Liver Structure and Function

Question 1

Describe the composition and function of bile, the mechanisms controlling its storage and release from the gallbladder and the mechanism whereby it is reabsorbed.

Answer 1

a

Question 2

Describe the anatomy of the liver.

Answer 2

• Largest organ in body: approx. 1.4kg (3lb)
• Located in upper right quadrant of abdomen (tucked against inferior surface of diaphragm).
• 2 major lobes: right > left.
• 2 minor lobes: caudate, quadrate.
• Entry/exit of blood vessels (hepatic portal vein, hepatic artery), lymphatic vessels, ducts (right/left hepatic ducts –> common hepatic duct), nerves (hepatic nerve plexus) via porta on inferior surface.
• Common hepatic duct joined by cystic duct from gallbladder –> common bile duct + pancreatic duct –> major duodenal papilla (Sphincter of Oddi).

Question 3

Describe the histology of the liver.

Answer 3

• Liver covered by connective tissue capsule and visceral peritoneum except for bare area (small area on diaphragmatic surface surrounded by coronary ligament – connects liver to diaphragm).
• At porta, connective tissue capsule becomes branching network of septa into body of liver providing support.
• Vessels, ducts and nerves follow septa throughout liver.
• Septa divide liver into hexagonal lobules.
• Portal triad (hepatic portal vein, hepatic artery, hepatic duct) located at each corner of hexagonal lobule (also nerves and lymph vessels).
• Central veins at centre of each lobule –> hepatic veins –> inferior vena cava.
• Hepatic cords (composed of hepatocytes) radiate out from central veins.
• Bile canaliculus (cleft-like lumen) lied between cells within each cord.
• Hepatic sinusoids: spaces (blood channels) between hepatic cords.

Question 4

Describe the function of the liver.

Answer 4

• Hepatocytes: bile synthesis, nutrient storage (glycogen, fats, vitamins (B12, A, D, E, K), Cu, Fe), nutrient interconversion, detoxification.

Six components of bile;
- Bile acids, lecithin, cholesterol: synthesised in liver, solubilise fat.
- Bile pigments: bilirubin (from haemoglobin).
- Toxic metals: detoxified in liver.
- Bicarbonate: neutralisation of acid chyme.
1 –> 5 secreted by hepatocytes.
HCO3 secreted by duct cells.

Question 5

Describe bile pigments.

Answer 5

• Breakdown of products of haemoglobin from old/damaged erythrocytes.
• Bilirubin (predominant bile pigment): extracted from blood by hepatocytes and secreted into bile –> yellow bile.
• Bilirubin modified by bacterial enzymes –> brown faeces.
• Reabsorbed bilirubin excreted in urine –> yellow urine.

Question 6

Describe the synthesis and secretion of bile acids.

Answer 6

• Synthesised in lover from cholesterol (0.5g/day).
• Before secretion, bile acids conjugated with glycine or taurine –> bile salts (increase in solubility).
• Secreted bile salts recycled via enterohepatic circulation.
  Liver –> bile duct –> duodenum –> ileum –> hepatic portal vein –> liver…

Question 7

Describe the control of bile secretion.

Answer 7

Sphincter of Oddi;

• Controls release of bile and pancreatic juice into duodenum.
• When contracted (closed) –> bile forced back into gallbladder.
• Gallbladder concentrates bile 5-20 times (absorbs Na and H2O).
• Fat in duodenum –> release of CCK –> Sphincter of Oddi relaxes, gallbladder contracts (stimulates secretion of secretin from acinar cells).

Question 8

Describe the control of bile secretion.

Answer 8

Sphincter of Oddi;

• Controls release of bile and pancreatic juice into duodenum.
• When contracted (closed) –> bile forced back into gallbladder.
• Gallbladder concentrates bile 5-20 times (absorbs Na and H2O).
• Fat in duodenum –> release of CCK –> Sphincter of Oddi relaxes, gallbladder contracts (stimulates secretion of secretin from acinar cells –> pancreatic enzyme and bile secretion).

Question 9

Summarise the actions of secretin.

Answer 9

Acid in duodenum –> secretin –>

• Decrease in gastric acid secretion
• Decrease in gastric emptying
• Increase in duodenal HCO3 secretion
• Increase in pancreatic HCO3 secretion
• Increase in bile duct HCO3 secretion
• -> neutralisation

Question 10

Summarise the actions of cholecystokinin (CCK).

Answer 10

Fat/amino acids in duodenum –> CCK –>

• Decrease in gastric emptying
• Increase in pancreatic enzyme secretion
• Gallbladder contraction
• Relaxation of Sphincter of Oddi
• -> digestion