant amount of mechanical digestion takes place in the mouth and stomach,
chemical digestion of food there is limited to a small amount of starch breakdown and
incomplete protein digestion in the stomach
- When chyme enters the small intestine, protein digestion stops when pepsin is inactivated at the
higher intestinal pH
o Pancreatic and brush border enzymes then finish digestion of peptides, carbohydrates,
and fats into smaller molecules that can be absorbed
Bile Salts Facilitate Fat Digestion
- Fats and related molecules in the western diet include triglycerides, cholesterol, phospholipids,
long-chain fatty acids, and the fat-soluble vitamins
- Nearly 90% of our fat calories come from triglycerides
o Are the primary form of lipid in both plants and animals
- Fat digestion is complicated by the fact that most lipids are not particularly water soluble
o The aqueous chyme leaving the stomach contains a coarse emulsion of large fat droplets
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